= zmq_recv(3)


== NAME
zmq_recv - receive a message part from a socket


== SYNOPSIS
*int zmq_recv (void '*socket', void '*buf', size_t 'len', int 'flags');*


== DESCRIPTION
The _zmq_recv()_ function shall receive a message from the socket referenced
by the 'socket' argument and store it in the buffer referenced by the 'buf'
argument. Any bytes exceeding the length specified by the 'len' argument shall
be truncated. If there are no messages available on the specified 'socket'
the _zmq_recv()_ function shall block until the request can be satisfied.
The 'flags' argument is a combination of the flags defined below: The 'buf'
argument may be null if len is zero.

*ZMQ_DONTWAIT*::
Specifies that the operation should be performed in non-blocking mode. If there
are no messages available on the specified 'socket', the _zmq_recv()_
function shall fail with 'errno' set to EAGAIN.


Multi-part messages
~~~~~~~~~~~~~~~~~~~
A 0MQ message is composed of 1 or more message parts. 0MQ ensures atomic
delivery of messages: peers shall receive either all _message parts_ of a
message or none at all. The total number of message parts is unlimited except
by available memory.

An application that processes multi-part messages must use the _ZMQ_RCVMORE_
xref:zmq_getsockopt.adoc[zmq_getsockopt] option after calling _zmq_recv()_ to determine if
there are further parts to receive.

== RETURN VALUE
The _zmq_recv()_ function shall return number of bytes in the message
if successful. Note that the value can exceed the value of the 'len' parameter
in case the message was truncated. If not successful the function shall return
`-1` and set 'errno' to one of the values defined below.


== ERRORS
*EAGAIN*::
Either the timeout set via the socket-option ZMQ_RCVTIMEO (see xref:zmq_setsockopt.adoc[zmq_setsockopt])
has been reached (flag ZMQ_DONTWAIT not set) without being able to read a message
from the socket or there are no messages available at the moment (flag ZMQ_DONTWAIT set)
and the operation would block.
*ENOTSUP*::
The _zmq_recv()_ operation is not supported by this socket type.
*EFSM*::
The _zmq_recv()_ operation cannot be performed on this socket at the moment
due to the socket not being in the appropriate state.  This error may occur with
socket types that switch between several states, such as ZMQ_REP.  See the
_messaging patterns_ section of xref:zmq_socket.adoc[zmq_socket] for more information.
*ETERM*::
The 0MQ 'context' associated with the specified 'socket' was terminated.
*ENOTSOCK*::
The provided 'socket' was invalid.
*EINTR*::
The operation was interrupted by delivery of a signal before a message was
available.


== EXAMPLE
.Receiving a message from a socket
----
char buf [256];
nbytes = zmq_recv (socket, buf, 256, 0);
assert (nbytes != -1);
----


== SEE ALSO
* xref:zmq_send.adoc[zmq_send]
* xref:zmq_getsockopt.adoc[zmq_getsockopt]
* xref:zmq_setsockopt.adoc[zmq_setsockopt]
* xref:zmq_socket.adoc[zmq_socket]
* xref:zmq.adoc[zmq]


== AUTHORS
This page was written by the 0MQ community. To make a change please
read the 0MQ Contribution Policy at <https://zeromq.org/how-to-contribute/>.
