MQ_RECEIVE

Section: Linux Programmer's Manual (3)
Updated: 2008-09-29
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NAME

mq_receive, mq_timedreceive - receive a message from a message queue

SYNOPSIS

#include <mqueue.h>

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

#define _XOPEN_SOURCE 600
#include <time.h>
#include <mqueue.h>

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

Link with -lrt.

DESCRIPTION

mq_receive() removes the oldest message with the highest priority from the message queue referred to by the 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 the mq_msgsize attribute of the queue (see mq_getattr(3)). If 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 a ceiling on the time for which the call will block. This ceiling is an absolute timeout in seconds and nanoseconds since the Epoch (midnight on the morning of 1 January 1970), 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

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.
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.

CONFORMING TO

POSIX.1-2001.

NOTES

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

COLOPHON

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