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<h1 id="pipes"><a class="header" href="#pipes">Pipes</a></h1>
<pre><code class="language-c">typedef struct nng_pipe_s nng_pipe;
</code></pre>
<p>An <a name="a001"></a><code>nng_pipe</code> is a handle to a <a name="a002"></a>pipe object, which can be thought of as a single <a name="a003"></a>connection.
(In most cases this is actually the case – the pipe is an abstraction for a single TCP or IPC connection.)
Pipes are associated with either the listener or dialer that created them,
and therefore are also automatically associated with a single socket.</p>
<div class="mdbook-alerts mdbook-alerts-note">
<p class="mdbook-alerts-title">
<span class="mdbook-alerts-icon"></span>
note
</p>
<p>The <code>nng_pipe</code> structure is always passed by value (both
for input parameters and return values), and should be treated opaquely.
Passing structures this way gives the compiler a chance to perform
accurate type checks in functions passing values of this type.</p>
</div>
<div class="mdbook-alerts mdbook-alerts-tip">
<p class="mdbook-alerts-title">
<span class="mdbook-alerts-icon"></span>
tip
</p>
<p>Most applications should never concern themselves with individual pipes.
However it is possible to access a pipe when more information about the
source of a message is needed, or when more control is required over message delivery.</p>
</div>
<p>Pipe objects are created by <a href="../TODO.html">dialers</a> and and <a href="../TODO.html">listeners</a>.</p>
<h2 id="initialization"><a class="header" href="#initialization">Initialization</a></h2>
<p>A pipe may be initialized using the macro <a name="a004"></a><code>NNG_PIPE_INITIALIZER</code>
before it is opened, to prevent confusion with valid open pipes.</p>
<p>For example:</p>
<pre><code class="language-c">nng_pipe p = NNG_PIPE_INITIALIZER;
</code></pre>
<h2 id="pipe-identity"><a class="header" href="#pipe-identity">Pipe Identity</a></h2>
<pre><code class="language-c">int nng_pipe_id(nng_pipe p);
</code></pre>
<p>The <a name="a005"></a><code>nng_pipe_id</code> function returns a positive identifier for the pipe <em>p</em>, if it is valid.
Otherwise it returns <code>-1</code>.</p>
<div class="mdbook-alerts mdbook-alerts-note">
<p class="mdbook-alerts-title">
<span class="mdbook-alerts-icon"></span>
note
</p>
<p>A pipe is considered valid if it was ever created by the socket.
Pipes that are allocated on the stack or statically should be initialized with the macro
<a href="../api/pipe.html#initialization"><code>NNG_PIPE_INITIALIZER</code></a> to ensure that they cannot be confused with a valid pipe.</p>
</div>
<h2 id="closing-a-pipe"><a class="header" href="#closing-a-pipe">Closing a Pipe</a></h2>
<pre><code class="language-c">nng_err nng_pipe_close(nng_pipe p);
</code></pre>
<p>The <a name="a006"></a><code>nng_pipe_close</code> function closes the supplied pipe, <em>p</em>.
Messages that have been submitted for sending may be flushed or delivered,
depending upon the transport.</p>
<p>Further attempts to use the pipe after this call returns will result in <a href="../api/errors.html#NNG_ECLOSED"><code>NNG_ECLOSED</code></a>.</p>
<div class="mdbook-alerts mdbook-alerts-tip">
<p class="mdbook-alerts-title">
<span class="mdbook-alerts-icon"></span>
tip
</p>
<p>Pipes are automatically closed when their creator closes, or when the
remote peer closes the underlying connection.</p>
</div>
<h2 id="pipe-creator"><a class="header" href="#pipe-creator">Pipe Creator</a></h2>
<pre><code class="language-c">nng_dialer nng_pipe_dialer(nng_pipe p);
nng_listener nng_pipe_listener(nng_pipe p);
nng_socket nng_pipe_socket(nng_pipe p);
</code></pre>
<p><a name="a007"></a>
<a name="a008"></a>
<a name="a009"></a>
These functions return the <a href="../TODO.html">socket</a>, <a href="../TODO.html">dialer</a>, or <a href="../TODO.html">listener</a> that created or owns the pipe.</p>
<p>If the pipe was does not have an associated dialer or listener, then the associated will
return [<code>NNG_DIALER_INITIALIZER</code>] or [<code>NNG_LISTENER_INITIALIZER</code>], as appropriate, and
either [<code>nng_dialer_id</code>] or [<code>nng_listener_id</code>] for the returned object will return -1.</p>
<div class="mdbook-alerts mdbook-alerts-note">
<p class="mdbook-alerts-title">
<span class="mdbook-alerts-icon"></span>
note
</p>
<p>The socket, or the endpoint, returned by these functions may be in the process of closing,
and might not be further usable as a result. (Other functions will result in <a href="../api/errors.html#NNG_ECLOSED"><code>NNG_ECLOSED</code></a>.)</p>
</div>
<h2 id="pipe-options"><a class="header" href="#pipe-options">Pipe Options</a></h2>
<pre><code class="language-c">nng_err nng_pipe_get_bool(nng_pipe p, const char *opt, bool *valp);
nng_err nng_pipe_get_int(nng_pipe p, const char *opt, int *valp);
nng_err nng_pipe_get_ms(nng_pipe p, const char *opt, nng_duration *valp);
nng_err nng_pipe_get_size(nng_pipe p, const char *opt, size_t *valp);
nng_err nng_pipe_get_addr(nng_pipe p, const char *opt, nng_sockaddr *valp);
nng_err nng_pipe_get_string(nng_pipe p, const char *opt, const char **valp);
nng_err nng_pipe_get_strcpy(nng_pipe p, const char *opt, char *val, size_t len);
nng_err nng_pipe_get_strdup(nng_pipe p, const char *opt, char **valp);
nng_err nng_pipe_get_strlen(nng_pipe p, const char *opt, size_t *lenp);
</code></pre>
<p><a name="a010"></a>
<a name="a011"></a>
<a name="a012"></a>
<a name="a013"></a>
<a name="a014"></a>
<a name="a015"></a>
<a name="a016"></a>
<a name="a017"></a>
These functions are used to obtain value of an option named <em>opt</em> from the pipe <em>p</em>, and store it in the location
referenced by <em>valp</em>.</p>
<p>These functions access an option as a specific type. The transport layer will have details about which options
are available, and which type they may be accessed using.</p>
<p>In the case of <code>nng_pipe_get_string</code>, the underlying string may only be valid for as long as the pipe is valid.
Thus, this function can only be safely called in a pipe event callback set up with <a href="../api/pipe.html#pipe-notifications"><code>nng_pipe_notify</code></a>.</p>
<p>The <code>nng_pipe_get_strdup</code> function is like <code>nng_pipe_get_string</code>, but makes a copy into a newly allocated buffer, so that the string must be freed by the caller using <a href="../api/memory.html#free-string"><code>nng_strfree</code></a>.</p>
<p>The <code>nng_pipe_get_strcpy</code> function is also like <code>nng_pipe_get_string</code>, but it makes a copy into a buffer
supplied by the caller. The buffer is passed in <em>val</em>, and the size of the buffer is passed in <em>len</em>.
The value of <em>len</em> must be large enough to hold the string and the terminating zero byte.</p>
<p>The <code>nng_pipe_get_strlen</code> function is used to obtain the length of the string. This can be useful
to find the size of the buffer needed by the <code>nng_pipe_get_strcpy</code> function for a property.
Note that like <code>strlen</code>, this size does not account for the zero byte to terminate the string.</p>
<h2 id="pipe-notifications"><a class="header" href="#pipe-notifications">Pipe Notifications</a></h2>
<pre><code class="language-c">typedef enum {
NNG_PIPE_EV_ADD_PRE,
NNG_PIPE_EV_ADD_POST,
NNG_PIPE_EV_REM_POST,
} nng_pipe_ev;
typedef void (*nng_pipe_cb)(nng_pipe, nng_pipe_ev, void *);
nng_err nng_pipe_notify(nng_socket s, nng_pipe_ev ev, nng_pipe_cb cb, void *arg);
</code></pre>
<p>The <a name="a018"></a><code>nng_pipe_notify</code> function registers the callback function <em>cb</em>
to be called whenever the pipe event specified by
<em>ev</em> occurs on the socket <em>s</em>.
The callback <em>cb</em> will be passed <em>arg</em> as its final argument.</p>
<p>A different callback may be supplied for each event.
Each event may have at most one callback registered.
Registering a callback implicitly unregisters any previously registered.</p>
<p>The following pipe events are supported:</p>
<div class="table-wrapper"><table><thead><tr><th>Event</th><th>Description</th></tr></thead><tbody>
<tr><td><a name="a019"></a><code>NNG_PIPE_EV_ADD_PRE</code><a name="NNG_PIPE_EV_ADD_PRE"></a></td><td>This event occurs after a connection and negotiation has completed, but before the pipe is added to the socket. If the pipe is closed (using <a href="../api/pipe.html#closing-a-pipe"><code>nng_pipe_close</code></a>) at this point, the socket will never see the pipe, and no further events will occur for the given pipe.</td></tr>
<tr><td><a name="a020"></a><code>NNG_PIPE_EV_ADD_POST</code><a name="NNG_PIPE_EV_ADD_POST"></a></td><td>This event occurs after the pipe is fully added to the socket. Prior to this time, it is not possible to communicate over the pipe with the socket.</td></tr>
<tr><td><a name="a021"></a><code>NNG_PIPE_EV_REM_POST</code><a name="NNG_PIPE_EV_REM_POST"></a></td><td>This event occurs after the pipe has been removed from the socket. The underlying transport may be closed at this point, and it is not possible communicate using this pipe.</td></tr>
</tbody></table>
</div><div class="mdbook-alerts mdbook-alerts-warning">
<p class="mdbook-alerts-title">
<span class="mdbook-alerts-icon"></span>
warning
</p>
<p>The callback <em>cb</em> function must <em>not</em> attempt to perform any
accesses to the socket, as it is called with a lock on the socket held!
Doing so would thus result in a deadlock.</p>
</div>
<div class="mdbook-alerts mdbook-alerts-tip">
<p class="mdbook-alerts-title">
<span class="mdbook-alerts-icon"></span>
tip
</p>
<p>The callback <em>cb</em> may close a pipe for any reason by simply closing it using <a href="../api/pipe.html#closing-a-pipe"><code>nng_pipe_close</code></a>.
For example, this might be done to prevent an unauthorized peer from connecting to the socket,
if an authorization check made during <code>NNG_PIPE_EV_ADD_PRE</code> fails.</p>
</div>
<div class="mdbook-alerts mdbook-alerts-note">
<p class="mdbook-alerts-title">
<span class="mdbook-alerts-icon"></span>
note
</p>
<p>This function ignores invalid values for <em>ev</em>.</p>
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