NAME

       mq_receive, mq_timedreceive - receive a message from a message queue

LIBRARY

       Real-time library (librt, -lrt)

SYNOPSIS

       #include <mqueue.h>

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

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

       mq_timedreceive():
           _POSIX_C_SOURCE >= 200112L

DESCRIPTION

       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 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

       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

       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.

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

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

ATTRIBUTES

       For an explanation of the terms used in this section, see attributes(7).

       ┌───────────────────────────────────────────────────────────────┬───────────────┬─────────┐
       │Interface                                                      │ Attribute     │ Value   │
       ├───────────────────────────────────────────────────────────────┼───────────────┼─────────┤
       │mq_receive(), mq_timedreceive()                                │ Thread safety │ MT-Safe │
       └───────────────────────────────────────────────────────────────┴───────────────┴─────────┘

STANDARDS

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

NOTES

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

SEE ALSO

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