## NAME

mq_send - send a message to a message queue

## SYNOPSIS

#include <mqueue.h>

int mq_send(mqd_t mqdes , const char *msg_ptr,
  size_t msg_len, unsigned int msg_prio );
struct mq_attr {
  long mq_flags; /* message queue flags */
  long mq_maxmsg; /* maximum number of messages */
  long mq_msgsize; /* maximum message size */
  long mq_curmsgs; /* number of messages currently queued */
  ...
};

## DESCRIPTION

mq_send() adds the message pointed to by msg_ptr to the message queue specified by mqdes. msg_len specifies the length of the message in bytes pointed to by msg_ptr . The value of msg_len must be less than or equal to the mq_msgsize attribute of the message queue, or mq_send() will fail.

If the specified message queue is not full, mq_send() behaves as if the message is inserted into the message queue at the position indicated by msg_prio . A message with a larger numeric value of msg_prio is inserted before messages with lower values of msg_prio . A message is inserted after other messages in the queue, if any, with equal msg_prio priority. The value of msg_prio must be greater than 0 , and less than or equal to {MQ_PRIO_MAX}.

If the specified message queue is full and if O_NONBLOCK is not set in the message queue description associated with mqdes (see mq_open() and mq_setattr()), mq_send() blocks, waiting until space becomes available to enqueue the message. If more than one thread is waiting to send when space becomes available in the message queue, then the thread of the highest priority which has been waiting the longest is unblocked to send its message. If the specified message queue is full and O_NONBLOCK is set in the message queue description associated with mqdes , the message is not queued, and mq_send() returns an error.

## RETURN VALUES

Upon successful completion, mq_send() returns a value of 0 ; otherwise, no message is enqueued, the function returns -1 , and sets errno to indicate the error condition.

## ERRORS

EAGAIN
  The O_NONBLOCK flag is set in the message queue description associated with mqdes , and the specified message queue is full.
EBADF
  mqdes is not a valid message queue descriptor open for writing.
EINTR
  A signal interrupted the call to mq_send()
EMSGSIZE
  The specified message length, msg_len , exceeds the message size attribute of the message queue.

## SEE ALSO

mq_open(), mq_receive(), mq_setattr(), mq_timedsend()

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