mq_send.2 (2010 09)

mq_send(2) mq_send(2)

NAME

mq_send - send a message to a message queue

SYNOPSIS

#include <sys/mqueue.h>

int mq_send(mqd_t mqdes,

const char *msg_ptr,

size_t msg_len,

unsigned int msg_prio

);

DESCRIPTION

The mq_send() system call adds a message pointed to by the argument msg_ptr to the message queue

speciﬁed by mqdes. The msg_len argument speciﬁes the length of the message in bytes. The value of

msg_len should 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() will insert the message into the queue at the posi-

tion indicated by the msg_prio argument. A message with priority, msg_prio , will be inserted behind any

other messages with larger or equal priority. The value of msg_prio should be less than

MQ_PRIO_MAX.

If the speciﬁed message queue is full and the

O_NONBLOCK ﬂag is not set in the message queue blocking

status associated with mqdes , mq_send() will block in priority order, until it can send a message on the

queue, or until mq_send() is interrupted by a signal. If the speciﬁed message queue is full and the

O_NONBLOCK ﬂag is set in the message queue blocking status associated with mqdes, the message will

not be enqueued, and mq_send() will return with an error.

To use this function, link in the realtime library by specifying

-lrt on the compiler or linker command

line.

RETURN VALUE

mq_send() returns the following values:

0 Successful completion. The message is enqueued.

-1 Failure. errno is set to indicate the error and the message is not enqueued.

ERRORS

mq_send() fails, errno is set to one of the following values:

[EAGAIN] The

O_NONBLOCK ﬂag is set in the message queue blocking status associated with

mqdes, and the 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().

[EINVAL] msg_ptr points to an invalid address, or the value of msg_prio is outside the valid

range.

[EMSGSIZE] The speciﬁed message length, msg_len , exceeds the message size attribute of the

message queue.

[ENOSYS] mq_send() is not supported by the implementation.