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 queue

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 *msg_ptr,
                          size_t msg_len, unsigned int *msg_prio,
                          const struct timespec *abs_timeout);

       Link with -lrt.

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

           _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

       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 structure:

           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 signal(7).

       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

              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 5.09 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                            2020-08-13                    MQ_RECEIVE(3)

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