mq_receive(3) — Linux manual page


MQ_RECEIVE(3)           Linux Programmer's Manual          MQ_RECEIVE(3)

NAME         top

       mq_receive, mq_timedreceive - receive a message from a message

SYNOPSIS         top

       #include <mqueue.h>

       ssize_t mq_receive(mqd_t mqdes, char *msg_ptr,
                          size_t msg_len, unsigned int *msg_prio);

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

       ssize_t mq_timedreceive(mqd_t mqdes, char *restrict msg_ptr,
                          size_t msg_len, unsigned int *restrict msg_prio,
                          const struct timespec *restrict abs_timeout);

       Link with -lrt.

   Feature Test Macro Requirements for glibc (see

           _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
       msg_ptr.  The msg_len argument specifies the size of the buffer
       pointed to by msg_ptr; this must be greater than or equal to the
       mq_msgsize attribute of the queue (see mq_getattr(3)).  If
       msg_prio 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 abs_timeout 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 the following

           struct timespec {
               time_t tv_sec;        /* seconds */
               long   tv_nsec;       /* nanoseconds */

       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 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

       EINVAL The call would have blocked, and abs_timeout was invalid,
              either because tv_sec was less than zero, or because
              tv_nsec was less than zero or greater than 1000 million.

              msg_len was less than the mq_msgsize attribute of the
              message queue.

              The call timed out before a message could be transferred.

ATTRIBUTES         top

       For an explanation of the terms used in this section, see

       │Interface                             Attribute     Value   │
       │mq_receive(), mq_timedreceive()       │ Thread safety │ MT-Safe │

CONFORMING TO         top

       POSIX.1-2001, POSIX.1-2008.

NOTES         top

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

SEE ALSO         top

       mq_close(3), mq_getattr(3), mq_notify(3), mq_open(3), mq_send(3),
       mq_unlink(3), mq_overview(7), time(7)

COLOPHON         top

       This page is part of release 5.13 of the Linux man-pages project.
       A description of the project, information about reporting bugs,
       and the latest version of this page, can be found at

Linux                          2021-03-22                  MQ_RECEIVE(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)