mq_receive(3) - Linux manual page (original) (raw)

mqreceive(3) Library Functions Manual mqreceive(3)

NAME top

   mq_receive, mq_timedreceive - receive a message from a message
   queue

LIBRARY top

   Real-time library (_librt_, _-lrt_)

SYNOPSIS top

   **#include <mqueue.h>**

   **ssize_t mq_receive(mqd_t** _mqdes_**, char** _msgptr_**[.**_msglen_**],**
                      **size_t** _msglen_**, unsigned int ***_msgprio_**);**

   **#include <time.h>**
   **#include <mqueue.h>**

   **ssize_t mq_timedreceive(mqd_t** _mqdes_**, char *restrict** _msgptr_**[.**_msglen_**],**
                      **size_t** _msglen_**, unsigned int *restrict** _msgprio_**,**
                      **const struct timespec *restrict** _abstimeout_**);**

Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

   **mq_timedreceive**():
       _POSIX_C_SOURCE >= 200112L

DESCRIPTION top

   **mq_receive**() removes the oldest message with the highest priority
   from the message queue referred to by the message queue descriptor
   _mqdes_, and places it in the buffer pointed to by _msgptr_.  The
   _msglen_ argument specifies the size of the buffer pointed to by
   _msgptr_; this must be greater than or equal to the _mqmsgsize_
   attribute of the queue (see [mq_getattr(3)](../man3/mq%5Fgetattr.3.html)).  If _msgprio_ is not
   NULL, then the buffer to which it points is used to return the
   priority associated with the received message.

   If the queue is empty, then, by default, **mq_receive**() blocks until
   a message becomes available, or the call is interrupted by a
   signal handler.  If the **O_NONBLOCK** flag is enabled for the message
   queue description, then the call instead fails immediately with
   the error **EAGAIN**.

   **mq_timedreceive**() behaves just like **mq_receive**(), except that if
   the queue is empty and the **O_NONBLOCK** flag is not enabled for the
   message queue description, then _abstimeout_ points to a structure
   which specifies how long the call will block.  This value is an
   absolute timeout in seconds and nanoseconds since the Epoch,
   1970-01-01 00:00:00 +0000 (UTC), specified in a **timespec**(3)
   structure.

   If no message is available, and the timeout has already expired by
   the time of the call, **mq_timedreceive**() returns immediately.

RETURN VALUE top

   On success, **mq_receive**() and **mq_timedreceive**() return the number
   of bytes in the received message; on error, -1 is returned, with
   _[errno](../man3/errno.3.html)_ set to indicate the error.

ERRORS top

   **EAGAIN** The queue was empty, and the **O_NONBLOCK** flag was set for
          the message queue description referred to by _mqdes_.

   **EBADF** The descriptor specified in _mqdes_ was invalid or not opened
          for reading.

   **EINTR** The call was interrupted by a signal handler; see
          [signal(7)](../man7/signal.7.html).

   **EINVAL** The call would have blocked, and _abstimeout_ was invalid,
          either because _tvsec_ was less than zero, or because
          _tvnsec_ was less than zero or greater than 1000 million.

   **EMSGSIZE**
          _msglen_ was less than the _mqmsgsize_ attribute of the
          message queue.

   **ETIMEDOUT**
          The call timed out before a message could be transferred.

ATTRIBUTES top

   For an explanation of the terms used in this section, see
   [attributes(7)](../man7/attributes.7.html).
   ┌──────────────────────────────────────┬───────────────┬─────────┐
   │ **Interface** │ **Attribute** │ **Value** │
   ├──────────────────────────────────────┼───────────────┼─────────┤
   │ **mq_receive**(), **mq_timedreceive**()      │ Thread safety │ MT-Safe │
   └──────────────────────────────────────┴───────────────┴─────────┘

VERSIONS top

   On Linux, **mq_timedreceive**() is a system call, and **mq_receive**() is
   a library function layered on top of that system call.

STANDARDS top

   POSIX.1-2008.

HISTORY top

   POSIX.1-2001.

SEE ALSO top

   [mq_close(3)](../man3/mq%5Fclose.3.html), [mq_getattr(3)](../man3/mq%5Fgetattr.3.html), [mq_notify(3)](../man3/mq%5Fnotify.3.html), [mq_open(3)](../man3/mq%5Fopen.3.html), [mq_send(3)](../man3/mq%5Fsend.3.html),
   [mq_unlink(3)](../man3/mq%5Funlink.3.html), **timespec**(3), [mq_overview(7)](../man7/mq%5Foverview.7.html), [time(7)](../man7/time.7.html)

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Linux man-pages 6.10 2024-07-23 mqreceive(3)

Pages that refer to this page:syscalls(2), mq_close(3), mq_getattr(3), mq_notify(3), mq_open(3), mq_send(3), mq_unlink(3), mq_overview(7), signal(7)