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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"> |
| — 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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