1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
|
<!DOCTYPE HTML>
<html lang="en" class="light sidebar-visible" dir="ltr">
<head>
<!-- Book generated using mdBook -->
<meta charset="UTF-8">
<title>Contexts - NNG Reference Manual (DRAFT)</title>
<!-- Custom HTML head -->
<meta name="description" content="">
<meta name="viewport" content="width=device-width, initial-scale=1">
<meta name="theme-color" content="#ffffff">
<link rel="icon" href="../favicon.svg">
<link rel="shortcut icon" href="../favicon.png">
<link rel="stylesheet" href="../css/variables.css">
<link rel="stylesheet" href="../css/general.css">
<link rel="stylesheet" href="../css/chrome.css">
<link rel="stylesheet" href="../css/print.css" media="print">
<!-- Fonts -->
<link rel="stylesheet" href="../FontAwesome/css/font-awesome.css">
<link rel="stylesheet" href="../fonts/fonts.css">
<!-- Highlight.js Stylesheets -->
<link rel="stylesheet" id="highlight-css" href="../highlight.css">
<link rel="stylesheet" id="tomorrow-night-css" href="../tomorrow-night.css">
<link rel="stylesheet" id="ayu-highlight-css" href="../ayu-highlight.css">
<!-- Custom theme stylesheets -->
<link rel="stylesheet" href="../theme/pagetoc.css">
<!-- Provide site root and default themes to javascript -->
<script>
const path_to_root = "../";
const default_light_theme = "light";
const default_dark_theme = "navy";
window.path_to_searchindex_js = "../searchindex.js";
</script>
<!-- Start loading toc.js asap -->
<script src="../toc.js"></script>
</head>
<body>
<div id="mdbook-help-container">
<div id="mdbook-help-popup">
<h2 class="mdbook-help-title">Keyboard shortcuts</h2>
<div>
<p>Press <kbd>←</kbd> or <kbd>→</kbd> to navigate between chapters</p>
<p>Press <kbd>S</kbd> or <kbd>/</kbd> to search in the book</p>
<p>Press <kbd>?</kbd> to show this help</p>
<p>Press <kbd>Esc</kbd> to hide this help</p>
</div>
</div>
</div>
<div id="body-container">
<!-- Work around some values being stored in localStorage wrapped in quotes -->
<script>
try {
let theme = localStorage.getItem('mdbook-theme');
let sidebar = localStorage.getItem('mdbook-sidebar');
if (theme.startsWith('"') && theme.endsWith('"')) {
localStorage.setItem('mdbook-theme', theme.slice(1, theme.length - 1));
}
if (sidebar.startsWith('"') && sidebar.endsWith('"')) {
localStorage.setItem('mdbook-sidebar', sidebar.slice(1, sidebar.length - 1));
}
} catch (e) { }
</script>
<!-- Set the theme before any content is loaded, prevents flash -->
<script>
const default_theme = window.matchMedia("(prefers-color-scheme: dark)").matches ? default_dark_theme : default_light_theme;
let theme;
try { theme = localStorage.getItem('mdbook-theme'); } catch(e) { }
if (theme === null || theme === undefined) { theme = default_theme; }
const html = document.documentElement;
html.classList.remove('light')
html.classList.add(theme);
html.classList.add("js");
</script>
<input type="checkbox" id="sidebar-toggle-anchor" class="hidden">
<!-- Hide / unhide sidebar before it is displayed -->
<script>
let sidebar = null;
const sidebar_toggle = document.getElementById("sidebar-toggle-anchor");
if (document.body.clientWidth >= 1080) {
try { sidebar = localStorage.getItem('mdbook-sidebar'); } catch(e) { }
sidebar = sidebar || 'visible';
} else {
sidebar = 'hidden';
sidebar_toggle.checked = false;
}
if (sidebar === 'visible') {
sidebar_toggle.checked = true;
} else {
html.classList.remove('sidebar-visible');
}
</script>
<nav id="sidebar" class="sidebar" aria-label="Table of contents">
<!-- populated by js -->
<mdbook-sidebar-scrollbox class="sidebar-scrollbox"></mdbook-sidebar-scrollbox>
<noscript>
<iframe class="sidebar-iframe-outer" src="../toc.html"></iframe>
</noscript>
<div id="sidebar-resize-handle" class="sidebar-resize-handle">
<div class="sidebar-resize-indicator"></div>
</div>
</nav>
<div id="page-wrapper" class="page-wrapper">
<div class="page">
<div id="menu-bar-hover-placeholder"></div>
<div id="menu-bar" class="menu-bar sticky">
<div class="left-buttons">
<label id="sidebar-toggle" class="icon-button" for="sidebar-toggle-anchor" title="Toggle Table of Contents" aria-label="Toggle Table of Contents" aria-controls="sidebar">
<i class="fa fa-bars"></i>
</label>
<button id="theme-toggle" class="icon-button" type="button" title="Change theme" aria-label="Change theme" aria-haspopup="true" aria-expanded="false" aria-controls="theme-list">
<i class="fa fa-paint-brush"></i>
</button>
<ul id="theme-list" class="theme-popup" aria-label="Themes" role="menu">
<li role="none"><button role="menuitem" class="theme" id="default_theme">Auto</button></li>
<li role="none"><button role="menuitem" class="theme" id="light">Light</button></li>
<li role="none"><button role="menuitem" class="theme" id="rust">Rust</button></li>
<li role="none"><button role="menuitem" class="theme" id="coal">Coal</button></li>
<li role="none"><button role="menuitem" class="theme" id="navy">Navy</button></li>
<li role="none"><button role="menuitem" class="theme" id="ayu">Ayu</button></li>
</ul>
<button id="search-toggle" class="icon-button" type="button" title="Search (`/`)" aria-label="Toggle Searchbar" aria-expanded="false" aria-keyshortcuts="/ s" aria-controls="searchbar">
<i class="fa fa-search"></i>
</button>
</div>
<h1 class="menu-title">NNG Reference Manual (DRAFT)</h1>
<div class="right-buttons">
<a href="../print.html" title="Print this book" aria-label="Print this book">
<i id="print-button" class="fa fa-print"></i>
</a>
</div>
</div>
<div id="search-wrapper" class="hidden">
<form id="searchbar-outer" class="searchbar-outer">
<div class="search-wrapper">
<input type="search" id="searchbar" name="searchbar" placeholder="Search this book ..." aria-controls="searchresults-outer" aria-describedby="searchresults-header">
<div class="spinner-wrapper">
<i class="fa fa-spinner fa-spin"></i>
</div>
</div>
</form>
<div id="searchresults-outer" class="searchresults-outer hidden">
<div id="searchresults-header" class="searchresults-header"></div>
<ul id="searchresults">
</ul>
</div>
</div>
<!-- Apply ARIA attributes after the sidebar and the sidebar toggle button are added to the DOM -->
<script>
document.getElementById('sidebar-toggle').setAttribute('aria-expanded', sidebar === 'visible');
document.getElementById('sidebar').setAttribute('aria-hidden', sidebar !== 'visible');
Array.from(document.querySelectorAll('#sidebar a')).forEach(function(link) {
link.setAttribute('tabIndex', sidebar === 'visible' ? 0 : -1);
});
</script>
<div id="content" class="content">
<main>
<style>
.mdbook-alerts {
padding: 8px 16px;
margin-bottom: 16px;
border-left: 0.25em solid var(--mdbook-alerts-color);
}
.mdbook-alerts > *:first-child {
margin-top: 0;
}
.mdbook-alerts > *:last-child {
margin-bottom: 0;
}
.mdbook-alerts-title {
display: flex;
font-weight: 600;
align-items: center;
line-height: 1;
color: var(--mdbook-alerts-color);
text-transform: capitalize;
}
.mdbook-alerts-icon {
display: inline-block;
width: 1em;
height: 1em;
margin-right: 0.2em;
background-color: currentColor;
-webkit-mask: no-repeat center / 100%;
mask: no-repeat center / 100%;
-webkit-mask-image: var(--mdbook-alerts-icon);
mask-image: var(--mdbook-alerts-icon);
}
.mdbook-alerts-note {
--mdbook-alerts-color: rgb(9, 105, 218);
/* https://icon-sets.iconify.design/material-symbols/info-outline-rounded/ */
--mdbook-alerts-icon: url('data:image/svg+xml,%3Csvg xmlns="http%3A%2F%2Fwww.w3.org%2F2000%2Fsvg" width="24" height="24" viewBox="0 0 24 24"%3E%3Cpath fill="currentColor" d="M12 17q.425 0 .713-.288T13 16v-4q0-.425-.288-.712T12 11q-.425 0-.712.288T11 12v4q0 .425.288.713T12 17m0-8q.425 0 .713-.288T13 8q0-.425-.288-.712T12 7q-.425 0-.712.288T11 8q0 .425.288.713T12 9m0 13q-2.075 0-3.9-.788t-3.175-2.137q-1.35-1.35-2.137-3.175T2 12q0-2.075.788-3.9t2.137-3.175q1.35-1.35 3.175-2.137T12 2q2.075 0 3.9.788t3.175 2.137q1.35 1.35 2.138 3.175T22 12q0 2.075-.788 3.9t-2.137 3.175q-1.35 1.35-3.175 2.138T12 22m0-2q3.35 0 5.675-2.325T20 12q0-3.35-2.325-5.675T12 4Q8.65 4 6.325 6.325T4 12q0 3.35 2.325 5.675T12 20m0-8"%2F%3E%3C%2Fsvg%3E');
}
.mdbook-alerts-tip {
--mdbook-alerts-color: rgb(26, 127, 55);
/* https://icon-sets.iconify.design/material-symbols/lightbulb-outline-rounded/ */
--mdbook-alerts-icon: url('data:image/svg+xml,%3Csvg xmlns="http%3A%2F%2Fwww.w3.org%2F2000%2Fsvg" width="24" height="24" viewBox="0 0 24 24"%3E%3Cpath fill="currentColor" d="M12 22q-.825 0-1.412-.587T10 20h4q0 .825-.587 1.413T12 22m-3-3q-.425 0-.712-.288T8 18q0-.425.288-.712T9 17h6q.425 0 .713.288T16 18q0 .425-.288.713T15 19zm-.75-3q-1.725-1.025-2.738-2.75T4.5 9.5q0-3.125 2.188-5.312T12 2q3.125 0 5.313 2.188T19.5 9.5q0 2.025-1.012 3.75T15.75 16zm.6-2h6.3q1.125-.8 1.738-1.975T17.5 9.5q0-2.3-1.6-3.9T12 4Q9.7 4 8.1 5.6T6.5 9.5q0 1.35.613 2.525T8.85 14M12 14"%2F%3E%3C%2Fsvg%3E');
}
.mdbook-alerts-important {
--mdbook-alerts-color: rgb(130, 80, 223);
/* https://icon-sets.iconify.design/material-symbols/chat-info-outline-rounded/ */
--mdbook-alerts-icon: url('data:image/svg+xml,%3Csvg xmlns="http%3A%2F%2Fwww.w3.org%2F2000%2Fsvg" width="24" height="24" viewBox="0 0 24 24"%3E%3Cpath fill="currentColor" d="M12 7q.425 0 .713-.288T13 6q0-.425-.288-.712T12 5q-.425 0-.712.288T11 6q0 .425.288.713T12 7m0 8q.425 0 .713-.288T13 14v-4q0-.425-.288-.712T12 9q-.425 0-.712.288T11 10v4q0 .425.288.713T12 15m-6 3l-2.3 2.3q-.475.475-1.088.213T2 19.575V4q0-.825.588-1.412T4 2h16q.825 0 1.413.588T22 4v12q0 .825-.587 1.413T20 18zm-.85-2H20V4H4v13.125zM4 16V4z"%2F%3E%3C%2Fsvg%3E');
}
.mdbook-alerts-warning {
--mdbook-alerts-color: rgb(154, 103, 0);
/* https://icon-sets.iconify.design/material-symbols/warning-outline-rounded/ */
--mdbook-alerts-icon: url('data:image/svg+xml,%3Csvg xmlns="http%3A%2F%2Fwww.w3.org%2F2000%2Fsvg" width="24" height="24" viewBox="0 0 24 24"%3E%3Cpath fill="currentColor" d="M2.725 21q-.275 0-.5-.137t-.35-.363q-.125-.225-.137-.488t.137-.512l9.25-16q.15-.25.388-.375T12 3q.25 0 .488.125t.387.375l9.25 16q.15.25.138.513t-.138.487q-.125.225-.35.363t-.5.137zm1.725-2h15.1L12 6zM12 18q.425 0 .713-.288T13 17q0-.425-.288-.712T12 16q-.425 0-.712.288T11 17q0 .425.288.713T12 18m0-3q.425 0 .713-.288T13 14v-3q0-.425-.288-.712T12 10q-.425 0-.712.288T11 11v3q0 .425.288.713T12 15m0-2.5"%2F%3E%3C%2Fsvg%3E');
}
.mdbook-alerts-caution {
--mdbook-alerts-color: rgb(207, 34, 46);
/* https://icon-sets.iconify.design/material-symbols/brightness-alert-outline-rounded/ */
--mdbook-alerts-icon: url('data:image/svg+xml,%3Csvg xmlns="http%3A%2F%2Fwww.w3.org%2F2000%2Fsvg" width="24" height="24" viewBox="0 0 24 24"%3E%3Cpath fill="currentColor" d="M12 17q.425 0 .713-.288T13 16q0-.425-.288-.712T12 15q-.425 0-.712.288T11 16q0 .425.288.713T12 17m0-4q.425 0 .713-.288T13 12V8q0-.425-.288-.712T12 7q-.425 0-.712.288T11 8v4q0 .425.288.713T12 13m-3.35 7H6q-.825 0-1.412-.587T4 18v-2.65L2.075 13.4q-.275-.3-.425-.662T1.5 12q0-.375.15-.737t.425-.663L4 8.65V6q0-.825.588-1.412T6 4h2.65l1.95-1.925q.3-.275.663-.425T12 1.5q.375 0 .738.15t.662.425L15.35 4H18q.825 0 1.413.588T20 6v2.65l1.925 1.95q.275.3.425.663t.15.737q0 .375-.15.738t-.425.662L20 15.35V18q0 .825-.587 1.413T18 20h-2.65l-1.95 1.925q-.3.275-.662.425T12 22.5q-.375 0-.737-.15t-.663-.425zm.85-2l2.5 2.5l2.5-2.5H18v-3.5l2.5-2.5L18 9.5V6h-3.5L12 3.5L9.5 6H6v3.5L3.5 12L6 14.5V18zm2.5-6"%2F%3E%3C%2Fsvg%3E');
}
</style>
<h1 id="contexts"><a class="header" href="#contexts">Contexts</a></h1>
<p>Contexts <a name="a001"></a> in Scalability Protocols provide for isolation of protocol-specific
state machines and associated data, allowing multiple <a name="a002"></a>concurrent <a name="a003"></a>state machines (or transactions)
to coexist on a single <a href="../TODO.html">socket</a>.</p>
<p>For example, a <a href="../proto/rep.html">REP</a> server may wish to allow many requests to be serviced concurrently,
even though some jobs may take significant time to process. Contexts provide for this ability.</p>
<p>Not all protocols have contexts, because many protocols simply have no state to manage.
The following protocols support contexts:</p>
<ul>
<li><a href="../proto/rep.html">REP</a></li>
<li><a href="../proto/req.html">REQ</a></li>
<li><a href="../proto/respondent.html">RESPONDENT</a></li>
<li><a href="../proto/surveyor.html">SURVEYOR</a></li>
<li><a href="../proto/sub.html">SUB</a></li>
</ul>
<p>For these protocols, the socket will also have a single, default, context that is used when
performing send or receive operations on the socket directly.</p>
<p>Other protocols are stateless, at least with respect to message processing, and have no use for
contexts. For the same reason, <a href="../api/sock.html#raw-mode-sockets">raw</a> mode sockets do not support contexts.</p>
<div class="mdbook-alerts mdbook-alerts-tip">
<p class="mdbook-alerts-title">
<span class="mdbook-alerts-icon"></span>
tip
</p>
<p>Developers with experience with <a href="https://github.com/nanomsg/nanomsg">libnanomsg</a> may be used to using <a href="../api/sock.html#raw-mode-sockets">raw</a> sockets for concurrency.
Contexts provide a superior solution, as they are much easier to use, less error prone, and allow
for easy control of the amount of concurrency used on any given socket.</p>
</div>
<p>One drawback of contexts is that they cannot be used with file descriptor polling using
<a href="../api/sock.html#polling-socket-events"><code>nng_socket_get_recv_poll_fd</code></a> or <a href="../api/sock.html#polling-socket-events"><code>nng_socket_get_send_poll_fd</code></a>.</p>
<h2 id="context-structure"><a class="header" href="#context-structure">Context Structure</a></h2>
<pre><code class="language-c">#define NNG_CTX_INITIALIZER // opaque value
typedef struct nng_ctx_s nng_ctx;
</code></pre>
<p>The <a name="a004"></a><code>nng_ctx</code> structure represents context. This is a handle, and
the members of it are opaque. However, unlike a pointer, it is passed by value.</p>
<p>A context may be initialized statically with the <a name="a005"></a><code>NNG_CTX_INITIALIZER</code> macro,
to ensure that it cannot be confused with a valid open context.</p>
<h2 id="creating-a-context"><a class="header" href="#creating-a-context">Creating a Context</a></h2>
<pre><code class="language-c">int nng_ctx_open(nng_ctx *ctxp, nng_socket s);
</code></pre>
<p>The <a name="a006"></a><code>nng_ctx_open</code> function creates a separate context to be used with the <a href="../TODO.html">socket</a> <em>s</em>,
and returns it at the location pointed by <em>ctxp</em>.</p>
<h2 id="context-identity"><a class="header" href="#context-identity">Context Identity</a></h2>
<pre><code class="language-c">int nng_ctx_id(nng_ctx c);
</code></pre>
<p>The <a name="a007"></a><code>nng_ctx_id</code> function returns a positive identifier for the context <em>c</em> if it is valid.
Otherwise it returns <code>-1</code>.</p>
<p>A context is considered valid if it was ever opened with <a href="../api/ctx.html#creating-a-context"><code>nng_ctx_open</code></a> function.
Contexts that are allocated on the stack or statically should be initialized with the macro <a href="../api/ctx.html#context-structure"><code>NNG_CTX_INITIALIZER</code></a>
to ensure that they cannot be confused with a valid context before they are opened.</p>
<h2 id="closing-a-context"><a class="header" href="#closing-a-context">Closing a Context</a></h2>
<pre><code class="language-c">int nng_ctx_close(nng_ctx ctx);
</code></pre>
<p>The <a name="a008"></a><code>nng_ctx_close</code> function closes the context <em>ctx</em>.
Messages that have been submitted for sending may be flushed or delivered,
depending upon the transport.</p>
<p>Further attempts to use the context after this call returns will result in <code>NNG_ECLOSED</code>.
Threads waiting for operations on the context when this
call is executed may also return with an <code>NNG_ECLOSED</code> result.</p>
<div class="mdbook-alerts mdbook-alerts-note">
<p class="mdbook-alerts-title">
<span class="mdbook-alerts-icon"></span>
note
</p>
<p>Closing the socket associated with <em>ctx</em> using <a href="../api/sock.html#closing-a-socket"><code>nng_socket_close</code></a> also closes this context.</p>
</div>
<h2 id="sending-messages"><a class="header" href="#sending-messages">Sending Messages</a></h2>
<pre><code class="language-c">int nng_ctx_sendmsg(nng_ctx ctx, nng_msg *msg, int flags);
void nng_ctx_send(nng_ctx ctx, nng_aio *aio);
</code></pre>
<p>These functions (<a name="a009"></a><code>nng_ctx_sendmsg</code> and <a name="a010"></a><code>nng_ctx_send</code>) send
messages over the socket <em>s</em>. The differences in their behaviors are as follows.</p>
<div class="mdbook-alerts mdbook-alerts-note">
<p class="mdbook-alerts-title">
<span class="mdbook-alerts-icon"></span>
note
</p>
<p>The semantics of what sending a message means varies from protocol to
protocol, so examination of the protocol documentation is encouraged.
Additionally, some protocols may not support sending at all or may require other pre-conditions first.
(For example, <a href="../proto/rep.html">REP</a> sockets cannot normally send data until they have first received a request,
while <a href="../proto/sub.html">SUB</a> sockets can only receive data and never send it.)</p>
</div>
<h3 id="nng_ctx_sendmsg"><a class="header" href="#nng_ctx_sendmsg">nng_ctx_sendmsg</a></h3>
<p>The <code>nng_ctx_sendmsg</code> function sends the <em>msg</em> over the context <em>ctx</em>.</p>
<p>If this function returns zero, then the socket will dispose of <em>msg</em> when the transmission is complete.
If the function returns a non-zero status, then the call retains the responsibility for disposing of <em>msg</em>.</p>
<p>The <em>flags</em> can contain the value <a href="../TODO.html"><code>NNG_FLAG_NONBLOCK</code></a>, indicating that the function should not wait if the socket
cannot accept more data for sending. In such a case, it will return <a href="../api/errors.html#NNG_EAGAIN"><code>NNG_EAGAIN</code></a>.</p>
<h3 id="nng_ctx_send"><a class="header" href="#nng_ctx_send">nng_ctx_send</a></h3>
<p>The <code>nng_ctx_send</code> function sends a message asynchronously, using the <a href="../api/aio.html#asynchronous-io-handle"><code>nng_aio</code></a> <em>aio</em>, over the context <em>ctx</em>.
The message to send must have been set on <em>aio</em> using the <a href="../api/aio.html#messages"><code>nng_aio_set_msg</code></a> function.</p>
<p>If the operation completes successfully, then the context will have disposed of the message.
However, if it fails, then callback of <em>aio</em> should arrange for a final disposition of the message.
(The message can be retrieved from <em>aio</em> with <a href="../api/aio.html#messages"><code>nng_aio_get_msg</code></a>.)</p>
<p>Note that callback associated with <em>aio</em> may be called <em>before</em> the message is finally delivered to the recipient.
For example, the message may be sitting in queue, or located in TCP buffers, or even in flight.</p>
<div class="mdbook-alerts mdbook-alerts-tip">
<p class="mdbook-alerts-title">
<span class="mdbook-alerts-icon"></span>
tip
</p>
<p>This is the preferred function to use for sending data on a context. While it does require a few extra
steps on the part of the application, the lowest latencies and highest performance will be achieved by using
this function instead of <a href="../api/ctx.html#nng_ctx_sendmsg"><code>nng_ctx_sendmsg</code></a>.</p>
</div>
<h2 id="receiving-messages"><a class="header" href="#receiving-messages">Receiving Messages</a></h2>
<pre><code class="language-c">int nng_ctx_recvmsg(nng_ctx ctx, nng_msg **msgp, int flags);
void nng_ctx_recv(nng_ctx ctx, nng_aio *aio);
</code></pre>
<p>These functions (, <a name="a011"></a><code>nng_ctx_recvmsg</code> and <a name="a012"></a><code>nng_ctx_recv</code>) receive
messages over the context <em>ctx</em>. The differences in their behaviors are as follows.</p>
<div class="mdbook-alerts mdbook-alerts-note">
<p class="mdbook-alerts-title">
<span class="mdbook-alerts-icon"></span>
note
</p>
<p>The semantics of what receiving a message means varies from protocol to
protocol, so examination of the protocol documentation is encouraged.
Additionally, some protocols may not support receiving at all or may require other pre-conditions first.
(For example, <a href="../proto/req.html">REQ</a> sockets cannot normally receive data until they have first sent a request.)</p>
</div>
<h3 id="nng_recvmsg"><a class="header" href="#nng_recvmsg">nng_recvmsg</a></h3>
<p>The <code>nng_ctx_recvmsg</code> function receives a message and stores a pointer to the <a href="../api/msg.html#message-structure"><code>nng_msg</code></a> for that message in <em>msgp</em>.</p>
<p>The <em>flags</em> can contain the value <a href="../TODO.html"><code>NNG_FLAG_NONBLOCK</code></a>, indicating that the function should not wait if the socket
has no messages available to receive. In such a case, it will return <a href="../api/errors.html#NNG_EAGAIN"><code>NNG_EAGAIN</code></a>.</p>
<h3 id="nng_socket_recv"><a class="header" href="#nng_socket_recv">nng_socket_recv</a></h3>
<p>The <code>nng_ctx_send</code> function receives a message asynchronously, using the <a href="../api/aio.html#asynchronous-io-handle"><code>nng_aio</code></a> <em>aio</em>, over the context <em>ctx</em>.
On success, the received message can be retrieved from the <em>aio</em> using the <a href="../api/aio.html#messages"><code>nng_aio_get_msg</code></a> function.</p>
<div class="mdbook-alerts mdbook-alerts-note">
<p class="mdbook-alerts-title">
<span class="mdbook-alerts-icon"></span>
note
</p>
<p>It is important that the application retrieves the message, and disposes of it accordingly.
Failure to do so will leak the memory.</p>
</div>
<div class="mdbook-alerts mdbook-alerts-tip">
<p class="mdbook-alerts-title">
<span class="mdbook-alerts-icon"></span>
tip
</p>
<p>This is the preferred function to use for receiving data on a context. While it does require a few extra
steps on the part of the application, the lowest latencies and highest performance will be achieved by using
this function instead of <a href="../api/ctx.html#nng_ctx_recvmsg"><code>nng_ctx_recvmsg</code></a>.</p>
</div>
<h2 id="context-options"><a class="header" href="#context-options">Context Options</a></h2>
<pre><code class="language-c">int nng_ctx_get_bool(nng_ctx ctx, const char *opt, bool *valp);
int nng_ctx_get_int(nng_ctx ctx, const char *opt, int *valp);
int nng_ctx_get_ms(nng_ctx ctx, const char *opt, nng_duration *valp);
int nng_ctx_get_size(nng_ctx ctx, const char *opt, size_t *valp);
int nng_ctx_set_bool(nng_ctx ctx, const char *opt, int val);
int nng_ctx_set_int(nng_ctx ctx, const char *opt, int val);
int nng_ctx_set_ms(nng_ctx ctx, const char *opt, nng_duration val);
int nng_ctx_set_size(nng_ctx ctx, const char *opt, size_t val);
</code></pre>
<p>Some protocols support certain options that affect the behavior of a specific context.
For example, most protocols will let you set the defaults timeouts associated with
send or receive separately for different contexts.</p>
<p>These functions are used to retrieve or change the value of an option named <em>opt</em> from the context <em>ctx</em>.
The <code>nng_ctx_get_</code> functions retrieve the value from <em>ctx</em>, and store it in the location <em>valp</em> references.
The <code>nng_ctx_set_</code> functions change the value for the <em>ctx</em>, taking it from <em>val</em>.</p>
<p>These functions access an option as a specific type. The protocol documentation will have details about which options
are available for contexts, whether they can be read or written, and the appropriate type to use.</p>
<h2 id="examples"><a class="header" href="#examples">Examples</a></h2>
<p>These examples show building blocks for a concurrent service based on contexts.</p>
<h3 id="example-1-context-echo-server"><a class="header" href="#example-1-context-echo-server">Example 1: Context Echo Server</a></h3>
<p>The following program fragment demonstrates the use of contexts to implement
a concurrent <a href="../proto/rep.html">REP</a> service that simply echos messages back
to the sender.</p>
<pre><code class="language-c">struct echo_context {
nng_ctx ctx;
nng_aio *aio;
enum { INIT, RECV, SEND } state;
};
void
echo(void *arg)
{
struct echo_context *ec = arg;
switch (ec->state) {
case INIT:
ec->state = RECV;
nng_ctx_recv(ec->ctx, ec->aio);
return;
case RECV:
if (nng_aio_result(ec->aio) != 0) {
// ... handle error
}
// We reuse the message on the ec->aio
ec->state = SEND;
nng_ctx_send(ec->ctx, ec->aio);
return;
case SEND:
if (nng_aio_result(ec->aio) != 0) {
// ... handle error
}
ec->state = RECV;
nng_ctx_recv(ec->ctx, ec->aio);
return;
}
}
</code></pre>
<h3 id="example-2-starting-the-echo-service"><a class="header" href="#example-2-starting-the-echo-service">Example 2: Starting the Echo Service</a></h3>
<p>Given the above fragment, the following example shows setting up the service.
It assumes that the <a href="../TODO.html">socket</a> has already been
created and any transports set up as well with functions such as <a href="../TODO.html"><code>nng_dial</code></a> or <a href="../TODO.html"><code>nng_listen</code></a>.</p>
<pre><code class="language-c">
#define CONCURRENCY 1024
static struct echo_context ecs[CONCURRENCY];
void
start_echo_service(nng_socket rep_socket)
{
for (int i = 0; i < CONCURRENCY; i++) {
// error checks elided for clarity
nng_ctx_open(&ecs[i].ctx, rep_socket);
nng_aio_alloc(&ecs[i].aio, echo, ecs+i);
ecs[i].state = INIT;
echo(ecs+i); // start it running
}
}
</code></pre>
<!-- NOTE: This assumes that any page referencing this is located
in a directory two levels down. Meaning ./api/somefile.md or
similar. mdbook cannot accommodate links that are called from
different levels in the hierarchy. -->
<!-- Symbol cross reference -->
<!-- HTTP -->
<!-- HTTP Status -->
<!-- TLS -->
<!-- Macros -->
<!-- Protocols -->
<!-- Transports -->
<!-- Concept index -->
</main>
<nav class="nav-wrapper" aria-label="Page navigation">
<!-- Mobile navigation buttons -->
<a rel="prev" href="../api/sock.html" class="mobile-nav-chapters previous" title="Previous chapter" aria-label="Previous chapter" aria-keyshortcuts="Left">
<i class="fa fa-angle-left"></i>
</a>
<a rel="next prefetch" href="../api/pipe.html" class="mobile-nav-chapters next" title="Next chapter" aria-label="Next chapter" aria-keyshortcuts="Right">
<i class="fa fa-angle-right"></i>
</a>
<div style="clear: both"></div>
</nav>
</div>
</div>
<nav class="nav-wide-wrapper" aria-label="Page navigation">
<a rel="prev" href="../api/sock.html" class="nav-chapters previous" title="Previous chapter" aria-label="Previous chapter" aria-keyshortcuts="Left">
<i class="fa fa-angle-left"></i>
</a>
<a rel="next prefetch" href="../api/pipe.html" class="nav-chapters next" title="Next chapter" aria-label="Next chapter" aria-keyshortcuts="Right">
<i class="fa fa-angle-right"></i>
</a>
</nav>
</div>
<script>
window.playground_copyable = true;
</script>
<script src="../elasticlunr.min.js"></script>
<script src="../mark.min.js"></script>
<script src="../searcher.js"></script>
<script src="../clipboard.min.js"></script>
<script src="../highlight.js"></script>
<script src="../book.js"></script>
<!-- Custom JS scripts -->
<script src="../theme/pagetoc.js"></script>
</div>
</body>
</html>
|
