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<h4 class="subsection">16.10.2 Receiving Datagrams</h4>

<p><a name="index-receiving-datagrams-1823"></a>
The <code>recvfrom</code> function reads a packet from a datagram socket and
also tells you where it was sent from.  This function is declared in
<samp><span class="file">sys/socket.h</span></samp>.

<!-- sys/socket.h -->
<!-- BSD -->
<div class="defun">
&mdash; Function: int <b>recvfrom</b> (<var>int socket, void *buffer, size_t size, int flags, struct sockaddr *addr, socklen_t *length-ptr</var>)<var><a name="index-recvfrom-1824"></a></var><br>
<blockquote><p>The <code>recvfrom</code> function reads one packet from the socket
<var>socket</var> into the buffer <var>buffer</var>.  The <var>size</var> argument
specifies the maximum number of bytes to be read.

        <p>If the packet is longer than <var>size</var> bytes, then you get the first
<var>size</var> bytes of the packet and the rest of the packet is lost. 
There's no way to read the rest of the packet.  Thus, when you use a
packet protocol, you must always know how long a packet to expect.

        <p>The <var>addr</var> and <var>length-ptr</var> arguments are used to return the
address where the packet came from.  See <a href="Socket-Addresses.html#Socket-Addresses">Socket Addresses</a>.  For a
socket in the local domain the address information won't be meaningful,
since you can't read the address of such a socket (see <a href="Local-Namespace.html#Local-Namespace">Local Namespace</a>).  You can specify a null pointer as the <var>addr</var> argument
if you are not interested in this information.

        <p>The <var>flags</var> are interpreted the same way as for <code>recv</code>
(see <a href="Socket-Data-Options.html#Socket-Data-Options">Socket Data Options</a>).  The return value and error conditions
are also the same as for <code>recv</code>.

        <p>This function is defined as a cancellation point in multi-threaded
programs, so one has to be prepared for this and make sure that
allocated resources (like memory, files descriptors, semaphores or
whatever) are freed even if the thread is canceled. 
<!-- @xref{pthread_cleanup_push}, for a method how to do this. -->
</p></blockquote></div>

   <p>You can use plain <code>recv</code> (see <a href="Receiving-Data.html#Receiving-Data">Receiving Data</a>) instead of
<code>recvfrom</code> if you don't need to find out who sent the packet
(either because you know where it should come from or because you
treat all possible senders alike).  Even <code>read</code> can be used if
you don't want to specify <var>flags</var> (see <a href="I_002fO-Primitives.html#I_002fO-Primitives">I/O Primitives</a>).

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