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authorgdamore <gdamore@users.noreply.github.com>2024-10-27 18:55:51 +0000
committergdamore <gdamore@users.noreply.github.com>2024-10-27 18:55:51 +0000
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+ <ol class="chapter"><li class="chapter-item expanded affix "><a href="../copyright.html">Copyright</a></li><li class="chapter-item expanded affix "><a href="../dedication.html">Dedication</a></li><li class="chapter-item expanded affix "><a href="../preface.html">Preface</a></li><li class="chapter-item expanded "><a href="../api/index.html"><strong aria-hidden="true">1.</strong> API Reference</a><a class="toggle"><div>❱</div></a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../api/msg.html" class="active"><strong aria-hidden="true">1.1.</strong> Messages</a></li><li class="chapter-item expanded "><a href="../api/memory.html"><strong aria-hidden="true">1.2.</strong> Memory</a></li><li class="chapter-item expanded "><a href="../api/time.html"><strong aria-hidden="true">1.3.</strong> Time</a></li><li class="chapter-item expanded "><a href="../api/url.html"><strong aria-hidden="true">1.4.</strong> URLs</a></li><li class="chapter-item expanded "><a href="../api/aio.html"><strong aria-hidden="true">1.5.</strong> Asynchronous I/O</a></li><li class="chapter-item expanded "><a href="../api/synch.html"><strong aria-hidden="true">1.6.</strong> Synchronization</a></li><li class="chapter-item expanded "><a href="../api/thread.html"><strong aria-hidden="true">1.7.</strong> Threads</a></li><li class="chapter-item expanded "><a href="../api/logging.html"><strong aria-hidden="true">1.8.</strong> Logging</a></li><li class="chapter-item expanded "><a href="../api/stats.html"><strong aria-hidden="true">1.9.</strong> Statistics</a></li><li class="chapter-item expanded "><a href="../api/errors.html"><strong aria-hidden="true">1.10.</strong> Errors</a></li><li class="chapter-item expanded "><a href="../api/misc.html"><strong aria-hidden="true">1.11.</strong> Miscellaneous</a></li><li class="chapter-item expanded "><a href="../api/id_map.html"><strong aria-hidden="true">1.12.</strong> ID Map</a></li><li class="chapter-item expanded "><a href="../api/cmd_opts.html"><strong aria-hidden="true">1.13.</strong> Command Options</a></li></ol></li><li class="chapter-item expanded "><a href="../proto/index.html"><strong aria-hidden="true">2.</strong> Protocols</a><a class="toggle"><div>❱</div></a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../proto/bus.html"><strong aria-hidden="true">2.1.</strong> BUS Protocol</a></li><li class="chapter-item expanded "><a href="../proto/pair.html"><strong aria-hidden="true">2.2.</strong> PAIR Protocol</a></li><li class="chapter-item expanded "><a href="../proto/pub.html"><strong aria-hidden="true">2.3.</strong> PUB Protocol</a></li><li class="chapter-item expanded "><a href="../proto/pull.html"><strong aria-hidden="true">2.4.</strong> PULL Protocol</a></li><li class="chapter-item expanded "><a href="../proto/push.html"><strong aria-hidden="true">2.5.</strong> PUSH Protocol</a></li><li class="chapter-item expanded "><a href="../proto/rep.html"><strong aria-hidden="true">2.6.</strong> REP Protocol</a></li><li class="chapter-item expanded "><a href="../proto/req.html"><strong aria-hidden="true">2.7.</strong> REQ Protocol</a></li><li class="chapter-item expanded "><a href="../proto/respondent.html"><strong aria-hidden="true">2.8.</strong> RESPONDENT Protocol</a></li><li class="chapter-item expanded "><a href="../proto/sub.html"><strong aria-hidden="true">2.9.</strong> SUB Protocol</a></li><li class="chapter-item expanded "><a href="../proto/surveyor.html"><strong aria-hidden="true">2.10.</strong> SURVEYOR Protocol</a></li></ol></li><li class="chapter-item expanded "><a href="../tran/index.html"><strong aria-hidden="true">3.</strong> Transports</a><a class="toggle"><div>❱</div></a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../tran/inproc.html"><strong aria-hidden="true">3.1.</strong> Intra-Process Transport</a></li><li class="chapter-item expanded "><a href="../tran/ipc.html"><strong aria-hidden="true">3.2.</strong> Inter-Process Transport</a></li><li class="chapter-item expanded "><a href="../tran/socket.html"><strong aria-hidden="true">3.3.</strong> BSD Socket (Experimental)</a></li><li class="chapter-item expanded "><a href="../tran/udp.html"><strong aria-hidden="true">3.4.</strong> UDP Transport (Experimental)</a></li></ol></li><li class="chapter-item expanded "><a href="../migrate/index.html"><strong aria-hidden="true">4.</strong> Migration Guides</a><a class="toggle"><div>❱</div></a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../migrate/nng1.html"><strong aria-hidden="true">4.1.</strong> Migrating from NNG 1.x</a></li><li class="chapter-item expanded "><a href="../migrate/nanomsg.html"><strong aria-hidden="true">4.2.</strong> Migrating from libnanomsg</a></li></ol></li><li class="chapter-item expanded "><a href="../indexing.html">Index</a></li></ol>
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+
+</style>
+<h1 id="messages"><a class="header" href="#messages">Messages</a></h1>
+<p>Messages <a name="a001"></a> in Scalability Protocols are the fundamental unit of
+transmission and reception, as these protocols are fundamentally message-oriented.</p>
+<p>Messages have a [body][nng_msg_body]<a name="a002"></a>, containing the application-supplied
+payload, and a [header][nng_msg_header]<a name="a003"></a>, containing protocol specific routing and similar
+related information.</p>
+<div class="mdbook-alerts mdbook-alerts-tip">
+<p class="mdbook-alerts-title">
+ <span class="mdbook-alerts-icon"></span>
+ tip
+</p>
+<p>Only applications using <a href="/TODO.html">raw mode</a> need to access the message header.
+Very few <em>NNG</em> applications do this.</p>
+</div>
+<h2 id="message-structure"><a class="header" href="#message-structure">Message Structure</a></h2>
+<pre><code class="language-c">typedef struct nng_msg nng_msg;
+</code></pre>
+<p>The <a name="a004"></a><code>nng_msg</code> structure represents a single message. It carries a body
+and a header.</p>
+<h3 id="create-a-message"><a class="header" href="#create-a-message">Create a Message</a></h3>
+<pre><code class="language-c">int nng_msg_alloc(nng_msg **msgp, size_t size);
+</code></pre>
+<p>The <a name="a005"></a><code>nng_msg_alloc</code> function allocates a new message.
+It takes a <em>size</em> argument, and returns a message
+with a preallocated body of that size in the <em>msgp</em> parameter.</p>
+<p>If it succeeds, it returns zero, otherwise this function may return <a href="/api/errors.html#NNG_ENOMEM"><code>NNG_ENOMEM</code></a>,
+indicating that insufficient memory is available to allocate a new message.</p>
+<h3 id="destroy-a-message"><a class="header" href="#destroy-a-message">Destroy a Message</a></h3>
+<pre><code class="language-c">void nng_msg_free(nng_msg *msg);
+</code></pre>
+<p>The <a name="a006"></a><code>nng_msg_free</code> function deallocates a message.</p>
+<h3 id="duplicate-a-message"><a class="header" href="#duplicate-a-message">Duplicate a Message</a></h3>
+<pre><code class="language-c">int nng_msg_dup(nng_msg **dup, nng_msg *msg);
+</code></pre>
+<p>The <a name="a007"></a><code>nng_msg_dup</code> function duplicates the message <em>msg</em>, storing a pointer
+to the new duplicate in <em>dup</em>. This function also returns zero on succes, or <a href="/api/errors.html#NNG_ENOMEM"><code>NNG_ENOMEM</code></a>
+if memory is exhausted.</p>
+<h2 id="message-size-and-capacity"><a class="header" href="#message-size-and-capacity">Message Size and Capacity</a></h2>
+<pre><code class="language-c">size_t nng_msg_capacity(nng_msg *msg);
+int nng_msg_realloc(nng_msg *msg, size_t size);
+int nng_msg_reserve(nng_msg *msg, size_t capacity);
+</code></pre>
+<p>Messages have a certain amount of pre-reserved space, which may exceed the total
+size of the message. This allows for content to be added to the message later,
+without necessarily performing a reallocation.</p>
+<p>The <a name="a008"></a><code>nng_msg_capacity</code> function returns the amount of prereserved space.
+If a message size change is required, and the new size will fit within the capacity
+reported by this function, then change will be done without a reallocation, and
+likely without a data copy as well.</p>
+<div class="mdbook-alerts mdbook-alerts-tip">
+<p class="mdbook-alerts-title">
+ <span class="mdbook-alerts-icon"></span>
+ tip
+</p>
+<p>The capacity reported by <code>nng_msg_capacity</code> may not include reserved headroom, which
+is present to allow a very limited amount of content to be inserted in front of the
+message without requiring the rest of the message to be copied.</p>
+</div>
+<p>The message size may be changed by use of the <a name="a009"></a><code>nng_msg_realloc</code> function. This
+function will reallocate the underlying memory for the message <em>msg</em>,
+preserving contents while doing so.
+If the new size is smaller than the original message, it will
+truncate the message, but not perform any allocations.
+If reallocation fails due to insufficient memory, then the original is left intact.</p>
+<p>The <a name="a010"></a><code>nng_msg_reserve</code> function ensures that the total message capacity
+is at least <em>capacity</em> bytes. Use of this function to ensure the total anticipated
+capacity is present in the message may help prevent many small allocations.</p>
+<p>Both <code>nng_msg_realloc</code> and <code>nng_msg_reserve</code> return zero on success, or may return
+<a href="/api/errors.html#NNG_ENOMEM"><code>NNG_ENOMEM</code></a> if insufficient memory exists to preform allocation.</p>
+<div class="mdbook-alerts mdbook-alerts-important">
+<p class="mdbook-alerts-title">
+ <span class="mdbook-alerts-icon"></span>
+ important
+</p>
+<p>Any pointers to message content obtained before a call to <code>nng_msg_realloc</code> or
+<code>nng_msg_reserve</code> (or any other function that changes the message size) should be
+treated as invalid, as the locations pointed to may be deallocated by these functions.</p>
+</div>
+<h2 id="message-body"><a class="header" href="#message-body">Message Body</a></h2>
+<pre><code class="language-c">void *nng_msg_body(nng_msg *msg);
+size_t nng_msg_len(nng_msg *msg);
+</code></pre>
+<p>The body and body length of <em>msg</em> are returned by <a name="a011"></a><code>nng_msg_body</code> and
+<a name="a012"></a><code>nng_msg_len</code>, respectively.</p>
+<h3 id="clear-the-body"><a class="header" href="#clear-the-body">Clear the Body</a></h3>
+<pre><code class="language-c">void *nng_msg_clear(nng_msg *msg);
+</code></pre>
+<p>The <a name="a013"></a><code>nng_msg_clear</code> simply resets the total message body length to zero, but does
+not affect the capacity. It does not change the underlying bytes of the message.</p>
+<h3 id="add-to-body"><a class="header" href="#add-to-body">Add to Body</a></h3>
+<pre><code class="language-c">int nng_msg_append(nng_msg *msg, const void *val, size_t size);
+int nng_msg_append_u16(nng_msg *msg, uint16_t val16);
+int nng_msg_append_u32(nng_msg *msg, uint32_t val32);
+int nng_msg_append_u64(nng_msg *msg, uint64_t val64);
+
+int nng_msg_insert(nng_msg *msg, const void *val, size_t size);
+int nng_msg_insert_u16(nng_msg *msg, uint16_t val16);
+int nng_msg_insert_u32(nng_msg *msg, uint32_t val32);
+int nng_msg_insert_u64(nng_msg *msg, uint64_t val64);
+</code></pre>
+<p>Appending data to a message body is done by using the <a name="a014"></a><code>nng_msg_append</code> functions.
+The base <code>nng_msg_append</code> function appends <em>size</em> bytes of untyped data to the end of the
+message.</p>
+<p>Use of the typed versions, ending in suffixes <code>_u16</code>, <code>_u32</code>, and <code>_u64</code> allows
+for unsigned integers to be appended directly. The integers are encoded in network byte order, with
+the most significant byte appearing first. The message body will by two, four, or eight
+bytes accordingly.</p>
+<p>Data may inserted before the rest of the message body by using the <a name="a015"></a><code>nng_msg_insert</code> functions.
+This will attempt to use “headroom” in the message to avoid a data copy.
+Otherwise they are like the <code>nng_msg_append</code> functions except that the put the data in front
+of the messages instead of at the end.</p>
+<div class="mdbook-alerts mdbook-alerts-tip">
+<p class="mdbook-alerts-title">
+ <span class="mdbook-alerts-icon"></span>
+ tip
+</p>
+<p>Message headroom is limited, so <code>nng_msg_insert</code> is best used sparingly.
+It is much more efficient to build the message content from start to end
+using <code>nng_msg_append</code>.</p>
+</div>
+<h3 id="consume-from-body"><a class="header" href="#consume-from-body">Consume From Body</a></h3>
+<pre><code class="language-c">int nng_msg_chop(nng_msg *msg, size_t size);
+int nng_msg_chop_u16(nng_msg *msg, uint16_t *val16);
+int nng_msg_chop_u32(nng_msg *msg, uint32_t *val32);
+int nng_msg_chop_u64(nng_msg *msg, uint64_t *val64);
+
+int nng_msg_trim(nng_msg *msg, size_t size);
+int nng_msg_trim_u16(nng_msg *msg, uint16_t *val16);
+int nng_msg_trim_u32(nng_msg *msg, uint32_t *val32);
+int nng_msg_trim_u64(nng_msg *msg, uint64_t *val64);
+</code></pre>
+<p>The <a name="a016"></a><code>nng_msg_chop</code> functions remove data from the end of the body of message <em>msg</em>,
+reducing the message length by either <em>size</em>, or the appropriate value size.</p>
+<p>The <a name="a017"></a><code>nng_msg_trim</code> functions remove data from the beginning of the message body of <em>msg</em>.
+but are otherwise just like the <code>nng_msg_chop</code> functions.</p>
+<p>If the message is not big enough to remove requisite amount of bytes, these functions
+return <code>NNG_EINVAL</code>. Otherwise they return zero.</p>
+<p>Additionally, functions with typed suffixes (<code>_u16</code>, <code>_u32</code>, <code>_u64</code>) decode the data and return it
+through the appropriate <em>val</em> pointer.</p>
+<p>The data is assumed to have been in network byte order in the message, but is returned in
+the native machine byte order. The appropriate number of bytes is consumed for each of these types,
+so two bytes for <code>_u16</code>, four bytes for <code>_u32</code>, and eight bytes for <code>_u64</code>.</p>
+<h2 id="message-header"><a class="header" href="#message-header">Message Header</a></h2>
+<pre><code class="language-c">void *nng_msg_header(nng_msg *msg);
+size_t nng_msg_header_len(nng_msg *msg);
+</code></pre>
+<p>The header and header length of <em>msg</em> are returned by <a name="a018"></a><code>nng_msg_header</code> and
+<a name="a019"></a><code>nng_msg_header_len</code>, respectively.</p>
+<p>The message headers are generally intended for limited use, to store protocol headers.</p>
+<div class="mdbook-alerts mdbook-alerts-important">
+<p class="mdbook-alerts-title">
+ <span class="mdbook-alerts-icon"></span>
+ important
+</p>
+<p>The message headers are for protocol and transport headers, and not for general
+application payloads. Misuse of the header may prevent the application from functioning
+properly.</p>
+</div>
+<h3 id="clear-the-header"><a class="header" href="#clear-the-header">Clear the Header</a></h3>
+<pre><code class="language-c">void *nng_msg_header_clear(nng_msg *msg);
+</code></pre>
+<p>The <a name="a020"></a><code>nng_msg_header_clear</code> simply resets the total message header length to zero.</p>
+<h3 id="append-or-insert-header"><a class="header" href="#append-or-insert-header">Append or Insert Header</a></h3>
+<p>Appending data to a message header is done by using the <a name="a021"></a><code>nng_msg_header_append</code> functions,
+and inserting data in the header is done using the <a name="a022"></a><code>nng_msg_header_insert</code> functions.</p>
+<p>These functions act just like the <a href="/api/msg.html#add-to-body"><code>nng_msg_append</code></a> and <a href="/api/msg.html#add-to-body"><code>nng_msg_insert</code></a> functions,
+except that they operate on the message header rather than the message body.</p>
+<h3 id="consume-from-header"><a class="header" href="#consume-from-header">Consume from Header</a></h3>
+<p>The <a name="a023"></a><code>nng_msg_header_trim</code> functions remove data from the beginning of the message header,
+and the <a name="a024"></a><code>nng_msg_header_chop</code> functions remove data from the end of the message header.</p>
+<p>These functions act just like the <a href="/api/msg.html#consume-from-body"><code>nng_msg_trim</code></a> and <a href="/api/msg.html#consume-from-body"><code>nng_msg_chop</code></a> functions,
+except that they operate the message header rather than the message body.</p>
+<h2 id="message-pipe"><a class="header" href="#message-pipe">Message Pipe</a></h2>
+<pre><code class="language-c">nng_pipe nng_msg_get_pipe(nng_msg *msg);
+void nng_msg_get_pipe(nng_msg *msg, nng_pipe p);
+</code></pre>
+<p>The <a name="a025"></a><code>nng_msg_set_pipe</code> function sets the <a href="/TODO.html">pipe</a> associated with <em>msg</em> to <em>p</em>.
+This is most often useful when used with protocols that support directing
+a message to a specific peer.
+For example the <a href="/proto/pair.html"><em>PAIR</em></a> version 1 protocol can do
+this when <code>NNG_OPT_PAIR1_POLY</code> mode is set.</p>
+<p>The <a name="a026"></a><code>nng_msg_get_pipe</code> function returns the pipe that was previously set on the message <em>m</em>,
+either directly by the application, or when the message was received by the protocol.</p>
+<div class="mdbook-alerts mdbook-alerts-note">
+<p class="mdbook-alerts-title">
+ <span class="mdbook-alerts-icon"></span>
+ note
+</p>
+<p>Not all protocols support overriding the destination pipe.</p>
+</div>
+<h2 id="examples"><a class="header" href="#examples">Examples</a></h2>
+<h3 id="example-1-preparing-a-message-for-use"><a class="header" href="#example-1-preparing-a-message-for-use">Example 1: Preparing a message for use</a></h3>
+<pre><code class="language-c"> #include &lt;nng/nng.h&gt;
+
+ nng_msg *m;
+ if (nng_msg_alloc(&amp;m, strlen("content") + 1) != 0) {
+ // handle error
+ }
+ strcpy(nng_msg_body(m), "content");
+</code></pre>
+<h3 id="example-2-preallocating-message-content"><a class="header" href="#example-2-preallocating-message-content">Example 2: Preallocating message content</a></h3>
+<pre><code class="language-c"> if (nng_msg_alloc(&amp;m, 1024) != 0) {
+ // handle error
+ }
+ while ((val64 = next_datum()) != 0) P
+ if (nng_msg_append_u64(m, val64) != 0) {
+ // handle error
+ }
+ }
+</code></pre>
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