1 <!doctype html>
2 <html>
3 <head>
4 <meta charset='UTF-8'><meta name='viewport' content='width=device-width initial-scale=1'>
5
6 <link href='https://fonts.loli.net/css?family=Open+Sans:400italic,700italic,700,400&subset=latin,latin-ext' rel='stylesheet' type='text/css' /><style type='text/css'>html {overflow-x: initial !important;}:root { --bg-color:#ffffff; --text-color:#333333; --select-text-bg-color:#B5D6FC; --select-text-font-color:auto; --monospace:"Lucida Console",Consolas,"Courier",monospace; --title-bar-height:20px; }
7 .mac-os-11 { --title-bar-height:28px; }
8 html { font-size: 14px; background-color: var(--bg-color); color: var(--text-color); font-family: "Helvetica Neue", Helvetica, Arial, sans-serif; -webkit-font-smoothing: antialiased; }
9 body { margin: 0px; padding: 0px; height: auto; inset: 0px; font-size: 1rem; line-height: 1.42857; overflow-x: hidden; background: inherit; tab-size: 4; }
10 iframe { margin: auto; }
11 a.url { word-break: break-all; }
12 a:active, a:hover { outline: 0px; }
13 .in-text-selection, ::selection { text-shadow: none; background: var(--select-text-bg-color); color: var(--select-text-font-color); }
14 #write { margin: 0px auto; height: auto; width: inherit; word-break: normal; overflow-wrap: break-word; position: relative; white-space: normal; overflow-x: visible; padding-top: 36px; }
15 #write.first-line-indent p { text-indent: 2em; }
16 #write.first-line-indent li p, #write.first-line-indent p * { text-indent: 0px; }
17 #write.first-line-indent li { margin-left: 2em; }
18 .for-image #write { padding-left: 8px; padding-right: 8px; }
19 body.typora-export { padding-left: 30px; padding-right: 30px; }
20 .typora-export .footnote-line, .typora-export li, .typora-export p { white-space: pre-wrap; }
21 .typora-export .task-list-item input { pointer-events: none; }
22 @media screen and (max-width: 500px) {
23 body.typora-export { padding-left: 0px; padding-right: 0px; }
24 #write { padding-left: 20px; padding-right: 20px; }
25 }
26 #write li > figure:last-child { margin-bottom: 0.5rem; }
27 #write ol, #write ul { position: relative; }
28 img { max-width: 100%; vertical-align: middle; image-orientation: from-image; }
29 button, input, select, textarea { color: inherit; font: inherit; }
30 input[type="checkbox"], input[type="radio"] { line-height: normal; padding: 0px; }
31 *, ::after, ::before { box-sizing: border-box; }
32 #write h1, #write h2, #write h3, #write h4, #write h5, #write h6, #write p, #write pre { width: inherit; }
33 #write h1, #write h2, #write h3, #write h4, #write h5, #write h6, #write p { position: relative; }
34 p { line-height: inherit; }
35 h1, h2, h3, h4, h5, h6 { break-after: avoid-page; break-inside: avoid; orphans: 4; }
36 p { orphans: 4; }
37 h1 { font-size: 2rem; }
38 h2 { font-size: 1.8rem; }
39 h3 { font-size: 1.6rem; }
40 h4 { font-size: 1.4rem; }
41 h5 { font-size: 1.2rem; }
42 h6 { font-size: 1rem; }
43 .md-math-block, .md-rawblock, h1, h2, h3, h4, h5, h6, p { margin-top: 1rem; margin-bottom: 1rem; }
44 .hidden { display: none; }
45 .md-blockmeta { color: rgb(204, 204, 204); font-weight: 700; font-style: italic; }
46 a { cursor: pointer; }
47 sup.md-footnote { padding: 2px 4px; background-color: rgba(238, 238, 238, 0.7); color: rgb(85, 85, 85); border-radius: 4px; cursor: pointer; }
48 sup.md-footnote a, sup.md-footnote a:hover { color: inherit; text-transform: inherit; text-decoration: inherit; }
49 #write input[type="checkbox"] { cursor: pointer; width: inherit; height: inherit; }
50 figure { overflow-x: auto; margin: 1.2em 0px; max-width: calc(100% + 16px); padding: 0px; }
51 figure > table { margin: 0px; }
52 thead, tr { break-inside: avoid; break-after: auto; }
53 thead { display: table-header-group; }
54 table { border-collapse: collapse; border-spacing: 0px; width: 100%; overflow: auto; break-inside: auto; text-align: left; }
55 table.md-table td { min-width: 32px; }
56 .CodeMirror-gutters { border-right: 0px; background-color: inherit; }
57 .CodeMirror-linenumber { user-select: none; }
58 .CodeMirror { text-align: left; }
59 .CodeMirror-placeholder { opacity: 0.3; }
60 .CodeMirror pre { padding: 0px 4px; }
61 .CodeMirror-lines { padding: 0px; }
62 div.hr:focus { cursor: none; }
63 #write pre { white-space: pre-wrap; }
64 #write.fences-no-line-wrapping pre { white-space: pre; }
65 #write pre.ty-contain-cm { white-space: normal; }
66 .CodeMirror-gutters { margin-right: 4px; }
67 .md-fences { font-size: 0.9rem; display: block; break-inside: avoid; text-align: left; overflow: visible; white-space: pre; background: inherit; position: relative !important; }
68 .md-fences-adv-panel { width: 100%; margin-top: 10px; text-align: center; padding-top: 0px; padding-bottom: 8px; overflow-x: auto; }
69 #write .md-fences.mock-cm { white-space: pre-wrap; }
70 .md-fences.md-fences-with-lineno { padding-left: 0px; }
71 #write.fences-no-line-wrapping .md-fences.mock-cm { white-space: pre; overflow-x: auto; }
72 .md-fences.mock-cm.md-fences-with-lineno { padding-left: 8px; }
73 .CodeMirror-line, twitterwidget { break-inside: avoid; }
74 svg { break-inside: avoid; }
75 .footnotes { opacity: 0.8; font-size: 0.9rem; margin-top: 1em; margin-bottom: 1em; }
76 .footnotes + .footnotes { margin-top: 0px; }
77 .md-reset { margin: 0px; padding: 0px; border: 0px; outline: 0px; vertical-align: top; background: 0px 0px; text-decoration: none; text-shadow: none; float: none; position: static; width: auto; height: auto; white-space: nowrap; cursor: inherit; -webkit-tap-highlight-color: transparent; line-height: normal; font-weight: 400; text-align: left; box-sizing: content-box; direction: ltr; }
78 li div { padding-top: 0px; }
79 blockquote { margin: 1rem 0px; }
80 li .mathjax-block, li p { margin: 0.5rem 0px; }
81 li blockquote { margin: 1rem 0px; }
82 li { margin: 0px; position: relative; }
83 blockquote > :last-child { margin-bottom: 0px; }
84 blockquote > :first-child, li > :first-child { margin-top: 0px; }
85 .footnotes-area { color: rgb(136, 136, 136); margin-top: 0.714rem; padding-bottom: 0.143rem; white-space: normal; }
86 #write .footnote-line { white-space: pre-wrap; }
87 @media print {
88 body, html { border: 1px solid transparent; height: 99%; break-after: avoid; break-before: avoid; font-variant-ligatures: no-common-ligatures; }
89 #write { margin-top: 0px; padding-top: 0px; border-color: transparent !important; padding-bottom: 0px !important; }
90 .typora-export * { -webkit-print-color-adjust: exact; }
91 .typora-export #write { break-after: avoid; }
92 .typora-export #write::after { height: 0px; }
93 .is-mac table { break-inside: avoid; }
94 .typora-export-show-outline .typora-export-sidebar { display: none; }
95 }
96 .footnote-line { margin-top: 0.714em; font-size: 0.7em; }
97 a img, img a { cursor: pointer; }
98 pre.md-meta-block { font-size: 0.8rem; min-height: 0.8rem; white-space: pre-wrap; background: rgb(204, 204, 204); display: block; overflow-x: hidden; }
99 p > .md-image:only-child:not(.md-img-error) img, p > img:only-child { display: block; margin: auto; }
100 #write.first-line-indent p > .md-image:only-child:not(.md-img-error) img { left: -2em; position: relative; }
101 p > .md-image:only-child { display: inline-block; width: 100%; }
102 #write .MathJax_Display { margin: 0.8em 0px 0px; }
103 .md-math-block { width: 100%; }
104 .md-math-block:not(:empty)::after { display: none; }
105 .MathJax_ref { fill: currentcolor; }
106 [contenteditable="true"]:active, [contenteditable="true"]:focus, [contenteditable="false"]:active, [contenteditable="false"]:focus { outline: 0px; box-shadow: none; }
107 .md-task-list-item { position: relative; list-style-type: none; }
108 .task-list-item.md-task-list-item { padding-left: 0px; }
109 .md-task-list-item > input { position: absolute; top: 0px; left: 0px; margin-left: -1.2em; margin-top: calc(1em - 10px); border: none; }
110 .math { font-size: 1rem; }
111 .md-toc { min-height: 3.58rem; position: relative; font-size: 0.9rem; border-radius: 10px; }
112 .md-toc-content { position: relative; margin-left: 0px; }
113 .md-toc-content::after, .md-toc::after { display: none; }
114 .md-toc-item { display: block; color: rgb(65, 131, 196); }
115 .md-toc-item a { text-decoration: none; }
116 .md-toc-inner:hover { text-decoration: underline; }
117 .md-toc-inner { display: inline-block; cursor: pointer; }
118 .md-toc-h1 .md-toc-inner { margin-left: 0px; font-weight: 700; }
119 .md-toc-h2 .md-toc-inner { margin-left: 2em; }
120 .md-toc-h3 .md-toc-inner { margin-left: 4em; }
121 .md-toc-h4 .md-toc-inner { margin-left: 6em; }
122 .md-toc-h5 .md-toc-inner { margin-left: 8em; }
123 .md-toc-h6 .md-toc-inner { margin-left: 10em; }
124 @media screen and (max-width: 48em) {
125 .md-toc-h3 .md-toc-inner { margin-left: 3.5em; }
126 .md-toc-h4 .md-toc-inner { margin-left: 5em; }
127 .md-toc-h5 .md-toc-inner { margin-left: 6.5em; }
128 .md-toc-h6 .md-toc-inner { margin-left: 8em; }
129 }
130 a.md-toc-inner { font-size: inherit; font-style: inherit; font-weight: inherit; line-height: inherit; }
131 .footnote-line a:not(.reversefootnote) { color: inherit; }
132 .reversefootnote { font-family: ui-monospace, sans-serif; }
133 .md-attr { display: none; }
134 .md-fn-count::after { content: "."; }
135 code, pre, samp, tt { font-family: var(--monospace); }
136 kbd { margin: 0px 0.1em; padding: 0.1em 0.6em; font-size: 0.8em; color: rgb(36, 39, 41); background: rgb(255, 255, 255); border: 1px solid rgb(173, 179, 185); border-radius: 3px; box-shadow: rgba(12, 13, 14, 0.2) 0px 1px 0px, rgb(255, 255, 255) 0px 0px 0px 2px inset; white-space: nowrap; vertical-align: middle; }
137 .md-comment { color: rgb(162, 127, 3); opacity: 0.6; font-family: var(--monospace); }
138 code { text-align: left; vertical-align: initial; }
139 a.md-print-anchor { white-space: pre !important; border-width: initial !important; border-style: none !important; border-color: initial !important; display: inline-block !important; position: absolute !important; width: 1px !important; right: 0px !important; outline: 0px !important; background: 0px 0px !important; text-decoration: initial !important; text-shadow: initial !important; }
140 .os-windows.monocolor-emoji .md-emoji { font-family: "Segoe UI Symbol", sans-serif; }
141 .md-diagram-panel > svg { max-width: 100%; }
142 [lang="flow"] svg, [lang="mermaid"] svg { max-width: 100%; height: auto; }
143 [lang="mermaid"] .node text { font-size: 1rem; }
144 table tr th { border-bottom: 0px; }
145 video { max-width: 100%; display: block; margin: 0px auto; }
146 iframe { max-width: 100%; width: 100%; border: none; }
147 .highlight td, .highlight tr { border: 0px; }
148 mark { background: rgb(255, 255, 0); color: rgb(0, 0, 0); }
149 .md-html-inline .md-plain, .md-html-inline strong, mark .md-inline-math, mark strong { color: inherit; }
150 .md-expand mark .md-meta { opacity: 0.3 !important; }
151 mark .md-meta { color: rgb(0, 0, 0); }
152 @media print {
153 .typora-export h1, .typora-export h2, .typora-export h3, .typora-export h4, .typora-export h5, .typora-export h6 { break-inside: avoid; }
154 }
155 .md-diagram-panel .messageText { stroke: none !important; }
156 .md-diagram-panel .start-state { fill: var(--node-fill); }
157 .md-diagram-panel .edgeLabel rect { opacity: 1 !important; }
158 .md-fences.md-fences-math { font-size: 1em; }
159 .md-fences-advanced:not(.md-focus) { padding: 0px; white-space: nowrap; border: 0px; }
160 .md-fences-advanced:not(.md-focus) { background: inherit; }
161 .typora-export-show-outline .typora-export-content { max-width: 1440px; margin: auto; display: flex; flex-direction: row; }
162 .typora-export-sidebar { width: 300px; font-size: 0.8rem; margin-top: 80px; margin-right: 18px; }
163 .typora-export-show-outline #write { --webkit-flex:2; flex: 2 1 0%; }
164 .typora-export-sidebar .outline-content { position: fixed; top: 0px; max-height: 100%; overflow: hidden auto; padding-bottom: 30px; padding-top: 60px; width: 300px; }
165 @media screen and (max-width: 1024px) {
166 .typora-export-sidebar, .typora-export-sidebar .outline-content { width: 240px; }
167 }
168 @media screen and (max-width: 800px) {
169 .typora-export-sidebar { display: none; }
170 }
171 .outline-content li, .outline-content ul { margin-left: 0px; margin-right: 0px; padding-left: 0px; padding-right: 0px; list-style: none; }
172 .outline-content ul { margin-top: 0px; margin-bottom: 0px; }
173 .outline-content strong { font-weight: 400; }
174 .outline-expander { width: 1rem; height: 1.42857rem; position: relative; display: table-cell; vertical-align: middle; cursor: pointer; padding-left: 4px; }
175 .outline-expander::before { content: ""; position: relative; font-family: Ionicons; display: inline-block; font-size: 8px; vertical-align: middle; }
176 .outline-item { padding-top: 3px; padding-bottom: 3px; cursor: pointer; }
177 .outline-expander:hover::before { content: ""; }
178 .outline-h1 > .outline-item { padding-left: 0px; }
179 .outline-h2 > .outline-item { padding-left: 1em; }
180 .outline-h3 > .outline-item { padding-left: 2em; }
181 .outline-h4 > .outline-item { padding-left: 3em; }
182 .outline-h5 > .outline-item { padding-left: 4em; }
183 .outline-h6 > .outline-item { padding-left: 5em; }
184 .outline-label { cursor: pointer; display: table-cell; vertical-align: middle; text-decoration: none; color: inherit; }
185 .outline-label:hover { text-decoration: underline; }
186 .outline-item:hover { border-color: rgb(245, 245, 245); background-color: var(--item-hover-bg-color); }
187 .outline-item:hover { margin-left: -28px; margin-right: -28px; border-left: 28px solid transparent; border-right: 28px solid transparent; }
188 .outline-item-single .outline-expander::before, .outline-item-single .outline-expander:hover::before { display: none; }
189 .outline-item-open > .outline-item > .outline-expander::before { content: ""; }
190 .outline-children { display: none; }
191 .info-panel-tab-wrapper { display: none; }
192 .outline-item-open > .outline-children { display: block; }
193 .typora-export .outline-item { padding-top: 1px; padding-bottom: 1px; }
194 .typora-export .outline-item:hover { margin-right: -8px; border-right: 8px solid transparent; }
195 .typora-export .outline-expander::before { content: "+"; font-family: inherit; top: -1px; }
196 .typora-export .outline-expander:hover::before, .typora-export .outline-item-open > .outline-item > .outline-expander::before { content: "−"; }
197 .typora-export-collapse-outline .outline-children { display: none; }
198 .typora-export-collapse-outline .outline-item-open > .outline-children, .typora-export-no-collapse-outline .outline-children { display: block; }
199 .typora-export-no-collapse-outline .outline-expander::before { content: "" !important; }
200 .typora-export-show-outline .outline-item-active > .outline-item .outline-label { font-weight: 700; }
201 .md-inline-math-container mjx-container { zoom: 0.95; }
202 mjx-container { break-inside: avoid; }
203
204
205 .CodeMirror { height: auto; }
206 .CodeMirror.cm-s-inner { background: inherit; }
207 .CodeMirror-scroll { overflow: auto hidden; z-index: 3; }
208 .CodeMirror-gutter-filler, .CodeMirror-scrollbar-filler { background-color: rgb(255, 255, 255); }
209 .CodeMirror-gutters { border-right: 1px solid rgb(221, 221, 221); background: inherit; white-space: nowrap; }
210 .CodeMirror-linenumber { padding: 0px 3px 0px 5px; text-align: right; color: rgb(153, 153, 153); }
211 .cm-s-inner .cm-keyword { color: rgb(119, 0, 136); }
212 .cm-s-inner .cm-atom, .cm-s-inner.cm-atom { color: rgb(34, 17, 153); }
213 .cm-s-inner .cm-number { color: rgb(17, 102, 68); }
214 .cm-s-inner .cm-def { color: rgb(0, 0, 255); }
215 .cm-s-inner .cm-variable { color: rgb(0, 0, 0); }
216 .cm-s-inner .cm-variable-2 { color: rgb(0, 85, 170); }
217 .cm-s-inner .cm-variable-3 { color: rgb(0, 136, 85); }
218 .cm-s-inner .cm-string { color: rgb(170, 17, 17); }
219 .cm-s-inner .cm-property { color: rgb(0, 0, 0); }
220 .cm-s-inner .cm-operator { color: rgb(152, 26, 26); }
221 .cm-s-inner .cm-comment, .cm-s-inner.cm-comment { color: rgb(170, 85, 0); }
222 .cm-s-inner .cm-string-2 { color: rgb(255, 85, 0); }
223 .cm-s-inner .cm-meta { color: rgb(85, 85, 85); }
224 .cm-s-inner .cm-qualifier { color: rgb(85, 85, 85); }
225 .cm-s-inner .cm-builtin { color: rgb(51, 0, 170); }
226 .cm-s-inner .cm-bracket { color: rgb(153, 153, 119); }
227 .cm-s-inner .cm-tag { color: rgb(17, 119, 0); }
228 .cm-s-inner .cm-attribute { color: rgb(0, 0, 204); }
229 .cm-s-inner .cm-header, .cm-s-inner.cm-header { color: rgb(0, 0, 255); }
230 .cm-s-inner .cm-quote, .cm-s-inner.cm-quote { color: rgb(0, 153, 0); }
231 .cm-s-inner .cm-hr, .cm-s-inner.cm-hr { color: rgb(153, 153, 153); }
232 .cm-s-inner .cm-link, .cm-s-inner.cm-link { color: rgb(0, 0, 204); }
233 .cm-negative { color: rgb(221, 68, 68); }
234 .cm-positive { color: rgb(34, 153, 34); }
235 .cm-header, .cm-strong { font-weight: 700; }
236 .cm-del { text-decoration: line-through; }
237 .cm-em { font-style: italic; }
238 .cm-link { text-decoration: underline; }
239 .cm-error { color: red; }
240 .cm-invalidchar { color: red; }
241 .cm-constant { color: rgb(38, 139, 210); }
242 .cm-defined { color: rgb(181, 137, 0); }
243 div.CodeMirror span.CodeMirror-matchingbracket { color: rgb(0, 255, 0); }
244 div.CodeMirror span.CodeMirror-nonmatchingbracket { color: rgb(255, 34, 34); }
245 .cm-s-inner .CodeMirror-activeline-background { background: inherit; }
246 .CodeMirror { position: relative; overflow: hidden; }
247 .CodeMirror-scroll { height: 100%; outline: 0px; position: relative; box-sizing: content-box; background: inherit; }
248 .CodeMirror-sizer { position: relative; }
249 .CodeMirror-gutter-filler, .CodeMirror-hscrollbar, .CodeMirror-scrollbar-filler, .CodeMirror-vscrollbar { position: absolute; z-index: 6; display: none; outline: 0px; }
250 .CodeMirror-vscrollbar { right: 0px; top: 0px; overflow: hidden; }
251 .CodeMirror-hscrollbar { bottom: 0px; left: 0px; overflow: auto hidden; }
252 .CodeMirror-scrollbar-filler { right: 0px; bottom: 0px; }
253 .CodeMirror-gutter-filler { left: 0px; bottom: 0px; }
254 .CodeMirror-gutters { position: absolute; left: 0px; top: 0px; padding-bottom: 10px; z-index: 3; overflow-y: hidden; }
255 .CodeMirror-gutter { white-space: normal; height: 100%; box-sizing: content-box; padding-bottom: 30px; margin-bottom: -32px; display: inline-block; }
256 .CodeMirror-gutter-wrapper { position: absolute; z-index: 4; background: 0px 0px !important; border: none !important; }
257 .CodeMirror-gutter-background { position: absolute; top: 0px; bottom: 0px; z-index: 4; }
258 .CodeMirror-gutter-elt { position: absolute; cursor: default; z-index: 4; }
259 .CodeMirror-lines { cursor: text; }
260 .CodeMirror pre { border-radius: 0px; border-width: 0px; background: 0px 0px; font-family: inherit; font-size: inherit; margin: 0px; white-space: pre; overflow-wrap: normal; color: inherit; z-index: 2; position: relative; overflow: visible; }
261 .CodeMirror-wrap pre { overflow-wrap: break-word; white-space: pre-wrap; word-break: normal; }
262 .CodeMirror-code pre { border-right: 30px solid transparent; width: fit-content; }
263 .CodeMirror-wrap .CodeMirror-code pre { border-right: none; width: auto; }
264 .CodeMirror-linebackground { position: absolute; inset: 0px; z-index: 0; }
265 .CodeMirror-linewidget { position: relative; z-index: 2; overflow: auto; }
266 .CodeMirror-wrap .CodeMirror-scroll { overflow-x: hidden; }
267 .CodeMirror-measure { position: absolute; width: 100%; height: 0px; overflow: hidden; visibility: hidden; }
268 .CodeMirror-measure pre { position: static; }
269 .CodeMirror div.CodeMirror-cursor { position: absolute; visibility: hidden; border-right: none; width: 0px; }
270 .CodeMirror div.CodeMirror-cursor { visibility: hidden; }
271 .CodeMirror-focused div.CodeMirror-cursor { visibility: inherit; }
272 .cm-searching { background: rgba(255, 255, 0, 0.4); }
273 span.cm-underlined { text-decoration: underline; }
274 span.cm-strikethrough { text-decoration: line-through; }
275 .cm-tw-syntaxerror { color: rgb(255, 255, 255); background-color: rgb(153, 0, 0); }
276 .cm-tw-deleted { text-decoration: line-through; }
277 .cm-tw-header5 { font-weight: 700; }
278 .cm-tw-listitem:first-child { padding-left: 10px; }
279 .cm-tw-box { border-style: solid; border-right-width: 1px; border-bottom-width: 1px; border-left-width: 1px; border-color: inherit; border-top-width: 0px !important; }
280 .cm-tw-underline { text-decoration: underline; }
281 @media print {
282 .CodeMirror div.CodeMirror-cursor { visibility: hidden; }
283 }
284
285
286 :root {
287 --side-bar-bg-color: #fafafa;
288 --control-text-color: #777;
289 }
290
291 @include-when-export url(https://fonts.loli.net/css?family=Open+Sans:400italic,700italic,700,400&subset=latin,latin-ext);
292
293 /* open-sans-regular - latin-ext_latin */
294 /* open-sans-italic - latin-ext_latin */
295 /* open-sans-700 - latin-ext_latin */
296 /* open-sans-700italic - latin-ext_latin */
297 html {
298 font-size: 16px;
299 -webkit-font-smoothing: antialiased;
300 }
301
302 body {
303 font-family: "Open Sans","Clear Sans", "Helvetica Neue", Helvetica, Arial, 'Segoe UI Emoji', sans-serif;
304 color: rgb(51, 51, 51);
305 line-height: 1.6;
306 }
307
308 #write {
309 max-width: 860px;
310 margin: 0 auto;
311 padding: 30px;
312 padding-bottom: 100px;
313 }
314
315 @media only screen and (min-width: 1400px) {
316 #write {
317 max-width: 1024px;
318 }
319 }
320
321 @media only screen and (min-width: 1800px) {
322 #write {
323 max-width: 1200px;
324 }
325 }
326
327 #write > ul:first-child,
328 #write > ol:first-child{
329 margin-top: 30px;
330 }
331
332 a {
333 color: #4183C4;
334 }
335 h1,
336 h2,
337 h3,
338 h4,
339 h5,
340 h6 {
341 position: relative;
342 margin-top: 1rem;
343 margin-bottom: 1rem;
344 font-weight: bold;
345 line-height: 1.4;
346 cursor: text;
347 }
348 h1:hover a.anchor,
349 h2:hover a.anchor,
350 h3:hover a.anchor,
351 h4:hover a.anchor,
352 h5:hover a.anchor,
353 h6:hover a.anchor {
354 text-decoration: none;
355 }
356 h1 tt,
357 h1 code {
358 font-size: inherit;
359 }
360 h2 tt,
361 h2 code {
362 font-size: inherit;
363 }
364 h3 tt,
365 h3 code {
366 font-size: inherit;
367 }
368 h4 tt,
369 h4 code {
370 font-size: inherit;
371 }
372 h5 tt,
373 h5 code {
374 font-size: inherit;
375 }
376 h6 tt,
377 h6 code {
378 font-size: inherit;
379 }
380 h1 {
381 font-size: 2.25em;
382 line-height: 1.2;
383 border-bottom: 1px solid #eee;
384 }
385 h2 {
386 font-size: 1.75em;
387 line-height: 1.225;
388 border-bottom: 1px solid #eee;
389 }
390
391 /*@media print {
392 .typora-export h1,
393 .typora-export h2 {
394 border-bottom: none;
395 padding-bottom: initial;
396 }
397
398 .typora-export h1::after,
399 .typora-export h2::after {
400 content: "";
401 display: block;
402 height: 100px;
403 margin-top: -96px;
404 border-top: 1px solid #eee;
405 }
406 }*/
407
408 h3 {
409 font-size: 1.5em;
410 line-height: 1.43;
411 }
412 h4 {
413 font-size: 1.25em;
414 }
415 h5 {
416 font-size: 1em;
417 }
418 h6 {
419 font-size: 1em;
420 color: #777;
421 }
422 p,
423 blockquote,
424 ul,
425 ol,
426 dl,
427 table{
428 margin: 0.8em 0;
429 }
430 li>ol,
431 li>ul {
432 margin: 0 0;
433 }
434 hr {
435 height: 2px;
436 padding: 0;
437 margin: 16px 0;
438 background-color: #e7e7e7;
439 border: 0 none;
440 overflow: hidden;
441 box-sizing: content-box;
442 }
443
444 li p.first {
445 display: inline-block;
446 }
447 ul,
448 ol {
449 padding-left: 30px;
450 }
451 ul:first-child,
452 ol:first-child {
453 margin-top: 0;
454 }
455 ul:last-child,
456 ol:last-child {
457 margin-bottom: 0;
458 }
459 blockquote {
460 border-left: 4px solid #dfe2e5;
461 padding: 0 15px;
462 color: #777777;
463 }
464 blockquote blockquote {
465 padding-right: 0;
466 }
467 table {
468 padding: 0;
469 word-break: initial;
470 }
471 table tr {
472 border: 1px solid #dfe2e5;
473 margin: 0;
474 padding: 0;
475 }
476 table tr:nth-child(2n),
477 thead {
478 background-color: #f8f8f8;
479 }
480 table th {
481 font-weight: bold;
482 border: 1px solid #dfe2e5;
483 border-bottom: 0;
484 margin: 0;
485 padding: 6px 13px;
486 }
487 table td {
488 border: 1px solid #dfe2e5;
489 margin: 0;
490 padding: 6px 13px;
491 }
492 table th:first-child,
493 table td:first-child {
494 margin-top: 0;
495 }
496 table th:last-child,
497 table td:last-child {
498 margin-bottom: 0;
499 }
500
501 .CodeMirror-lines {
502 padding-left: 4px;
503 }
504
505 .code-tooltip {
506 box-shadow: 0 1px 1px 0 rgba(0,28,36,.3);
507 border-top: 1px solid #eef2f2;
508 }
509
510 .md-fences,
511 code,
512 tt {
513 border: 1px solid #e7eaed;
514 background-color: #f8f8f8;
515 border-radius: 3px;
516 padding: 0;
517 padding: 2px 4px 0px 4px;
518 font-size: 0.9em;
519 }
520
521 code {
522 background-color: #f3f4f4;
523 padding: 0 2px 0 2px;
524 }
525
526 .md-fences {
527 margin-bottom: 15px;
528 margin-top: 15px;
529 padding-top: 8px;
530 padding-bottom: 6px;
531 }
532
533
534 .md-task-list-item > input {
535 margin-left: -1.3em;
536 }
537
538 @media print {
539 html {
540 font-size: 13px;
541 }
542 pre {
543 page-break-inside: avoid;
544 word-wrap: break-word;
545 }
546 }
547
548 .md-fences {
549 background-color: #f8f8f8;
550 }
551 #write pre.md-meta-block {
552 padding: 1rem;
553 font-size: 85%;
554 line-height: 1.45;
555 background-color: #f7f7f7;
556 border: 0;
557 border-radius: 3px;
558 color: #777777;
559 margin-top: 0 !important;
560 }
561
562 .mathjax-block>.code-tooltip {
563 bottom: .375rem;
564 }
565
566 .md-mathjax-midline {
567 background: #fafafa;
568 }
569
570 #write>h3.md-focus:before{
571 left: -1.5625rem;
572 top: .375rem;
573 }
574 #write>h4.md-focus:before{
575 left: -1.5625rem;
576 top: .285714286rem;
577 }
578 #write>h5.md-focus:before{
579 left: -1.5625rem;
580 top: .285714286rem;
581 }
582 #write>h6.md-focus:before{
583 left: -1.5625rem;
584 top: .285714286rem;
585 }
586 .md-image>.md-meta {
587 /*border: 1px solid #ddd;*/
588 border-radius: 3px;
589 padding: 2px 0px 0px 4px;
590 font-size: 0.9em;
591 color: inherit;
592 }
593
594 .md-tag {
595 color: #a7a7a7;
596 opacity: 1;
597 }
598
599 .md-toc {
600 margin-top:20px;
601 padding-bottom:20px;
602 }
603
604 .sidebar-tabs {
605 border-bottom: none;
606 }
607
608 #typora-quick-open {
609 border: 1px solid #ddd;
610 background-color: #f8f8f8;
611 }
612
613 #typora-quick-open-item {
614 background-color: #FAFAFA;
615 border-color: #FEFEFE #e5e5e5 #e5e5e5 #eee;
616 border-style: solid;
617 border-width: 1px;
618 }
619
620 /** focus mode */
621 .on-focus-mode blockquote {
622 border-left-color: rgba(85, 85, 85, 0.12);
623 }
624
625 header, .context-menu, .megamenu-content, footer{
626 font-family: "Segoe UI", "Arial", sans-serif;
627 }
628
629 .file-node-content:hover .file-node-icon,
630 .file-node-content:hover .file-node-open-state{
631 visibility: visible;
632 }
633
634 .mac-seamless-mode #typora-sidebar {
635 background-color: #fafafa;
636 background-color: var(--side-bar-bg-color);
637 }
638
639 .md-lang {
640 color: #b4654d;
641 }
642
643 /*.html-for-mac {
644 --item-hover-bg-color: #E6F0FE;
645 }*/
646
647 #md-notification .btn {
648 border: 0;
649 }
650
651 .dropdown-menu .divider {
652 border-color: #e5e5e5;
653 opacity: 0.4;
654 }
655
656 .ty-preferences .window-content {
657 background-color: #fafafa;
658 }
659
660 .ty-preferences .nav-group-item.active {
661 color: white;
662 background: #999;
663 }
664
665 .menu-item-container a.menu-style-btn {
666 background-color: #f5f8fa;
667 background-image: linear-gradient( 180deg , hsla(0, 0%, 100%, 0.8), hsla(0, 0%, 100%, 0));
668 }
669
670
671
672 </style><title>panama_ffi</title>
673 </head>
674 <body class='typora-export'><div class='typora-export-content'>
675 <div id='write' class=''><h2 id='state-of-foreign-function-support'><span>State of foreign function support</span></h2><p><strong><span>January 2023</span></strong></p><p><strong><span>Maurizio Cimadamore</span></strong></p><p><span>The Foreign Function & Memory API (FFM API in short) provides access to foreign functions through the </span><code>Linker</code><span> interface, which has been available as an </span><a href='https://openjdk.java.net/jeps/11'><span>incubating</span></a><span> API since Java </span><a href='https://openjdk.java.net/jeps/389'><span>16</span></a><span>. A linker allows clients to construct </span><em><span>downcall</span></em><span> method handles — that is, method handles whose invocation targets a native function defined in some native library. In other words, FFM API's foreign function support is completely expressed in terms of Java code and no intermediate native code is required.</span></p><h3 id='zero-length-memory-segments'><span>Zero-length memory segments</span></h3><p><span>Before we dive into the specifics of the foreign function support, it would be useful to briefly recap some of the main concepts we have learned when exploring the </span><a href='panama_memaccess.md'><span>foreign memory access support</span></a><span>. The Foreign Memory Access API allows client to create and manipulate </span><em><span>memory segments</span></em><span>. A memory segment is a view over a memory source (either on- or off-heap) which is spatially bounded, temporally bounded and thread-confined. The guarantees ensure that dereferencing a segment that has been created by Java code is always </span><em><span>safe</span></em><span>, and can never result in a VM crash, or, worse, in silent memory corruption.</span></p><p><span>Now, in the case of memory segments, the above properties (spatial bounds, temporal bounds and confinement) can be known </span><em><span>in full</span></em><span> when the segment is created. But when we interact with native libraries we often receive </span><em><span>raw</span></em><span> pointers; such pointers have no spatial bounds (does a </span><code>char*</code><span> in C refer to one </span><code>char</code><span>, or a </span><code>char</code><span> array of a given size?), no notion of temporal bounds, nor thread-confinement. Raw addresses in the FFM API are modelled using </span><em><span>zero-length memory segments</span></em><span>.</span></p><p><span>If clients want to dereference a zero-length memory segment, they can do so </span><em><span>unsafely</span></em><span> in two ways. First, the client can create a new memory segment from the zero-length memory segment </span><em><span>unsafely</span></em><span>, using the </span><code>MemorySegment::ofAddress</code><span> factory. This method is </span><em><span>restricted</span></em><span> and will generate runtime warnings if called without specifying the </span><code>--enable-native-access</code><span> command-line flag. By calling </span><code>MemorySegment::ofAddress</code><span> a client inject extra knowledge about spatial bounds which might be available in the native library the client is interacting with:</span></p><pre class="md-fences md-end-block ty-contain-cm modeLoaded" spellcheck="false" lang="java"><div class="CodeMirror cm-s-inner cm-s-null-scroll" lang="java"><div style="overflow: hidden; position: relative; width: 3px; height: 0px; top: 9.52344px; left: 8px;"><textarea autocorrect="off" autocapitalize="off" spellcheck="false" tabindex="0" style="position: absolute; bottom: -1em; padding: 0px; width: 1000px; height: 1em; outline: none;"></textarea></div><div class="CodeMirror-scrollbar-filler" cm-not-content="true"></div><div class="CodeMirror-gutter-filler" cm-not-content="true"></div><div class="CodeMirror-scroll" tabindex="-1"><div class="CodeMirror-sizer" style="margin-left: 0px; margin-bottom: 0px; border-right-width: 0px; min-width: 758.438px; padding-right: 0px; padding-bottom: 0px;"><div style="position: relative; top: 0px;"><div class="CodeMirror-lines" role="presentation"><div role="presentation" style="position: relative; outline: none;"><div class="CodeMirror-measure"></div><div class="CodeMirror-measure"></div><div style="position: relative; z-index: 1;"></div><div class="CodeMirror-code" role="presentation" style=""><div class="CodeMirror-activeline" style="position: relative;"><div class="CodeMirror-activeline-background CodeMirror-linebackground"></div><div class="CodeMirror-gutter-background CodeMirror-activeline-gutter" style="left: 0px; width: 0px;"></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-variable">MemorySegment</span> <span class="cm-variable">raw</span> <span class="cm-operator">=</span> ... <span class="cm-comment">//obtain address from native code</span></span></pre></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-keyword">try</span> (<span class="cm-variable">Arena</span> <span class="cm-variable">arena</span> <span class="cm-operator">=</span> <span class="cm-variable">Arena</span>.<span class="cm-variable">openConfined</span>()) {</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> <span class="cm-variable">MemorySegment</span> <span class="cm-variable">segment</span> <span class="cm-operator">=</span> <span class="cm-variable">MemorySegment</span>.<span class="cm-variable">ofAddress</span>(<span class="cm-variable">raw</span>.<span class="cm-variable">address</span>(), <span class="cm-number">100</span>, <span class="cm-variable">arena</span>.<span class="cm-variable">scope</span>());</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> <span class="cm-variable-3">int</span> <span class="cm-variable">x</span> <span class="cm-operator">=</span> <span class="cm-variable">segment</span>.<span class="cm-variable">get</span>(<span class="cm-variable">JAVA_INT</span>, <span class="cm-number">0</span>);</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">}</span></pre></div></div></div></div></div><div style="position: absolute; height: 0px; width: 1px; border-bottom: 0px solid transparent; top: 115px;"></div><div class="CodeMirror-gutters" style="display: none; height: 115px;"></div></div></div></pre><p><span>Alternatively, clients can obtain an </span><em><span>unbounded</span></em><span> address value layout. This is done using the </span><code>ValueLayout.OfAddress::asUnbounded</code><span> method (which is also a restricted method). When an access operation uses an unbounded address value layouts, the runtime will wrap any corresponding raw addresses with native segments with </span><em><span>maximal</span></em><span> size (i.e. </span><code>Long.MAX_VALUE</code><span>). As such, these segments can be accessed directly, as follows:</span></p><pre class="md-fences md-end-block ty-contain-cm modeLoaded" spellcheck="false" lang="java"><div class="CodeMirror cm-s-inner cm-s-null-scroll" lang="java"><div style="overflow: hidden; position: relative; width: 3px; height: 0px; top: 9.52344px; left: 8px;"><textarea autocorrect="off" autocapitalize="off" spellcheck="false" tabindex="0" style="position: absolute; bottom: -1em; padding: 0px; width: 1000px; height: 1em; outline: none;"></textarea></div><div class="CodeMirror-scrollbar-filler" cm-not-content="true"></div><div class="CodeMirror-gutter-filler" cm-not-content="true"></div><div class="CodeMirror-scroll" tabindex="-1"><div class="CodeMirror-sizer" style="margin-left: 0px; margin-bottom: 0px; border-right-width: 0px; min-width: 1138.39px; padding-right: 0px; padding-bottom: 0px;"><div style="position: relative; top: 0px;"><div class="CodeMirror-lines" role="presentation"><div role="presentation" style="position: relative; outline: none;"><div class="CodeMirror-measure"></div><div class="CodeMirror-measure"></div><div style="position: relative; z-index: 1;"></div><div class="CodeMirror-code" role="presentation"><div class="CodeMirror-activeline" style="position: relative;"><div class="CodeMirror-activeline-background CodeMirror-linebackground"></div><div class="CodeMirror-gutter-background CodeMirror-activeline-gutter" style="left: 0px; width: 0px;"></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-variable">MemorySegment</span> <span class="cm-variable">foreign</span> <span class="cm-operator">=</span> <span class="cm-variable">someSegment</span>.<span class="cm-variable">get</span>(<span class="cm-variable">ValueLayout</span>.<span class="cm-variable">ADDRESS</span>.<span class="cm-variable">asUnbounded</span>(), <span class="cm-number">0</span>); <span class="cm-comment">// wrap address into segment (size = Long.MAX_VALUE)</span></span></pre></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-variable-3">int</span> <span class="cm-variable">x</span> <span class="cm-operator">=</span> <span class="cm-variable">foreign</span>.<span class="cm-variable">get</span>(<span class="cm-variable">ValueLayout</span>.<span class="cm-variable">JAVA_INT</span>, <span class="cm-number">0</span>); <span class="cm-comment">//ok</span></span></pre></div></div></div></div></div><div style="position: absolute; height: 0px; width: 1px; border-bottom: 0px solid transparent; top: 46px;"></div><div class="CodeMirror-gutters" style="display: none; height: 46px;"></div></div></div></pre><p><span>Which approach is taken largely depends on the information that a client has available when obtaining a memory segment wrapping a native pointer. For instance, if such pointer points to a C struct, the client might prefer to resize the segment unsafely, to match the size of the struct (so that out-of-bounds access will be detected by the API). In other instances, however, there will be no, or little information as to what spatial and/or temporal bounds should be associated with a given native pointer. In these cases using an unbounded address layout might be preferable.</span></p><h3 id='segment-allocators'><span>Segment allocators</span></h3><p><span>Idiomatic C code implicitly relies on stack allocation to allow for concise variable declarations; consider this example:</span></p><pre class="md-fences md-end-block ty-contain-cm modeLoaded" spellcheck="false" lang="c"><div class="CodeMirror cm-s-inner cm-s-null-scroll" lang="c"><div style="overflow: hidden; position: relative; width: 3px; height: 0px; top: 9.52344px; left: 267.219px;"><textarea autocorrect="off" autocapitalize="off" spellcheck="false" tabindex="0" style="position: absolute; bottom: -1em; padding: 0px; width: 1000px; height: 1em; outline: none;"></textarea></div><div class="CodeMirror-scrollbar-filler" cm-not-content="true"></div><div class="CodeMirror-gutter-filler" cm-not-content="true"></div><div class="CodeMirror-scroll" tabindex="-1"><div class="CodeMirror-sizer" style="margin-left: 0px; margin-bottom: 0px; border-right-width: 0px; min-width: 266.219px; padding-right: 0px; padding-bottom: 0px;"><div style="position: relative; top: 0px;"><div class="CodeMirror-lines" role="presentation"><div role="presentation" style="position: relative; outline: none;"><div class="CodeMirror-measure"><pre>x</pre></div><div class="CodeMirror-measure"></div><div style="position: relative; z-index: 1;"></div><div class="CodeMirror-code" role="presentation"><div class="CodeMirror-activeline" style="position: relative;"><div class="CodeMirror-activeline-background CodeMirror-linebackground"></div><div class="CodeMirror-gutter-background CodeMirror-activeline-gutter" style="left: 0px; width: 0px;"></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-variable-3">int</span> <span class="cm-variable">arr</span>[] <span class="cm-operator">=</span> { <span class="cm-number">0</span>, <span class="cm-number">1</span>, <span class="cm-number">2</span>, <span class="cm-number">3</span>, <span class="cm-number">4</span> };</span></pre></div></div></div></div></div></div><div style="position: absolute; height: 0px; width: 1px; border-bottom: 0px solid transparent; top: 23px;"></div><div class="CodeMirror-gutters" style="display: none; height: 23px;"></div></div></div></pre><p><span>A variable initializer such as the one above can be implemented as follows, using the Foreign Memory Access API:</span></p><pre class="md-fences md-end-block ty-contain-cm modeLoaded" spellcheck="false" lang="java"><div class="CodeMirror cm-s-inner cm-s-null-scroll" lang="java"><div style="overflow: hidden; position: relative; width: 3px; height: 0px; top: 9.52344px; left: 8px;"><textarea autocorrect="off" autocapitalize="off" spellcheck="false" tabindex="0" style="position: absolute; bottom: -1em; padding: 0px; width: 1000px; height: 1em; outline: none;"></textarea></div><div class="CodeMirror-scrollbar-filler" cm-not-content="true"></div><div class="CodeMirror-gutter-filler" cm-not-content="true"></div><div class="CodeMirror-scroll" tabindex="-1"><div class="CodeMirror-sizer" style="margin-left: 0px; margin-bottom: 0px; border-right-width: 0px; min-width: 957.078px; padding-right: 0px; padding-bottom: 0px;"><div style="position: relative; top: 0px;"><div class="CodeMirror-lines" role="presentation"><div role="presentation" style="position: relative; outline: none;"><div class="CodeMirror-measure"></div><div class="CodeMirror-measure"></div><div style="position: relative; z-index: 1;"></div><div class="CodeMirror-code" role="presentation" style=""><div class="CodeMirror-activeline" style="position: relative;"><div class="CodeMirror-activeline-background CodeMirror-linebackground"></div><div class="CodeMirror-gutter-background CodeMirror-activeline-gutter" style="left: 0px; width: 0px;"></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-keyword">try</span> (<span class="cm-variable">Arena</span> <span class="cm-variable">arena</span> <span class="cm-operator">=</span> <span class="cm-variable">Arena</span> <span class="cm-def">openConfined</span>()) {</span></pre></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> <span class="cm-variable">MemorySegment</span> <span class="cm-variable">arr</span> <span class="cm-operator">=</span> <span class="cm-variable">MemorySegment</span>.<span class="cm-variable">allocateNative</span>(<span class="cm-variable">MemoryLayout</span>.<span class="cm-variable">sequenceLayout</span>(<span class="cm-number">5</span>, <span class="cm-variable">JAVA_INT</span>), <span class="cm-variable">arena</span>.<span class="cm-variable">scope</span>());</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> <span class="cm-keyword">for</span> (<span class="cm-variable-3">int</span> <span class="cm-variable">i</span> <span class="cm-operator">=</span> <span class="cm-number">0</span> ; <span class="cm-variable">i</span> <span class="cm-operator"><</span> <span class="cm-number">5</span> ; <span class="cm-variable">i</span><span class="cm-operator">++</span>) {</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> <span class="cm-variable">arr</span>.<span class="cm-variable">setAtIndex</span>(<span class="cm-variable">JAVA_INT</span>, <span class="cm-variable">i</span>, <span class="cm-variable">i</span>);</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> }</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">}</span></pre></div></div></div></div></div><div style="position: absolute; height: 0px; width: 1px; border-bottom: 0px solid transparent; top: 138px;"></div><div class="CodeMirror-gutters" style="display: none; height: 138px;"></div></div></div></pre><p><span>There are a number of issues with the above code snippet:</span></p><ul><li><span>compared to the C code, it is more verbose — the native array has to be initialized </span><em><span>element by element</span></em></li><li><span>allocation is very slow compared to C; allocating the </span><code>arr</code><span> variable now takes a full </span><code>malloc</code><span>, while in C the variable was simply stack-allocated</span></li><li><span>when having multiple declarations like the one above, it might become increasingly harder to manage the lifecycle of the various segments</span></li></ul><p><span>To address these problems, the FFM API provides a </span><code>SegmentAllocator</code><span> abstraction, a functional interface which provides methods to allocate commonly used values. Since </span><code>Arena</code><span> implements the </span><code>SegmentAllocator</code><span> interface, the above code can be rewritten conveniently as follows:</span></p><pre class="md-fences md-end-block ty-contain-cm modeLoaded" spellcheck="false" lang="java"><div class="CodeMirror cm-s-inner cm-s-null-scroll" lang="java"><div style="overflow: hidden; position: relative; width: 3px; height: 0px; top: 55.6172px; left: 241.188px;"><textarea autocorrect="off" autocapitalize="off" spellcheck="false" tabindex="0" style="position: absolute; bottom: -1em; padding: 0px; width: 1000px; height: 1em; outline: none;"></textarea></div><div class="CodeMirror-scrollbar-filler" cm-not-content="true"></div><div class="CodeMirror-gutter-filler" cm-not-content="true"></div><div class="CodeMirror-scroll" tabindex="-1"><div class="CodeMirror-sizer" style="margin-left: 0px; margin-bottom: 0px; border-right-width: 0px; min-width: 603.016px; padding-right: 0px; padding-bottom: 0px;"><div style="position: relative; top: 0px;"><div class="CodeMirror-lines" role="presentation"><div role="presentation" style="position: relative; outline: none;"><div class="CodeMirror-measure"><pre>x</pre></div><div class="CodeMirror-measure"></div><div style="position: relative; z-index: 1;"></div><div class="CodeMirror-code" role="presentation"><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-keyword">try</span> (<span class="cm-variable">Arena</span> <span class="cm-variable">arena</span> <span class="cm-operator">=</span> <span class="cm-variable">Arena</span>.<span class="cm-variable">openConfined</span>()) {</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> <span class="cm-variable">MemorySegment</span> <span class="cm-variable">arr</span> <span class="cm-operator">=</span> <span class="cm-variable">arena</span>.<span class="cm-variable">allocateArray</span>(<span class="cm-variable">JAVA_INT</span>, <span class="cm-number">0</span>, <span class="cm-number">1</span>, <span class="cm-number">2</span>, <span class="cm-number">3</span>, <span class="cm-number">4</span>);</span></pre><div class="CodeMirror-activeline" style="position: relative;"><div class="CodeMirror-activeline-background CodeMirror-linebackground"></div><div class="CodeMirror-gutter-background CodeMirror-activeline-gutter" style="left: 0px; width: 0px;"></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">} <span class="cm-comment">// 'arr' is released here</span></span></pre></div></div></div></div></div></div><div style="position: absolute; height: 0px; width: 1px; border-bottom: 0px solid transparent; top: 69px;"></div><div class="CodeMirror-gutters" style="display: none; height: 69px;"></div></div></div></pre><p><span>In the above code, the arena acts as a </span><em><span>native</span></em><span> allocator (that is, an allocator built on top of </span><code>MemorySegment::allocateNative</code><span>). The arena is then used to create a native array, initialized to the values </span><code>0, 1, 2, 3, 4</code><span>. The array initialization is more efficient, compared to the previous snippet, as the Java array is copied </span><em><span>in bulk</span></em><span> into the memory region associated with the newly allocated memory segment. The returned segment is associated with the scope of the arena which performed the allocation, meaning that the segment will no longer be accessible after the try-with-resource construct.</span></p><p><span>Custom segment allocators are also critical to achieve optimal allocation performance; for this reason, a number of predefined allocators are available via factories in the </span><code>SegmentAllocator</code><span> interface. For example, the following code creates a </span><em><span>slicing</span></em><span> allocator and uses it to allocate a segment whose content is initialized from a Java </span><code>int</code><span> array:</span></p><pre class="md-fences md-end-block ty-contain-cm modeLoaded" spellcheck="false" lang="java"><div class="CodeMirror cm-s-inner cm-s-null-scroll" lang="java"><div style="overflow: hidden; position: relative; width: 3px; height: 0px; top: 170.852px; left: 379.359px;"><textarea autocorrect="off" autocapitalize="off" spellcheck="false" tabindex="0" style="position: absolute; bottom: -1em; padding: 0px; width: 1000px; height: 1em; outline: none;"></textarea></div><div class="CodeMirror-scrollbar-filler" cm-not-content="true"></div><div class="CodeMirror-gutter-filler" cm-not-content="true"></div><div class="CodeMirror-scroll" tabindex="-1"><div class="CodeMirror-sizer" style="margin-left: 0px; margin-bottom: 0px; border-right-width: 0px; min-width: 784.406px; padding-right: 0px; padding-bottom: 0px;"><div style="position: relative; top: 0px;"><div class="CodeMirror-lines" role="presentation"><div role="presentation" style="position: relative; outline: none;"><div class="CodeMirror-measure"><pre>x</pre></div><div class="CodeMirror-measure"></div><div style="position: relative; z-index: 1;"></div><div class="CodeMirror-code" role="presentation" style=""><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-keyword">try</span> (<span class="cm-variable">Arena</span> <span class="cm-variable">arena</span> <span class="cm-operator">=</span> <span class="cm-variable">Arena</span>.<span class="cm-variable">openConfined</span>()) {</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> <span class="cm-variable">SegmentAllocator</span> <span class="cm-variable">allocator</span> <span class="cm-operator">=</span> <span class="cm-variable">SegmentAllocator</span>.<span class="cm-variable">slicingAllocator</span>(<span class="cm-variable">arena</span>.<span class="cm-variable">allocate</span>(<span class="cm-number">1024</span>));</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> <span class="cm-keyword">for</span> (<span class="cm-variable-3">int</span> <span class="cm-variable">i</span> <span class="cm-operator">=</span> <span class="cm-number">0</span> ; <span class="cm-variable">i</span> <span class="cm-operator"><</span> <span class="cm-number">10</span> ; <span class="cm-variable">i</span><span class="cm-operator">++</span>) {</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> <span class="cm-variable">MemorySegment</span> <span class="cm-variable">s</span> <span class="cm-operator">=</span> <span class="cm-variable">allocator</span>.<span class="cm-variable">allocateArray</span>(<span class="cm-variable">JAVA_INT</span>, <span class="cm-keyword">new</span> <span class="cm-variable-3">int</span>[] { <span class="cm-number">1</span>, <span class="cm-number">2</span>, <span class="cm-number">3</span>, <span class="cm-number">4</span>, <span class="cm-number">5</span> });</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> ...</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> }</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> ...</span></pre><div class="CodeMirror-activeline" style="position: relative;"><div class="CodeMirror-activeline-background CodeMirror-linebackground"></div><div class="CodeMirror-gutter-background CodeMirror-activeline-gutter" style="left: 0px; width: 0px;"></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> } <span class="cm-comment">// all memory allocated is released here</span></span></pre></div></div></div></div></div></div><div style="position: absolute; height: 0px; width: 1px; border-bottom: 0px solid transparent; top: 184px;"></div><div class="CodeMirror-gutters" style="display: none; height: 184px;"></div></div></div></pre><p><span>This code creates a native segment whose size is 1024 bytes. The segment is then used to create a slicing allocator, which responds to allocation requests by returning slices of that pre-allocated segment. If the current segment does not have sufficient space to accommodate an allocation request, an exception is thrown. All of the memory associated with the segments created by the allocator (i.e., in the body of the for loop) is deallocated atomically when the arena is closed. This technique combines the advantages of deterministic deallocation, provided by the </span><code>Arena</code><span> abstraction, with a more flexible and scalable allocation scheme. It can be very useful when writing code which manages a large number of off-heap segments.</span></p><p><span>All the methods in the FFM API which </span><em><span>produce</span></em><span> memory segments (see </span><code>VaList::nextVarg</code><span> and downcall method handles), allow for an allocator parameter to be provided — this is key in ensuring that an application using the FFM API achieves optimal allocation performances, especially in non-trivial use cases.</span></p><h3 id='symbol-lookups'><span>Symbol lookups</span></h3><p><span>The first ingredient of any foreign function support is a mechanism to lookup symbols in native libraries. In traditional Java/JNI, this is done via the </span><code>System::loadLibrary</code><span> and </span><code>System::load</code><span> methods. Unfortunately, these methods do not provide a way for clients to obtain the </span><em><span>address</span></em><span> associated with a given library symbol. For this reason, the Foreign Linker API introduces a new abstraction, namely </span><code>SymbolLookup</code><span> (similar in spirit to a method handle lookup), which provides capabilities to lookup named symbols; we can obtain a symbol lookup in 3 different ways:</span></p><ul><li><code>SymbolLookup::libraryLookup(String, SegmentScope)</code><span> — creates a symbol lookup which can be used to search symbol in a library with the given name. The provided segment scope parameter controls the library lifecycle: that is, when the scope is not longer alive, the library referred to by the lookup will also be closed;</span></li><li><code>SymbolLookup::loaderLookup</code><span> — creates a symbol lookup which can be used to search symbols in all the libraries loaded by the caller's classloader (e.g. using </span><code>System::loadLibrary</code><span> or </span><code>System::load</code><span>)</span></li><li><code>Linker::defaultLookup</code><span> — returns the default symbol lookup associated with a </span><code>Linker</code><span> instance. For instance, the default lookup of the native linker (see </span><code>Linker::nativeLinker</code><span>) can be used to look up platform-specific symbols in the standard C library (such as </span><code>strlen</code><span>, or </span><code>getpid</code><span>).</span></li></ul><p><span>Once a lookup has been obtained, a client can use it to retrieve handles to library symbols (either global variables or functions) using the </span><code>find(String)</code><span> method, which returns an </span><code>Optional<MemorySegment></code><span>. The memory segments returned by the </span><code>lookup</code><span> are zero-length segments, whose base address is the address of the function or variable in the library.</span></p><p><span>For instance, the following code can be used to look up the </span><code>clang_getClangVersion</code><span> function provided by the </span><code>clang</code><span> library; it does so by creating a </span><em><span>library lookup</span></em><span> whose lifecycle is associated to that of a confined arena.</span></p><pre class="md-fences md-end-block ty-contain-cm modeLoaded" spellcheck="false" lang="java"><div class="CodeMirror cm-s-inner cm-s-null-scroll" lang="java"><div style="overflow: hidden; position: relative; width: 3px; height: 0px; top: 9.52344px; left: 8px;"><textarea autocorrect="off" autocapitalize="off" spellcheck="false" tabindex="0" style="position: absolute; bottom: -1em; padding: 0px; width: 1000px; height: 1em; outline: none;"></textarea></div><div class="CodeMirror-scrollbar-filler" cm-not-content="true"></div><div class="CodeMirror-gutter-filler" cm-not-content="true"></div><div class="CodeMirror-scroll" tabindex="-1"><div class="CodeMirror-sizer" style="margin-left: 0px; margin-bottom: 0px; border-right-width: 0px; min-width: 741.141px; padding-right: 0px; padding-bottom: 0px;"><div style="position: relative; top: 0px;"><div class="CodeMirror-lines" role="presentation"><div role="presentation" style="position: relative; outline: none;"><div class="CodeMirror-measure"></div><div class="CodeMirror-measure"></div><div style="position: relative; z-index: 1;"></div><div class="CodeMirror-code" role="presentation"><div class="CodeMirror-activeline" style="position: relative;"><div class="CodeMirror-activeline-background CodeMirror-linebackground"></div><div class="CodeMirror-gutter-background CodeMirror-activeline-gutter" style="left: 0px; width: 0px;"></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-keyword">try</span> (<span class="cm-variable">Arena</span> <span class="cm-variable">arena</span> <span class="cm-operator">=</span> <span class="cm-variable">Arena</span>.<span class="cm-variable">openConfined</span>()) {</span></pre></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> <span class="cm-variable">SymbolLookup</span> <span class="cm-variable">libclang</span> <span class="cm-operator">=</span> <span class="cm-variable">SymbolLookup</span>.<span class="cm-variable">libraryLookup</span>(<span class="cm-string">"libclang.so"</span>, <span class="cm-variable">arena</span>.<span class="cm-variable">scope</span>());</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> <span class="cm-variable">MemorySegment</span> <span class="cm-variable">clangVersion</span> <span class="cm-operator">=</span> <span class="cm-variable">libclang</span>.<span class="cm-variable">find</span>(<span class="cm-string">"clang_getClangVersion"</span>).<span class="cm-variable">get</span>();</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">}</span></pre></div></div></div></div></div><div style="position: absolute; height: 0px; width: 1px; border-bottom: 0px solid transparent; top: 92px;"></div><div class="CodeMirror-gutters" style="display: none; height: 92px;"></div></div></div></pre><h3 id='linker'><span>Linker</span></h3><p><span>At the core of the FFM API's foreign function support we find the </span><code>Linker</code><span> abstraction. This abstraction plays a dual role: first, for downcalls, it allows modelling foreign function calls as plain </span><code>MethodHandle</code><span> calls (see </span><code>Linker::downcallHandle</code><span>); second, for upcalls, it allows to convert an existing </span><code>MethodHandle</code><span> (which might point to some Java method) into a </span><code>MemorySegment</code><span> which could then be passed to foreign functions as a function pointer (see </span><code>Linker::upcallStub</code><span>):</span></p><pre class="md-fences md-end-block ty-contain-cm modeLoaded" spellcheck="false" lang="java"><div class="CodeMirror cm-s-inner cm-s-null-scroll" lang="java"><div style="overflow: hidden; position: relative; width: 3px; height: 0px; top: 9.52344px; left: 8px;"><textarea autocorrect="off" autocapitalize="off" spellcheck="false" tabindex="0" style="position: absolute; bottom: -1em; padding: 0px; width: 1000px; height: 1em; outline: none;"></textarea></div><div class="CodeMirror-scrollbar-filler" cm-not-content="true"></div><div class="CodeMirror-gutter-filler" cm-not-content="true"></div><div class="CodeMirror-scroll" tabindex="-1"><div class="CodeMirror-sizer" style="margin-left: 0px; margin-bottom: 0px; border-right-width: 0px; min-width: 862.062px; padding-right: 0px; padding-bottom: 0px;"><div style="position: relative; top: 0px;"><div class="CodeMirror-lines" role="presentation"><div role="presentation" style="position: relative; outline: none;"><div class="CodeMirror-measure"><span><span></span>x</span></div><div class="CodeMirror-measure"></div><div style="position: relative; z-index: 1;"></div><div class="CodeMirror-code" role="presentation" style=""><div class="CodeMirror-activeline" style="position: relative;"><div class="CodeMirror-activeline-background CodeMirror-linebackground"></div><div class="CodeMirror-gutter-background CodeMirror-activeline-gutter" style="left: 0px; width: 0px;"></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-keyword">interface</span> <span class="cm-def">Linker</span> {</span></pre></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> <span class="cm-variable">MethodHandle</span> <span class="cm-variable">downcallHandle</span>(<span class="cm-variable">Addressable</span> <span class="cm-variable">symbol</span>, <span class="cm-variable">FunctionDescriptor</span> <span class="cm-variable">function</span>);</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> <span class="cm-variable">MemorySegment</span> <span class="cm-variable">upcallStub</span>(<span class="cm-variable">MethodHandle</span> <span class="cm-variable">target</span>, <span class="cm-variable">FunctionDescriptor</span> <span class="cm-variable">function</span>, <span class="cm-variable">SegmentScope</span> <span class="cm-variable">scope</span>);</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> ... <span class="cm-comment">// some overloads omitted here</span></span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="" cm-zwsp="">
676 </span></span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> <span class="cm-keyword">static</span> <span class="cm-variable">Linker</span> <span class="cm-variable">nativeLinker</span>() { ... }</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">}</span></pre></div></div></div></div></div><div style="position: absolute; height: 0px; width: 1px; border-bottom: 0px solid transparent; top: 161px;"></div><div class="CodeMirror-gutters" style="display: none; height: 161px;"></div></div></div></pre><p><span>Both functions take a </span><code>FunctionDescriptor</code><span> instance — essentially an aggregate of memory layouts which is used to describe the argument and return types of a foreign function in full. Supported layouts are </span><em><span>value layouts</span></em><span> (for scalars and pointers) and </span><em><span>group layouts</span></em><span> (for structs/unions). Each layout in a function descriptor is associated with a carrier Java type (see table below); together, all the carrier types associated with layouts in a function descriptor will determine a unique Java </span><code>MethodType</code><span> — that is, the Java signature that clients will be using when interacting with said downcall handles, or upcall stubs.</span></p><p><span>The </span><code>Linker::nativeLinker</code><span> factory is used to obtain a </span><code>Linker</code><span> implementation for the ABI associated with the OS and processor where the Java runtime is currently executing. As such, the native linker can be used to call C functions. The following table shows the mapping between C types, layouts and Java carriers under the Linux/macOS native linker implementation; note that the mappings can be platform dependent: on Windows/x64, the C type </span><code>long</code><span> is 32-bit, so the </span><code>JAVA_INT</code><span> layout (and the Java carrier </span><code>int.class</code><span>) would have to be used instead:</span></p><figure><table><thead><tr><th><span>C type</span></th><th><span>Layout</span></th><th><span>Java carrier</span></th></tr></thead><tbody><tr><td><code>bool</code></td><td><code>JAVA_BOOLEAN</code></td><td><code>byte</code></td></tr><tr><td><code>char</code></td><td><code>JAVA_BYTE</code></td><td><code>byte</code></td></tr><tr><td><code>short</code></td><td><code>JAVA_SHORT</code></td><td><code>short</code><span>, </span><code>char</code></td></tr><tr><td><code>int</code></td><td><code>JAVA_INT</code></td><td><code>int</code></td></tr><tr><td><code>long</code></td><td><code>JAVA_LONG</code></td><td><code>long</code></td></tr><tr><td><code>long long</code></td><td><code>JAVA_LONG</code></td><td><code>long</code></td></tr><tr><td><code>float</code></td><td><code>JAVA_FLOAT</code></td><td><code>float</code></td></tr><tr><td><code>double</code></td><td><code>JAVA_DOUBLE</code></td><td><code>double</code></td></tr><tr><td><code>char*</code><br><code>int**</code><br><span> ...</span></td><td><code>ADDRESS</code></td><td><code>MemorySegment</code></td></tr><tr><td><code>struct Point { int x; int y; };</code><br><code>union Choice { float a; int b; };</code><br><span>...</span></td><td><code>MemoryLayout.structLayout(...)</code><br><code>MemoryLayout.unionLayout(...)</code><br></td><td><code>MemorySegment</code></td></tr></tbody></table></figure><p><span>Both C structs/unions and pointers are modelled using the </span><code>MemorySegment</code><span> carrier type. However, C structs/unions are modelled in function descriptors with memory layouts of type </span><code>GroupLayout</code><span>, whereas pointers are modelled using the </span><code>ADDRESS</code><span> value layout constant (whose size is platform-specific). Moreover, the behavior of a downcall method handle returning a struct/union type is radically different from that of a downcall method handle returning a C pointer:</span></p><ul><li><span>downcall method handles returning C pointers will wrap the pointer address into a fresh zero-length memory segment (unless an unbounded address layout is specified);</span></li><li><span>downcall method handles returning a C struct/union type will return a </span><em><span>new</span></em><span> segment, of given size (the size of the struct/union). The segment is allocated using a user-provided </span><code>SegmentAllocator</code><span>, which is provided using an additional prefix parameter inserted in the downcall method handle signature.</span></li></ul><p><span>A tool, such as </span><code>jextract</code><span>, will generate all the required C layouts (for scalars and structs/unions) </span><em><span>automatically</span></em><span>, so that clients do not have to worry about platform-dependent details such as sizes, alignment constraints and padding.</span></p><h3 id='downcalls'><span>Downcalls</span></h3><p><span>We will now look at how foreign functions can be called from Java using the native linker. Assume we wanted to call the following function from the standard C library:</span></p><pre class="md-fences md-end-block ty-contain-cm modeLoaded" spellcheck="false" lang="c"><div class="CodeMirror cm-s-inner cm-s-null-scroll" lang="c"><div style="overflow: hidden; position: relative; width: 3px; height: 0px; top: 9.52344px; left: 8px;"><textarea autocorrect="off" autocapitalize="off" spellcheck="false" tabindex="0" style="position: absolute; bottom: -1em; padding: 0px; width: 1000px; height: 1em; outline: none;"></textarea></div><div class="CodeMirror-scrollbar-filler" cm-not-content="true"></div><div class="CodeMirror-gutter-filler" cm-not-content="true"></div><div class="CodeMirror-scroll" tabindex="-1"><div class="CodeMirror-sizer" style="margin-left: 0px; margin-bottom: 0px; border-right-width: 0px; min-width: 257.516px; padding-right: 0px; padding-bottom: 0px;"><div style="position: relative; top: 0px;"><div class="CodeMirror-lines" role="presentation"><div role="presentation" style="position: relative; outline: none;"><div class="CodeMirror-measure"></div><div class="CodeMirror-measure"></div><div style="position: relative; z-index: 1;"></div><div class="CodeMirror-code" role="presentation"><div class="CodeMirror-activeline" style="position: relative;"><div class="CodeMirror-activeline-background CodeMirror-linebackground"></div><div class="CodeMirror-gutter-background CodeMirror-activeline-gutter" style="left: 0px; width: 0px;"></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-variable-3">size_t</span> <span class="cm-def">strlen</span>(<span class="cm-keyword">const</span> <span class="cm-variable-3">char</span> <span class="cm-variable-3">*</span><span class="cm-variable">s</span>);</span></pre></div></div></div></div></div></div><div style="position: absolute; height: 0px; width: 1px; border-bottom: 0px solid transparent; top: 23px;"></div><div class="CodeMirror-gutters" style="display: none; height: 23px;"></div></div></div></pre><p><span>In order to do that, we have to:</span></p><ul><li><span>lookup the </span><code>strlen</code><span> symbol</span></li><li><span>describe the signature of the C function using a function descriptor</span></li><li><span>create a </span><em><span>downcall</span></em><span> native method handle with the above information, using the native linker</span></li></ul><p><span>Here's an example of how we might want to do that (a full listing of all the examples in this and subsequent sections will be provided in the </span><a href='#appendix-full-source-code'><span>appendix</span></a><span>):</span></p><pre class="md-fences md-end-block ty-contain-cm modeLoaded" spellcheck="false" lang="java"><div class="CodeMirror cm-s-inner cm-s-null-scroll" lang="java"><div style="overflow: hidden; position: relative; width: 3px; height: 0px; top: 9.52344px; left: 8px;"><textarea autocorrect="off" autocapitalize="off" spellcheck="false" tabindex="0" style="position: absolute; bottom: -1em; padding: 0px; width: 1000px; height: 1em; outline: none;"></textarea></div><div class="CodeMirror-scrollbar-filler" cm-not-content="true"></div><div class="CodeMirror-gutter-filler" cm-not-content="true"></div><div class="CodeMirror-scroll" tabindex="-1"><div class="CodeMirror-sizer" style="margin-left: 0px; margin-bottom: 0px; border-right-width: 0px; min-width: 456.156px; padding-right: 0px; padding-bottom: 0px;"><div style="position: relative; top: 0px;"><div class="CodeMirror-lines" role="presentation"><div role="presentation" style="position: relative; outline: none;"><div class="CodeMirror-measure"></div><div class="CodeMirror-measure"></div><div style="position: relative; z-index: 1;"></div><div class="CodeMirror-code" role="presentation" style=""><div class="CodeMirror-activeline" style="position: relative;"><div class="CodeMirror-activeline-background CodeMirror-linebackground"></div><div class="CodeMirror-gutter-background CodeMirror-activeline-gutter" style="left: 0px; width: 0px;"></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-variable">Linker</span> <span class="cm-variable">linker</span> <span class="cm-operator">=</span> <span class="cm-variable">Linker</span>.<span class="cm-variable">nativeLinker</span>();</span></pre></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-variable">MethodHandle</span> <span class="cm-variable">strlen</span> <span class="cm-operator">=</span> <span class="cm-variable">linker</span>.<span class="cm-variable">downcallHandle</span>(</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> <span class="cm-variable">linker</span>.<span class="cm-variable">defaultLookup</span>().<span class="cm-variable">find</span>(<span class="cm-string">"strlen"</span>).<span class="cm-variable">get</span>(),</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> <span class="cm-variable">FunctionDescriptor</span>.<span class="cm-variable">of</span>(<span class="cm-variable">JAVA_LONG</span>, <span class="cm-variable">ADDRESS</span>)</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">);</span></pre></div></div></div></div></div><div style="position: absolute; height: 0px; width: 1px; border-bottom: 0px solid transparent; top: 115px;"></div><div class="CodeMirror-gutters" style="display: none; height: 115px;"></div></div></div></pre><p><span>Note that, since the function </span><code>strlen</code><span> is part of the standard C library, which is loaded with the VM, we can just use the default lookup of the native linker to look it up. The rest is pretty straightforward — the only tricky detail is how to model </span><code>size_t</code><span>: typically this type has the size of a pointer, so we can use </span><code>JAVA_LONG</code><span> both Linux and Windows. On the Java side, we model the </span><code>size_t</code><span> using a </span><code>long</code><span> and the pointer is modelled using an </span><code>Addressable</code><span> parameter.</span></p><p><span>Once we have obtained the downcall method handle, we can just use it as any other method handle</span><a href="#2"><sup><span>1</span></sup></a><span>:</span></p><pre class="md-fences md-end-block ty-contain-cm modeLoaded" spellcheck="false" lang="java"><div class="CodeMirror cm-s-inner cm-s-null-scroll" lang="java"><div style="overflow: hidden; position: relative; width: 3px; height: 0px; top: 9.52344px; left: 8px;"><textarea autocorrect="off" autocapitalize="off" spellcheck="false" tabindex="0" style="position: absolute; bottom: -1em; padding: 0px; width: 1000px; height: 1em; outline: none;"></textarea></div><div class="CodeMirror-scrollbar-filler" cm-not-content="true"></div><div class="CodeMirror-gutter-filler" cm-not-content="true"></div><div class="CodeMirror-scroll" tabindex="-1"><div class="CodeMirror-sizer" style="margin-left: 0px; margin-bottom: 0px; border-right-width: 0px; min-width: 602.984px; padding-right: 0px; padding-bottom: 0px;"><div style="position: relative; top: 0px;"><div class="CodeMirror-lines" role="presentation"><div role="presentation" style="position: relative; outline: none;"><div class="CodeMirror-measure"></div><div class="CodeMirror-measure"></div><div style="position: relative; z-index: 1;"></div><div class="CodeMirror-code" role="presentation"><div class="CodeMirror-activeline" style="position: relative;"><div class="CodeMirror-activeline-background CodeMirror-linebackground"></div><div class="CodeMirror-gutter-background CodeMirror-activeline-gutter" style="left: 0px; width: 0px;"></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-keyword">try</span> (<span class="cm-variable">Arena</span> <span class="cm-variable">arena</span> <span class="cm-operator">=</span> <span class="cm-variable">Arena</span>.<span class="cm-variable">openConfined</span>()) {</span></pre></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> <span class="cm-variable-3">long</span> <span class="cm-variable">len</span> <span class="cm-operator">=</span> <span class="cm-variable">strlen</span>.<span class="cm-variable">invoke</span>(<span class="cm-variable">arena</span>.<span class="cm-variable">allocateUtf8String</span>(<span class="cm-string">"Hello"</span>)); <span class="cm-comment">// 5</span></span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">}</span></pre></div></div></div></div></div><div style="position: absolute; height: 0px; width: 1px; border-bottom: 0px solid transparent; top: 69px;"></div><div class="CodeMirror-gutters" style="display: none; height: 69px;"></div></div></div></pre><p><span>Here we are using a confined arena to convert a Java string into an off-heap memory segment which contains a </span><code>NULL</code><span> terminated C string. We then pass that segment to the method handle and retrieve our result in a Java </span><code>long</code><span>. Note how all this is possible </span><em><span>without</span></em><span> any piece of intervening native code — all the interop code can be expressed in (low level) Java. Note also how we use an arena to control the lifecycle of the allocated C string, which ensures timely deallocation of the memory segment holding the native string.</span></p><p><span>The </span><code>Linker</code><span> interface also supports linking of native functions without an address known at link time; when that happens, an address (of type </span><code>MemorySegment</code><span>) must be provided when the method handle returned by the linker is invoked — this is very useful to support </span><em><span>virtual calls</span></em><span>. For instance, the above code can be rewritten as follows:</span></p><pre class="md-fences md-end-block ty-contain-cm modeLoaded" spellcheck="false" lang="java"><div class="CodeMirror cm-s-inner cm-s-null-scroll" lang="java"><div style="overflow: hidden; position: relative; width: 3px; height: 0px; top: 9.52344px; left: 8px;"><textarea autocorrect="off" autocapitalize="off" spellcheck="false" tabindex="0" style="position: absolute; bottom: -1em; padding: 0px; width: 1000px; height: 1em; outline: none;"></textarea></div><div class="CodeMirror-scrollbar-filler" cm-not-content="true"></div><div class="CodeMirror-gutter-filler" cm-not-content="true"></div><div class="CodeMirror-scroll" tabindex="-1"><div class="CodeMirror-sizer" style="margin-left: 0px; margin-bottom: 0px; border-right-width: 0px; min-width: 715.219px; padding-right: 0px; padding-bottom: 0px;"><div style="position: relative; top: 0px;"><div class="CodeMirror-lines" role="presentation"><div role="presentation" style="position: relative; outline: none;"><div class="CodeMirror-measure"></div><div class="CodeMirror-measure"></div><div style="position: relative; z-index: 1;"></div><div class="CodeMirror-code" role="presentation" style=""><div class="CodeMirror-activeline" style="position: relative;"><div class="CodeMirror-activeline-background CodeMirror-linebackground"></div><div class="CodeMirror-gutter-background CodeMirror-activeline-gutter" style="left: 0px; width: 0px;"></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-variable">MethodHandle</span> <span class="cm-variable">strlen_virtual</span> <span class="cm-operator">=</span> <span class="cm-variable">linker</span>.<span class="cm-variable">downcallHandle</span>( <span class="cm-comment">// address parameter missing!</span></span></pre></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-tab" role="presentation" cm-text=" "> </span><span class="cm-tab" role="presentation" cm-text=" "> </span><span class="cm-variable">FunctionDescriptor</span>.<span class="cm-variable">of</span>(<span class="cm-variable">JAVA_LONG</span>, <span class="cm-variable">ADDRESS</span>)</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">);</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="" cm-zwsp="">
677 </span></span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-keyword">try</span> (<span class="cm-variable">Arena</span> <span class="cm-variable">arena</span> <span class="cm-operator">=</span> <span class="cm-variable">Arena</span> <span class="cm-def">openConfined</span>()) {</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> <span class="cm-variable-3">long</span> <span class="cm-variable">len</span> <span class="cm-operator">=</span> <span class="cm-variable">strlen_virtual</span>.<span class="cm-variable">invoke</span>(</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> <span class="cm-variable">linker</span>.<span class="cm-variable">defaultLookup</span>().<span class="cm-variable">find</span>(<span class="cm-string">"strlen"</span>).<span class="cm-variable">get</span>() <span class="cm-comment">// address provided here!</span></span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> <span class="cm-variable">arena</span>.<span class="cm-variable">allocateUtf8String</span>(<span class="cm-string">"Hello"</span>)</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> ); <span class="cm-comment">// 5</span></span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">}</span></pre></div></div></div></div></div><div style="position: absolute; height: 0px; width: 1px; border-bottom: 0px solid transparent; top: 230px;"></div><div class="CodeMirror-gutters" style="display: none; height: 230px;"></div></div></div></pre><p><span>It is important to note that, albeit the interop code is written in Java, the above code can </span><em><span>not</span></em><span> be considered 100% safe. There are many arbitrary decisions to be made when setting up downcall method handles such as the one above, some of which might be obvious to us (e.g. how many parameters does the function take), but which cannot ultimately be verified by the Java runtime. After all, a symbol in a dynamic library is nothing but a numeric offset and, unless we are using a shared library with debugging information, no type information is attached to a given library symbol. This means that the Java runtime has to </span><em><span>trust</span></em><span> the function descriptor passed in</span><a href="#2"><sup><span>2</span></sup></a><span>; for this reason, the </span><code>Linker::nativeLinker</code><span> factory is also a restricted method.</span></p><p><span>When working with shared arenas, it is always possible for the arena associated with a memory segment passed </span><em><span>by reference</span></em><span> to a native function to be closed (by another thread) </span><em><span>while</span></em><span> the native function is executing. When this happens, the native code is at risk of dereferencing already-freed memory, which might trigger a JVM crash, or even result in silent memory corruption. For this reason, the </span><code>Linker</code><span> API provides some basic temporal safety guarantees: any </span><code>MemorySegment</code><span> instance passed by reference to a downcall method handle will be </span><em><span>kept alive</span></em><span> for the entire duration of the call. In other words, it's as if the call to the downcall method handle occurred inside an invisible call to </span><code>SegmentScope::whileAlive</code><span>.</span></p><p><span>Performance-wise, the reader might ask how efficient calling a foreign function using a native method handle is; the answer is </span><em><span>very</span></em><span>. The JVM comes with some special support for native method handles, so that, if a give method handle is invoked many times (e.g, inside a </span><em><span>hot</span></em><span> loop), the JIT compiler might decide to generate a snippet of assembly code required to call the native function, and execute that directly. In most cases, invoking native function this way is as efficient as doing so through JNI.</span></p><h3 id='upcalls'><span>Upcalls</span></h3><p><span>Sometimes, it is useful to pass Java code as a function pointer to some native function; we can achieve that by using foreign linker support for upcalls. To demonstrate this, let's consider the following function from the C standard library:</span></p><pre class="md-fences md-end-block ty-contain-cm modeLoaded" spellcheck="false" lang="c"><div class="CodeMirror cm-s-inner cm-s-null-scroll" lang="c"><div style="overflow: hidden; position: relative; width: 3px; height: 0px; top: 9.52344px; left: 8px;"><textarea autocorrect="off" autocapitalize="off" spellcheck="false" tabindex="0" style="position: absolute; bottom: -1em; padding: 0px; width: 1000px; height: 1em; outline: none;"></textarea></div><div class="CodeMirror-scrollbar-filler" cm-not-content="true"></div><div class="CodeMirror-gutter-filler" cm-not-content="true"></div><div class="CodeMirror-scroll" tabindex="-1"><div class="CodeMirror-sizer" style="margin-left: 0px; margin-bottom: 0px; border-right-width: 0px; min-width: 473.469px; padding-right: 0px; padding-bottom: 0px;"><div style="position: relative; top: 0px;"><div class="CodeMirror-lines" role="presentation"><div role="presentation" style="position: relative; outline: none;"><div class="CodeMirror-measure"></div><div class="CodeMirror-measure"></div><div style="position: relative; z-index: 1;"></div><div class="CodeMirror-code" role="presentation"><div class="CodeMirror-activeline" style="position: relative;"><div class="CodeMirror-activeline-background CodeMirror-linebackground"></div><div class="CodeMirror-gutter-background CodeMirror-activeline-gutter" style="left: 0px; width: 0px;"></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-variable-3">void</span> <span class="cm-def">qsort</span>(<span class="cm-variable-3">void</span> <span class="cm-variable-3">*</span><span class="cm-variable">base</span>, <span class="cm-variable-3">size_t</span> <span class="cm-variable">nmemb</span>, <span class="cm-variable-3">size_t</span> <span class="cm-variable">size</span>,</span></pre></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> <span class="cm-variable-3">int</span> (<span class="cm-operator">*</span><span class="cm-variable">compar</span>)(<span class="cm-keyword">const</span> <span class="cm-variable-3">void</span> <span class="cm-variable-3">*</span>, <span class="cm-keyword">const</span> <span class="cm-variable-3">void</span> <span class="cm-variable-3">*</span>));</span></pre></div></div></div></div></div><div style="position: absolute; height: 0px; width: 1px; border-bottom: 0px solid transparent; top: 46px;"></div><div class="CodeMirror-gutters" style="display: none; height: 46px;"></div></div></div></pre><p><span>The </span><code>qsort</code><span> function can be used to sort the contents of an array, using a custom comparator function — </span><code>compar</code><span> — which is passed as a function pointer. To be able to call the </span><code>qsort</code><span> function from Java we have first to create a downcall method handle for it:</span></p><pre class="md-fences md-end-block ty-contain-cm modeLoaded" spellcheck="false" lang="java"><div class="CodeMirror cm-s-inner cm-s-null-scroll" lang="java"><div style="overflow: hidden; position: relative; width: 3px; height: 0px; top: 9.52344px; left: 8px;"><textarea autocorrect="off" autocapitalize="off" spellcheck="false" tabindex="0" style="position: absolute; bottom: -1em; padding: 0px; width: 1000px; height: 1em; outline: none;"></textarea></div><div class="CodeMirror-scrollbar-filler" cm-not-content="true"></div><div class="CodeMirror-gutter-filler" cm-not-content="true"></div><div class="CodeMirror-scroll" tabindex="-1"><div class="CodeMirror-sizer" style="margin-left: 0px; margin-bottom: 0px; border-right-width: 0px; min-width: 637.516px; padding-right: 0px; padding-bottom: 0px;"><div style="position: relative; top: 0px;"><div class="CodeMirror-lines" role="presentation"><div role="presentation" style="position: relative; outline: none;"><div class="CodeMirror-measure"></div><div class="CodeMirror-measure"></div><div style="position: relative; z-index: 1;"></div><div class="CodeMirror-code" role="presentation" style=""><div class="CodeMirror-activeline" style="position: relative;"><div class="CodeMirror-activeline-background CodeMirror-linebackground"></div><div class="CodeMirror-gutter-background CodeMirror-activeline-gutter" style="left: 0px; width: 0px;"></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-variable">Linker</span> <span class="cm-variable">linker</span> <span class="cm-operator">=</span> <span class="cm-variable">Linker</span>.<span class="cm-variable">nativeLinker</span>();</span></pre></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-variable">MethodHandle</span> <span class="cm-variable">qsort</span> <span class="cm-operator">=</span> <span class="cm-variable">linker</span>.<span class="cm-variable">downcallHandle</span>(</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-tab" role="presentation" cm-text=" "> </span><span class="cm-tab" role="presentation" cm-text=" "> </span><span class="cm-variable">linker</span>.<span class="cm-variable">defaultLookup</span>().<span class="cm-variable">lookup</span>(<span class="cm-string">"qsort"</span>).<span class="cm-variable">get</span>(),</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> <span class="cm-variable">FunctionDescriptor</span>.<span class="cm-variable">ofVoid</span>(<span class="cm-variable">ADDRESS</span>, <span class="cm-variable">JAVA_LONG</span>, <span class="cm-variable">JAVA_LONG</span>, <span class="cm-variable">ADDRESS</span>)</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">);</span></pre></div></div></div></div></div><div style="position: absolute; height: 0px; width: 1px; border-bottom: 0px solid transparent; top: 115px;"></div><div class="CodeMirror-gutters" style="display: none; height: 115px;"></div></div></div></pre><p><span>As before, we use </span><code>JAVA_LONG</code><span> and </span><code>long.class</code><span> to map the C </span><code>size_t</code><span> type, and </span><code>ADDRESS</code><span> for both the first pointer parameter (the array pointer) and the last parameter (the function pointer).</span></p><p><span>This time, in order to invoke the </span><code>qsort</code><span> downcall handle, we need a </span><em><span>function pointer</span></em><span> to be passed as the last parameter; this is where the upcall support in foreign linker comes in handy, as it allows us to create a function pointer out of an existing method handle. First, let's write a function that can compare two int elements (passed as pointers):</span></p><pre class="md-fences md-end-block ty-contain-cm modeLoaded" spellcheck="false" lang="java"><div class="CodeMirror cm-s-inner cm-s-null-scroll" lang="java"><div style="overflow: hidden; position: relative; width: 3px; height: 0px; top: 9.52344px; left: 8px;"><textarea autocorrect="off" autocapitalize="off" spellcheck="false" tabindex="0" style="position: absolute; bottom: -1em; padding: 0px; width: 1000px; height: 1em; outline: none;"></textarea></div><div class="CodeMirror-scrollbar-filler" cm-not-content="true"></div><div class="CodeMirror-gutter-filler" cm-not-content="true"></div><div class="CodeMirror-scroll" tabindex="-1"><div class="CodeMirror-sizer" style="margin-left: 0px; margin-bottom: 0px; border-right-width: 0px; min-width: 620.25px; padding-right: 0px; padding-bottom: 0px;"><div style="position: relative; top: 0px;"><div class="CodeMirror-lines" role="presentation"><div role="presentation" style="position: relative; outline: none;"><div class="CodeMirror-measure"></div><div class="CodeMirror-measure"></div><div style="position: relative; z-index: 1;"></div><div class="CodeMirror-code" role="presentation" style=""><div class="CodeMirror-activeline" style="position: relative;"><div class="CodeMirror-activeline-background CodeMirror-linebackground"></div><div class="CodeMirror-gutter-background CodeMirror-activeline-gutter" style="left: 0px; width: 0px;"></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-keyword">class</span> <span class="cm-def">Qsort</span> {</span></pre></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-tab" role="presentation" cm-text=" "> </span><span class="cm-keyword">static</span> <span class="cm-variable-3">int</span> <span class="cm-variable">qsortCompare</span>(<span class="cm-variable">MemoryAddress</span> <span class="cm-variable">addr1</span>, <span class="cm-variable">MemoryAddress</span> <span class="cm-variable">addr2</span>) {</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-tab" role="presentation" cm-text=" "> </span><span class="cm-tab" role="presentation" cm-text=" "> </span><span class="cm-keyword">return</span> <span class="cm-variable">addr1</span>.<span class="cm-variable">get</span>(<span class="cm-variable">JAVA_INT</span>, <span class="cm-number">0</span>) <span class="cm-operator">-</span> <span class="cm-variable">addr2</span>.<span class="cm-variable">get</span>(<span class="cm-variable">JAVA_INT</span>, <span class="cm-number">0</span>);</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-tab" role="presentation" cm-text=" "> </span>}</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">}</span></pre></div></div></div></div></div><div style="position: absolute; height: 0px; width: 1px; border-bottom: 0px solid transparent; top: 115px;"></div><div class="CodeMirror-gutters" style="display: none; height: 115px;"></div></div></div></pre><p><span>Here we can see that the function is performing some </span><em><span>unsafe</span></em><span> dereference of the pointer contents.</span></p><p><span>Now let's create a method handle pointing to the comparator function above:</span></p><pre class="md-fences md-end-block ty-contain-cm modeLoaded" spellcheck="false" lang="java"><div class="CodeMirror cm-s-inner cm-s-null-scroll" lang="java"><div style="overflow: hidden; position: relative; width: 3px; height: 0px; top: 9.52344px; left: 8px;"><textarea autocorrect="off" autocapitalize="off" spellcheck="false" tabindex="0" style="position: absolute; bottom: -1em; padding: 0px; width: 1000px; height: 1em; outline: none;"></textarea></div><div class="CodeMirror-scrollbar-filler" cm-not-content="true"></div><div class="CodeMirror-gutter-filler" cm-not-content="true"></div><div class="CodeMirror-scroll" tabindex="-1"><div class="CodeMirror-sizer" style="margin-left: 0px; margin-bottom: 0px; border-right-width: 0px; min-width: 741.078px; padding-right: 0px; padding-bottom: 0px;"><div style="position: relative; top: 0px;"><div class="CodeMirror-lines" role="presentation"><div role="presentation" style="position: relative; outline: none;"><div class="CodeMirror-measure"></div><div class="CodeMirror-measure"></div><div style="position: relative; z-index: 1;"></div><div class="CodeMirror-code" role="presentation"><div class="CodeMirror-activeline" style="position: relative;"><div class="CodeMirror-activeline-background CodeMirror-linebackground"></div><div class="CodeMirror-gutter-background CodeMirror-activeline-gutter" style="left: 0px; width: 0px;"></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-variable">FunctionDescriptor</span> <span class="cm-variable">comparDesc</span> <span class="cm-operator">=</span> <span class="cm-variable">FunctionDescriptor</span>.<span class="cm-variable">of</span>(<span class="cm-variable">JAVA_INT</span>, <span class="cm-variable">ADDRESS</span>, <span class="cm-variable">ADDRESS</span>);</span></pre></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-variable">MethodHandle</span> <span class="cm-variable">comparHandle</span> <span class="cm-operator">=</span> <span class="cm-variable">MethodHandles</span>.<span class="cm-variable">lookup</span>()</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> .<span class="cm-variable">findStatic</span>(<span class="cm-variable">Qsort</span>.<span class="cm-keyword">class</span>, <span class="cm-string">"qsortCompare"</span>,</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> <span class="cm-variable">CLinker</span>.<span class="cm-variable">upcallType</span>(<span class="cm-variable">comparDesc</span>));</span></pre></div></div></div></div></div><div style="position: absolute; height: 0px; width: 1px; border-bottom: 0px solid transparent; top: 92px;"></div><div class="CodeMirror-gutters" style="display: none; height: 92px;"></div></div></div></pre><p><span>To do that, we first create a function descriptor for the function pointer type, and then we use the </span><code>CLinker::upcallType</code><span> to turn that function descriptor into a suitable </span><code>MethodType</code><span> instance to be used in a method handle lookup. Now that we have a method handle for our Java comparator function, we finally have all the ingredients to create an upcall stub, and pass it to the </span><code>qsort</code><span> downcall handle:</span></p><pre class="md-fences md-end-block ty-contain-cm modeLoaded" spellcheck="false" lang="java"><div class="CodeMirror cm-s-inner cm-s-null-scroll" lang="java"><div style="overflow: hidden; position: relative; width: 3px; height: 0px; top: 9.52344px; left: 8px;"><textarea autocorrect="off" autocapitalize="off" spellcheck="false" tabindex="0" style="position: absolute; bottom: -1em; padding: 0px; width: 1000px; height: 1em; outline: none;"></textarea></div><div class="CodeMirror-scrollbar-filler" cm-not-content="true"></div><div class="CodeMirror-gutter-filler" cm-not-content="true"></div><div class="CodeMirror-scroll" tabindex="-1"><div class="CodeMirror-sizer" style="margin-left: 0px; margin-bottom: 0px; border-right-width: 0px; min-width: 732.5px; padding-right: 0px; padding-bottom: 0px;"><div style="position: relative; top: 0px;"><div class="CodeMirror-lines" role="presentation"><div role="presentation" style="position: relative; outline: none;"><div class="CodeMirror-measure"></div><div class="CodeMirror-measure"></div><div style="position: relative; z-index: 1;"></div><div class="CodeMirror-code" role="presentation" style=""><div class="CodeMirror-activeline" style="position: relative;"><div class="CodeMirror-activeline-background CodeMirror-linebackground"></div><div class="CodeMirror-gutter-background CodeMirror-activeline-gutter" style="left: 0px; width: 0px;"></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-keyword">try</span> (<span class="cm-variable">Arena</span> <span class="cm-variable">arena</span> <span class="cm-operator">=</span> <span class="cm-variable">Arena</span>.<span class="cm-variable">openConfined</span>()) {</span></pre></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> <span class="cm-variable">MemorySegment</span> <span class="cm-variable">comparFunc</span> <span class="cm-operator">=</span> <span class="cm-variable">linker</span>.<span class="cm-variable">upcallStub</span>(<span class="cm-variable">comparHandle</span>, <span class="cm-variable">comparDesc</span>, <span class="cm-variable">session</span>);</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> <span class="cm-variable">MemorySegment</span> <span class="cm-variable">array</span> <span class="cm-operator">=</span> <span class="cm-variable">session</span>.<span class="cm-variable">allocateArray</span>(<span class="cm-number">0</span>, <span class="cm-number">9</span>, <span class="cm-number">3</span>, <span class="cm-number">4</span>, <span class="cm-number">6</span>, <span class="cm-number">5</span>, <span class="cm-number">1</span>, <span class="cm-number">8</span>, <span class="cm-number">2</span>, <span class="cm-number">7</span>);</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> <span class="cm-variable">qsort</span>.<span class="cm-variable">invoke</span>(<span class="cm-variable">array</span>, <span class="cm-number">10L</span>, <span class="cm-number">4L</span>, <span class="cm-variable">comparFunc</span>);</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> <span class="cm-variable-3">int</span>[] <span class="cm-variable">sorted</span> <span class="cm-operator">=</span> <span class="cm-variable">array</span>.<span class="cm-variable">toArray</span>(<span class="cm-variable">JAVA_INT</span>); <span class="cm-comment">// [ 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 ]</span></span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">}</span></pre></div></div></div></div></div><div style="position: absolute; height: 0px; width: 1px; border-bottom: 0px solid transparent; top: 138px;"></div><div class="CodeMirror-gutters" style="display: none; height: 138px;"></div></div></div></pre><p><span>The above code creates an upcall stub — </span><code>comparFunc</code><span> — a function pointer that can be used to invoke our Java comparator function, of type </span><code>MemorySegment</code><span>. The upcall stub is associated with the provided segment scope instance; this means that the stub will be uninstalled when the arena is closed.</span></p><p><span>The snippet then creates an off-heap array from a Java array, which is then passed to the </span><code>qsort</code><span> handle, along with the comparator function we obtained from the foreign linker. As a side effect, after the call, the contents of the off-heap array will be sorted (as instructed by our comparator function, written in Java). We can than extract a new Java array from the segment, which contains the sorted elements. This is a more advanced example, but one that shows how powerful the native interop support provided by the foreign linker abstraction is, allowing full bidirectional interop support between Java and native.</span></p><h3 id='varargs'><span>Varargs</span></h3><p><span>Some C functions are </span><em><span>variadic</span></em><span> and can take an arbitrary number of arguments. Perhaps the most common example of this is the </span><code>printf</code><span> function, defined in the C standard library:</span></p><pre class="md-fences md-end-block ty-contain-cm modeLoaded" spellcheck="false" lang="c"><div class="CodeMirror cm-s-inner cm-s-null-scroll" lang="c"><div style="overflow: hidden; position: relative; width: 3px; height: 0px; top: 9.52344px; left: 8px;"><textarea autocorrect="off" autocapitalize="off" spellcheck="false" tabindex="0" style="position: absolute; bottom: -1em; padding: 0px; width: 1000px; height: 1em; outline: none;"></textarea></div><div class="CodeMirror-scrollbar-filler" cm-not-content="true"></div><div class="CodeMirror-gutter-filler" cm-not-content="true"></div><div class="CodeMirror-scroll" tabindex="-1"><div class="CodeMirror-sizer" style="margin-left: 0px; margin-bottom: 0px; border-right-width: 0px; min-width: 317.969px; padding-right: 0px; padding-bottom: 0px;"><div style="position: relative; top: 0px;"><div class="CodeMirror-lines" role="presentation"><div role="presentation" style="position: relative; outline: none;"><div class="CodeMirror-measure"></div><div class="CodeMirror-measure"></div><div style="position: relative; z-index: 1;"></div><div class="CodeMirror-code" role="presentation"><div class="CodeMirror-activeline" style="position: relative;"><div class="CodeMirror-activeline-background CodeMirror-linebackground"></div><div class="CodeMirror-gutter-background CodeMirror-activeline-gutter" style="left: 0px; width: 0px;"></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-variable-3">int</span> <span class="cm-def">printf</span>(<span class="cm-keyword">const</span> <span class="cm-variable-3">char</span> <span class="cm-variable-3">*</span><span class="cm-variable">format</span>, ...);</span></pre></div></div></div></div></div></div><div style="position: absolute; height: 0px; width: 1px; border-bottom: 0px solid transparent; top: 23px;"></div><div class="CodeMirror-gutters" style="display: none; height: 23px;"></div></div></div></pre><p><span>This function takes a format string, which features zero or more </span><em><span>holes</span></em><span>, and then can take a number of additional arguments that is identical to the number of holes in the format string.</span></p><p><span>The foreign function support can support variadic calls, but with a caveat: the client must provide a specialized Java signature, and a specialized description of the C signature. For instance, let's say we wanted to model the following C call:</span></p><pre class="md-fences md-end-block ty-contain-cm modeLoaded" spellcheck="false" lang="C"><div class="CodeMirror cm-s-inner cm-s-null-scroll" lang="c"><div style="overflow: hidden; position: relative; width: 3px; height: 0px; top: 9.52344px; left: 8px;"><textarea autocorrect="off" autocapitalize="off" spellcheck="false" tabindex="0" style="position: absolute; bottom: -1em; padding: 0px; width: 1000px; height: 1em; outline: none;"></textarea></div><div class="CodeMirror-scrollbar-filler" cm-not-content="true"></div><div class="CodeMirror-gutter-filler" cm-not-content="true"></div><div class="CodeMirror-scroll" tabindex="-1"><div class="CodeMirror-sizer" style="margin-left: 0px; margin-bottom: 0px; border-right-width: 0px; min-width: 352.516px; padding-right: 0px; padding-bottom: 0px;"><div style="position: relative; top: 0px;"><div class="CodeMirror-lines" role="presentation"><div role="presentation" style="position: relative; outline: none;"><div class="CodeMirror-measure"></div><div class="CodeMirror-measure"></div><div style="position: relative; z-index: 1;"></div><div class="CodeMirror-code" role="presentation"><div class="CodeMirror-activeline" style="position: relative;"><div class="CodeMirror-activeline-background CodeMirror-linebackground"></div><div class="CodeMirror-gutter-background CodeMirror-activeline-gutter" style="left: 0px; width: 0px;"></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-variable">printf</span>(<span class="cm-string">"%d plus %d equals %d"</span>, <span class="cm-number">2</span>, <span class="cm-number">2</span>, <span class="cm-number">4</span>);</span></pre></div></div></div></div></div></div><div style="position: absolute; height: 0px; width: 1px; border-bottom: 0px solid transparent; top: 23px;"></div><div class="CodeMirror-gutters" style="display: none; height: 23px;"></div></div></div></pre><p><span>To do this using the foreign function support provided by the FFM API we would have to build a </span><em><span>specialized</span></em><span> downcall handle for that call shape, using a linker option to specify the position of the first variadic layout, as follows:</span></p><pre class="md-fences md-end-block ty-contain-cm modeLoaded" spellcheck="false" lang="java"><div class="CodeMirror cm-s-inner cm-s-null-scroll" lang="java"><div style="overflow: hidden; position: relative; width: 3px; height: 0px; top: 9.52344px; left: 8px;"><textarea autocorrect="off" autocapitalize="off" spellcheck="false" tabindex="0" style="position: absolute; bottom: -1em; padding: 0px; width: 1000px; height: 1em; outline: none;"></textarea></div><div class="CodeMirror-scrollbar-filler" cm-not-content="true"></div><div class="CodeMirror-gutter-filler" cm-not-content="true"></div><div class="CodeMirror-scroll" tabindex="-1"><div class="CodeMirror-sizer" style="margin-left: 0px; margin-bottom: 0px; border-right-width: 0px; min-width: 680.703px; padding-right: 0px; padding-bottom: 0px;"><div style="position: relative; top: 0px;"><div class="CodeMirror-lines" role="presentation"><div role="presentation" style="position: relative; outline: none;"><div class="CodeMirror-measure"><pre>x</pre></div><div class="CodeMirror-measure"></div><div style="position: relative; z-index: 1;"></div><div class="CodeMirror-code" role="presentation" style=""><div class="CodeMirror-activeline" style="position: relative;"><div class="CodeMirror-activeline-background CodeMirror-linebackground"></div><div class="CodeMirror-gutter-background CodeMirror-activeline-gutter" style="left: 0px; width: 0px;"></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-variable">Linker</span> <span class="cm-variable">linker</span> <span class="cm-operator">=</span> <span class="cm-variable">Linker</span>.<span class="cm-variable">nativeLinker</span>();</span></pre></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-variable">MethodHandle</span> <span class="cm-variable">printf</span> <span class="cm-operator">=</span> <span class="cm-variable">linker</span>.<span class="cm-variable">downcallHandle</span>(</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-tab" role="presentation" cm-text=" "> </span><span class="cm-tab" role="presentation" cm-text=" "> </span><span class="cm-variable">linker</span>.<span class="cm-variable">defaultLookup</span>().<span class="cm-variable">lookup</span>(<span class="cm-string">"printf"</span>).<span class="cm-variable">get</span>(),</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> <span class="cm-variable">FunctionDescriptor</span>.<span class="cm-variable">of</span>(<span class="cm-variable">JAVA_INT</span>, <span class="cm-variable">ADDRESS</span>, <span class="cm-variable">JAVA_INT</span>, <span class="cm-variable">JAVA_INT</span>, <span class="cm-variable">JAVA_INT</span>)</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> <span class="cm-variable">Linker</span>.<span class="cm-variable">Option</span>.<span class="cm-variable">firstVariadicArg</span>(<span class="cm-number">1</span>) <span class="cm-comment">// first int is variadic</span></span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">);</span></pre></div></div></div></div></div><div style="position: absolute; height: 0px; width: 1px; border-bottom: 0px solid transparent; top: 138px;"></div><div class="CodeMirror-gutters" style="display: none; height: 138px;"></div></div></div></pre><p><span>Then we can call the specialized downcall handle as usual:</span></p><pre class="md-fences md-end-block ty-contain-cm modeLoaded" spellcheck="false" lang="java"><div class="CodeMirror cm-s-inner cm-s-null-scroll" lang="java"><div style="overflow: hidden; position: relative; width: 3px; height: 0px; top: 9.52344px; left: 8px;"><textarea autocorrect="off" autocapitalize="off" spellcheck="false" tabindex="0" style="position: absolute; bottom: -1em; padding: 0px; width: 1000px; height: 1em; outline: none;"></textarea></div><div class="CodeMirror-scrollbar-filler" cm-not-content="true"></div><div class="CodeMirror-gutter-filler" cm-not-content="true"></div><div class="CodeMirror-scroll" tabindex="-1"><div class="CodeMirror-sizer" style="margin-left: 0px; margin-bottom: 0px; border-right-width: 0px; min-width: 922.547px; padding-right: 0px; padding-bottom: 0px;"><div style="position: relative; top: 0px;"><div class="CodeMirror-lines" role="presentation"><div role="presentation" style="position: relative; outline: none;"><div class="CodeMirror-measure"></div><div class="CodeMirror-measure"></div><div style="position: relative; z-index: 1;"></div><div class="CodeMirror-code" role="presentation"><div class="CodeMirror-activeline" style="position: relative;"><div class="CodeMirror-activeline-background CodeMirror-linebackground"></div><div class="CodeMirror-gutter-background CodeMirror-activeline-gutter" style="left: 0px; width: 0px;"></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-keyword">try</span> (<span class="cm-variable">Arena</span> <span class="cm-variable">arena</span> <span class="cm-operator">=</span> <span class="cm-variable">Arena</span>.<span class="cm-variable">openConfined</span>()) {</span></pre></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> <span class="cm-variable">printf</span>.<span class="cm-variable">invoke</span>(<span class="cm-variable">arena</span>.<span class="cm-variable">allocateUtf8String</span>(<span class="cm-string">"%d plus %d equals %d"</span>), <span class="cm-number">2</span>, <span class="cm-number">2</span>, <span class="cm-number">4</span>); <span class="cm-comment">//prints "2 plus 2 equals 4"</span></span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">}</span></pre></div></div></div></div></div><div style="position: absolute; height: 0px; width: 1px; border-bottom: 0px solid transparent; top: 69px;"></div><div class="CodeMirror-gutters" style="display: none; height: 69px;"></div></div></div></pre><p><span>While this works, and provides optimal performance, it has some limitations</span><a href="#3"><sup><span>3</span></sup></a><span>:</span></p><ul><li><span>If the variadic function needs to be called with many shapes, we have to create many downcall handles</span></li><li><span>while this approach works for downcalls (since the Java code is in charge of determining which and how many arguments should be passed) it fails to scale to upcalls; in that case, the call comes from native code, so we have no way to guarantee that the shape of the upcall stub we have created will match that required by the native function.</span></li></ul><h3 id='appendix-full-source-code'><span>Appendix: full source code</span></h3><p><span>The full source code containing most of the code shown throughout this document can be seen below:</span></p><pre class="md-fences md-end-block ty-contain-cm modeLoaded" spellcheck="false" lang="java" style="break-inside: unset;"><div class="CodeMirror cm-s-inner cm-s-null-scroll" lang="java"><div style="overflow: hidden; position: relative; width: 3px; height: 0px; top: 9.52344px; left: 8px;"><textarea autocorrect="off" autocapitalize="off" spellcheck="false" tabindex="0" style="position: absolute; bottom: -1em; padding: 0px; width: 1000px; height: 1em; outline: none;"></textarea></div><div class="CodeMirror-scrollbar-filler" cm-not-content="true"></div><div class="CodeMirror-gutter-filler" cm-not-content="true"></div><div class="CodeMirror-scroll" tabindex="-1"><div class="CodeMirror-sizer" style="margin-left: 0px; margin-bottom: 0px; border-right-width: 0px; min-width: 1026.14px; padding-right: 0px; padding-bottom: 0px;"><div style="position: relative; top: 0px;"><div class="CodeMirror-lines" role="presentation"><div role="presentation" style="position: relative; outline: none;"><div class="CodeMirror-measure"><pre>x</pre></div><div class="CodeMirror-measure"></div><div style="position: relative; z-index: 1;"></div><div class="CodeMirror-code" role="presentation" style=""><div class="CodeMirror-activeline" style="position: relative;"><div class="CodeMirror-activeline-background CodeMirror-linebackground"></div><div class="CodeMirror-gutter-background CodeMirror-activeline-gutter" style="left: 0px; width: 0px;"></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-keyword">import</span> <span class="cm-variable">java</span>.<span class="cm-variable">lang</span>.<span class="cm-variable">foreign</span>.<span class="cm-variable">Arena</span>;</span></pre></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-keyword">import</span> <span class="cm-variable">java</span>.<span class="cm-variable">lang</span>.<span class="cm-variable">foreign</span>.<span class="cm-variable">Linker</span>;</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-keyword">import</span> <span class="cm-variable">java</span>.<span class="cm-variable">lang</span>.<span class="cm-variable">foreign</span>.<span class="cm-variable">FunctionDescriptor</span>;</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-keyword">import</span> <span class="cm-variable">java</span>.<span class="cm-variable">lang</span>.<span class="cm-variable">foreign</span>.<span class="cm-variable">SymbolLookup</span>;</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-keyword">import</span> <span class="cm-variable">java</span>.<span class="cm-variable">lang</span>.<span class="cm-variable">foreign</span>.<span class="cm-variable">MemorySegment</span>;</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-keyword">import</span> <span class="cm-variable">java</span>.<span class="cm-variable">lang</span>.<span class="cm-variable">foreign</span>.<span class="cm-variable">VaList</span>;</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="" cm-zwsp="">
678 </span></span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-keyword">import</span> <span class="cm-variable">java</span>.<span class="cm-variable">lang</span>.<span class="cm-variable">invoke</span>.<span class="cm-variable">MethodHandle</span>;</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-keyword">import</span> <span class="cm-variable">java</span>.<span class="cm-variable">lang</span>.<span class="cm-variable">invoke</span>.<span class="cm-variable">MethodHandles</span>;</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-keyword">import</span> <span class="cm-variable">java</span>.<span class="cm-variable">util</span>.<span class="cm-variable">Arrays</span>;</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="" cm-zwsp="">
679 </span></span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-keyword">import</span> <span class="cm-keyword">static</span> <span class="cm-variable">java</span>.<span class="cm-variable">lang</span>.<span class="cm-variable">foreign</span>.<span class="cm-variable">ValueLayout</span>.<span class="cm-operator">*</span>;</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="" cm-zwsp="">
680 </span></span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-keyword">public</span> <span class="cm-keyword">class</span> <span class="cm-def">Examples</span> {</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="" cm-zwsp="">
681 </span></span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> <span class="cm-keyword">static</span> <span class="cm-variable">Linker</span> <span class="cm-variable">LINKER</span> <span class="cm-operator">=</span> <span class="cm-variable">Linker</span>.<span class="cm-variable">nativeLinker</span>();</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> <span class="cm-keyword">static</span> <span class="cm-variable">SymbolLookup</span> <span class="cm-variable">STDLIB</span> <span class="cm-operator">=</span> <span class="cm-variable">LINKER</span>.<span class="cm-variable">defaultLookup</span>();</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="" cm-zwsp="">
682 </span></span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> <span class="cm-keyword">public</span> <span class="cm-keyword">static</span> <span class="cm-variable-3">void</span> <span class="cm-variable">main</span>(<span class="cm-variable-3">String</span>[] <span class="cm-variable">args</span>) <span class="cm-keyword">throws</span> <span class="cm-variable">Throwable</span> {</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> <span class="cm-variable">strlen</span>();</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> <span class="cm-variable">strlen_virtual</span>();</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> <span class="cm-variable">qsort</span>();</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> <span class="cm-variable">printf</span>();</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> }</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="" cm-zwsp="">
683 </span></span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> <span class="cm-keyword">public</span> <span class="cm-keyword">static</span> <span class="cm-variable-3">void</span> <span class="cm-variable">strlen</span>() <span class="cm-keyword">throws</span> <span class="cm-variable">Throwable</span> {</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> <span class="cm-variable">MethodHandle</span> <span class="cm-variable">strlen</span> <span class="cm-operator">=</span> <span class="cm-variable">LINKER</span>.<span class="cm-variable">downcallHandle</span>(</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> <span class="cm-variable">STDLIB</span>.<span class="cm-variable">find</span>(<span class="cm-string">"strlen"</span>).<span class="cm-variable">get</span>(),</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> <span class="cm-variable">FunctionDescriptor</span>.<span class="cm-variable">of</span>(<span class="cm-variable">JAVA_LONG</span>, <span class="cm-variable">ADDRESS</span>)</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> );</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="" cm-zwsp="">
684 </span></span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> <span class="cm-keyword">try</span> (<span class="cm-variable">Arena</span> <span class="cm-variable">arena</span> <span class="cm-operator">=</span> <span class="cm-variable">Arena</span>.<span class="cm-variable">openConfined</span>()) {</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> <span class="cm-variable">MemorySegment</span> <span class="cm-variable">hello</span> <span class="cm-operator">=</span> <span class="cm-variable">arena</span>.<span class="cm-variable">allocateUtf8String</span>(<span class="cm-string">"Hello"</span>);</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> <span class="cm-variable-3">long</span> <span class="cm-variable">len</span> <span class="cm-operator">=</span> (<span class="cm-variable-3">long</span>) <span class="cm-variable">strlen</span>.<span class="cm-variable">invoke</span>(<span class="cm-variable">hello</span>); <span class="cm-comment">// 5</span></span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> <span class="cm-variable">System</span>.<span class="cm-variable">out</span>.<span class="cm-variable">println</span>(<span class="cm-variable">len</span>);</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> }</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> }</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="" cm-zwsp="">
685 </span></span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> <span class="cm-keyword">public</span> <span class="cm-keyword">static</span> <span class="cm-variable-3">void</span> <span class="cm-variable">strlen_virtual</span>() <span class="cm-keyword">throws</span> <span class="cm-variable">Throwable</span> {</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> <span class="cm-variable">MethodHandle</span> <span class="cm-variable">strlen_virtual</span> <span class="cm-operator">=</span> <span class="cm-variable">LINKER</span>.<span class="cm-variable">downcallHandle</span>(</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> <span class="cm-variable">FunctionDescriptor</span>.<span class="cm-variable">of</span>(<span class="cm-variable">JAVA_LONG</span>, <span class="cm-variable">ADDRESS</span>)</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> );</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="" cm-zwsp="">
686 </span></span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> <span class="cm-keyword">try</span> (<span class="cm-variable">Arena</span> <span class="cm-variable">arena</span> <span class="cm-operator">=</span> <span class="cm-variable">Arena</span>.<span class="cm-variable">openConfined</span>()) {</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> <span class="cm-variable">MemorySegment</span> <span class="cm-variable">hello</span> <span class="cm-operator">=</span> <span class="cm-variable">arena</span>.<span class="cm-variable">allocateUtf8String</span>(<span class="cm-string">"Hello"</span>);</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> <span class="cm-variable-3">long</span> <span class="cm-variable">len</span> <span class="cm-operator">=</span> (<span class="cm-variable-3">long</span>) <span class="cm-variable">strlen_virtual</span>.<span class="cm-variable">invoke</span>(</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> <span class="cm-variable">STDLIB</span>.<span class="cm-variable">find</span>(<span class="cm-string">"strlen"</span>).<span class="cm-variable">get</span>(),</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> <span class="cm-variable">hello</span>); <span class="cm-comment">// 5</span></span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> <span class="cm-variable">System</span>.<span class="cm-variable">out</span>.<span class="cm-variable">println</span>(<span class="cm-variable">len</span>);</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> }</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> }</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="" cm-zwsp="">
687 </span></span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> <span class="cm-keyword">static</span> <span class="cm-keyword">class</span> <span class="cm-def">Qsort</span> {</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> <span class="cm-keyword">static</span> <span class="cm-variable-3">int</span> <span class="cm-variable">qsortCompare</span>(<span class="cm-variable">MemorySegment</span> <span class="cm-variable">addr1</span>, <span class="cm-variable">MemorySegment</span> <span class="cm-variable">addr2</span>) {</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> <span class="cm-keyword">return</span> <span class="cm-variable">addr1</span>.<span class="cm-variable">get</span>(<span class="cm-variable">JAVA_INT</span>, <span class="cm-number">0</span>) <span class="cm-operator">-</span> <span class="cm-variable">addr2</span>.<span class="cm-variable">get</span>(<span class="cm-variable">JAVA_INT</span>, <span class="cm-number">0</span>);</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> }</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> }</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="" cm-zwsp="">
688 </span></span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> <span class="cm-keyword">public</span> <span class="cm-keyword">static</span> <span class="cm-variable-3">void</span> <span class="cm-variable">qsort</span>() <span class="cm-keyword">throws</span> <span class="cm-variable">Throwable</span> {</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> <span class="cm-variable">MethodHandle</span> <span class="cm-variable">qsort</span> <span class="cm-operator">=</span> <span class="cm-variable">LINKER</span>.<span class="cm-variable">downcallHandle</span>(</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> <span class="cm-variable">STDLIB</span>.<span class="cm-variable">find</span>(<span class="cm-string">"qsort"</span>).<span class="cm-variable">get</span>(),</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> <span class="cm-variable">FunctionDescriptor</span>.<span class="cm-variable">ofVoid</span>(<span class="cm-variable">ADDRESS</span>, <span class="cm-variable">JAVA_LONG</span>, <span class="cm-variable">JAVA_LONG</span>, <span class="cm-variable">ADDRESS</span>)</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> );</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> <span class="cm-variable">FunctionDescriptor</span> <span class="cm-variable">comparDesc</span> <span class="cm-operator">=</span> <span class="cm-variable">FunctionDescriptor</span>.<span class="cm-variable">of</span>(<span class="cm-variable">JAVA_INT</span>, <span class="cm-variable">ADDRESS</span>.<span class="cm-variable">asUnbounded</span>(), <span class="cm-variable">ADDRESS</span>.<span class="cm-variable">asUnbounded</span>());</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> <span class="cm-variable">MethodHandle</span> <span class="cm-variable">comparHandle</span> <span class="cm-operator">=</span> <span class="cm-variable">MethodHandles</span>.<span class="cm-variable">lookup</span>()</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> .<span class="cm-variable">findStatic</span>(<span class="cm-variable">Qsort</span>.<span class="cm-keyword">class</span>, <span class="cm-string">"qsortCompare"</span>,</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> <span class="cm-variable">comparDesc</span>.<span class="cm-variable">toMethodType</span>());</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="" cm-zwsp="">
689 </span></span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> <span class="cm-keyword">try</span> (<span class="cm-variable">Arena</span> <span class="cm-variable">arena</span> <span class="cm-operator">=</span> <span class="cm-variable">Arena</span>.<span class="cm-variable">openConfined</span>()) {</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> <span class="cm-variable">MemorySegment</span> <span class="cm-variable">comparFunc</span> <span class="cm-operator">=</span> <span class="cm-variable">LINKER</span>.<span class="cm-variable">upcallStub</span>(</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> <span class="cm-variable">comparHandle</span>, <span class="cm-variable">comparDesc</span>, <span class="cm-variable">arena</span>.<span class="cm-variable">scope</span>());</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="" cm-zwsp="">
690 </span></span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> <span class="cm-variable">MemorySegment</span> <span class="cm-variable">array</span> <span class="cm-operator">=</span> <span class="cm-variable">arena</span>.<span class="cm-variable">allocateArray</span>(<span class="cm-variable">JAVA_INT</span>, <span class="cm-number">0</span>, <span class="cm-number">9</span>, <span class="cm-number">3</span>, <span class="cm-number">4</span>, <span class="cm-number">6</span>, <span class="cm-number">5</span>, <span class="cm-number">1</span>, <span class="cm-number">8</span>, <span class="cm-number">2</span>, <span class="cm-number">7</span>);</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> <span class="cm-variable">qsort</span>.<span class="cm-variable">invoke</span>(<span class="cm-variable">array</span>, <span class="cm-number">10L</span>, <span class="cm-number">4L</span>, <span class="cm-variable">comparFunc</span>);</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> <span class="cm-variable-3">int</span>[] <span class="cm-variable">sorted</span> <span class="cm-operator">=</span> <span class="cm-variable">array</span>.<span class="cm-variable">toArray</span>(<span class="cm-variable">JAVA_INT</span>); <span class="cm-comment">// [ 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 ]</span></span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> <span class="cm-variable">System</span>.<span class="cm-variable">out</span>.<span class="cm-variable">println</span>(<span class="cm-variable">Arrays</span>.<span class="cm-variable">toString</span>(<span class="cm-variable">sorted</span>));</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> }</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> }</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="" cm-zwsp="">
691 </span></span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> <span class="cm-keyword">public</span> <span class="cm-keyword">static</span> <span class="cm-variable-3">void</span> <span class="cm-variable">printf</span>() <span class="cm-keyword">throws</span> <span class="cm-variable">Throwable</span> {</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> <span class="cm-variable">MethodHandle</span> <span class="cm-variable">printf</span> <span class="cm-operator">=</span> <span class="cm-variable">LINKER</span>.<span class="cm-variable">downcallHandle</span>(</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> <span class="cm-variable">STDLIB</span>.<span class="cm-variable">find</span>(<span class="cm-string">"printf"</span>).<span class="cm-variable">get</span>(),</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> <span class="cm-variable">FunctionDescriptor</span>.<span class="cm-variable">of</span>(<span class="cm-variable">JAVA_INT</span>, <span class="cm-variable">ADDRESS</span>, <span class="cm-variable">JAVA_INT</span>, <span class="cm-variable">JAVA_INT</span>, <span class="cm-variable">JAVA_INT</span>),</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> <span class="cm-variable">Linker</span>.<span class="cm-variable">Option</span>.<span class="cm-variable">firstVariadicArg</span>(<span class="cm-number">1</span>) <span class="cm-comment">// first int is variadic</span></span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> );</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> <span class="cm-keyword">try</span> (<span class="cm-variable">Arena</span> <span class="cm-variable">arena</span> <span class="cm-operator">=</span> <span class="cm-variable">Arena</span>.<span class="cm-variable">openConfined</span>()) {</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> <span class="cm-variable">MemorySegment</span> <span class="cm-variable">s</span> <span class="cm-operator">=</span> <span class="cm-variable">arena</span>.<span class="cm-variable">allocateUtf8String</span>(<span class="cm-string">"%d plus %d equals %d\n"</span>);</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> <span class="cm-variable">printf</span>.<span class="cm-variable">invoke</span>(<span class="cm-variable">s</span>, <span class="cm-number">2</span>, <span class="cm-number">2</span>, <span class="cm-number">4</span>);</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> }</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> }</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">}</span></pre></div></div></div></div></div><div style="position: absolute; height: 0px; width: 1px; border-bottom: 0px solid transparent; top: 2097px;"></div><div class="CodeMirror-gutters" style="display: none; height: 2097px;"></div></div></div></pre><p> </p><ul><li><a id="1"/><span>(</span><sup><span>1</span></sup><span>):</span><small><span> For simplicity, the examples shown in this document use </span><code>MethodHandle::invoke</code><span> rather than </span><code>MethodHandle::invokeExact</code><span>; by doing so we avoid having to cast by-reference arguments back to </span><code>Addressable</code><span>. With </span><code>invokeExact</code><span> the method handle invocation should be rewritten as </span><code>strlen.invokeExact((Addressable)session.allocateUtf8String("Hello"));</code></small></li><li><a id="2"/><span>(</span><sup><span>2</span></sup><span>):</span><small><span> In reality this is not entirely new; even in JNI, when you call a </span><code>native</code><span> method the VM trusts that the corresponding implementing function in C will feature compatible parameter types and return values; if not a crash might occur.</span></small></li><li><a id="3"/><span>(</span><sup><span>3</span></sup><span>):</span><small><span> Previous iterations of the FFM API provided a </span><code>VaList</code><span> class that could be used to model a C </span><code>va_list</code><span>. This class was later dropped from the FFM API as too implementation specific. It is possible that a future version of the </span><code>jextract</code><span> tool might provide higher-level bindings for variadic calls. </span></small></li></ul></div></div>
692 </body>
693 </html>