---
name: "xs_recvmsg"
section: "3"
platform: "linux"
package: "libxs-dev"
version: "1.2.0-2"
release: "Debian 10 (Buster)"
platforms: ["linux"]
url: "https://new.carta.tech/xs_recvmsg"
---

# xs\_recvmsg(3)

receive a message part from a socket (zero-copy)

## NAME

xs\_recvmsg - receive a message part from a socket (zero-copy)

## SYNOPSIS

**int xs\_recvmsg (void** *\*socket***, xs\_msg\_t** *\*msg***, int** *flags***);**

## DESCRIPTION

The *xs\_recvmsg()* function shall receive a message part from the socket referenced by the *socket* argument and store it in the message referenced by the *msg* argument. Any content previously stored in *msg* shall be properly deallocated. If there are no message parts available on the specified *socket* the *xs\_recvmsg()* function shall block until the request can be satisfied. The *flags* argument is a combination of the flags defined below:

**XS\_DONTWAIT**

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

### Multi-part messages

A Crossroads message is composed of 1 or more message parts. Each message part is an independent *xs\_msg\_t* in its own right. Crossroads 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 multipart messages must use the *XS\_RCVMORE* [**xs\_getsockopt**(3)](/xs_getsockopt/3.md) option after calling *xs\_recvmsg()* to determine if there are further parts to receive.

## RETURN VALUE

The *xs\_recvmsg()* function shall return the number of bytes in the received message if successful. Otherwise it shall return -1 and set *errno* to one of the values defined below.

## ERRORS

**EAGAIN**

Non-blocking mode was requested and no messages are available at the moment.

**ENOTSUP**

The *xs\_recvmsg()* operation is not supported by this socket type.

**EFSM**

The *xs\_recvmsg()* 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 XS\_REP. See the *messaging patterns* section of [**xs\_socket**(3)](/xs_socket/3.md) for more information.

**ETERM**

The *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.

**EFAULT**

The message passed to the function was invalid.

**ETIMEDOUT**

Survey has timed out. (Applicable only to XS\_SURVEYOR socket.)

## EXAMPLE

**Receiving a message from a socket**.

```
/* Create an empty message */
xs_msg_t msg;
int rc = xs_msg_init (&msg);
assert (rc == 0);
/* Block until a message is available to be received from socket */
rc = xs_recvmsg (socket, &msg, 0);
assert (rc != -1);
/* Release message */
xs_msg_close (&msg);
```

**Receiving a multi-part message**.

```
int64_t more;
size_t more_size = sizeof more;
do {
    /* Create an empty message to hold the message part */
    xs_msg_t part;
    int rc = xs_msg_init (&part);
    assert (rc == 0);
    /* Block until a message is available to be received from socket */
    rc = xs_recvmsg (socket, &part, 0);
    assert (rc != -1);
    /* Determine if more message parts are to follow */
    rc = xs_getsockopt (socket, XS_RCVMORE, &more, &more_size);
    assert (rc == 0);
    xs_msg_close (&part);
} while (more);
```

## SEE ALSO

Applications that do not require zero-copy messaging can use the simpler [**xs\_recv**(3)](/xs_recv/3.md) instead of *xs\_recvmsg()*.

[**xs\_sendmsg**(3)](/xs_sendmsg/3.md) [**xs\_getsockopt**(3)](/xs_getsockopt/3.md) **xs\_socket**(7) [**xs**(7)](/xs/7.md)

## AUTHORS

This man page was written by Martin Sustrik \<**sustrik@250bpm.com**\[1\]\>, Martin Lucina \<**martin@lucina.net**\[2\]\> and Pieter Hintjens \<**ph@imatix.com**\[3\]\>.

## NOTES

- 1\.

  sustrik@250bpm.com

  mailto:sustrik@250bpm.com

- 2\.

  martin@lucina.net

  mailto:martin@lucina.net

- 3\.

  ph@imatix.com

  mailto:ph@imatix.com