Sends a message to a message queue (REALTIME).
#include <mqueue.h>
int mq_send(mqd_t mqdes, const char *msg_ptr,
size_t msg_len, unsigned *msg_prio,
#include <mqueue.h>
#include <time.h>
int mq_timedsend(mqd_t mqdes, const char *msg_ptr,
size_t msg_len, unsigned msg_prio,
const struct timespec *abs_timeout); The mq_send() function adds the 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, pointed to by msg_ptr. The value of msg_len is less than or equal to the mq_msgsize attribute of the message queue, or mq_send() fails.
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 the msg_prio argument. 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 values. The value of msg_prio is less than {MQ_PRIO_MAX}.
If the specified message queue is full and O_NONBLOCK is not set in the message queue description associated with mqdes, mq_send() blocks until space becomes available to enqueue the message, or until mq_send() is interrupted by a signal. If more than one thread is waiting to send when space becomes available in the message queue and the Priority Scheduling option is supported, then the thread of the highest priority that has been waiting the longest is unblocked to send its message. Otherwise, it is unspecified which waiting thread is unblocked. 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.
The mq_timedsend() function adds a message to the message queue specified by mqdes in the manner defined for the mq_send() function. However, if the specified message queue is full and O_NONBLOCK is not set in the message queue description associated with mqdes, the wait for sufficient room in the queue is terminated when the specified timeout expires. If O_NONBLOCK is set in the message queue description, this function matches mq_send().
The timeout expires when the absolute time specified by abs_timeout passes—as measured by the clock on which timeouts are based (that is, when the value of that clock equals or exceeds abs_timeout)—or when the absolute time specified by abs_timeout has already been passed at the time of the call.
If the Timers option is supported, the timeout is based on the CLOCK_REALTIME clock; if the Timers option is not supported, the timeout is based on the system clock as returned by the time() function.
The operation never fails with a timeout if there is sufficient room in the queue to add the message immediately. The validity of the abs_timeout parameter does not need to be checked when there is sufficient room in the queue.
The value of the symbol {MQ_PRIO_MAX} limits the number of priority levels supported by the application. Message priorities range from 0 to {MQ_PRIO_MAX}-1.
Upon successful completion, the mq_send() and mq_timedsend() functions return a value of 0. Otherwise, no message is enqueued, the functions return -1, and errno is set to indicate the error.
The mq_send() and mq_timedsend() functions fail if:
| Item | Description |
|---|---|
| [EAGAIN] | The O_NONBLOCK flag is set in the message queue description associated with mqdes, and the specified message queue is full. |
| [EBADF] | The mqdes argument is not a valid message queue descriptor open for writing. |
| [EFAULT] | abs_timeout references invalid memory. |
| [EIDRM] | Specified message queue was removed during required operation. |
| [EINTR] | A signal interrupted the call to mq_send() or mq_timedsend(). |
| [EINVAL] | The value of msg_prio was outside the valid range. |
| [EINVAL] | msg_ptr value was null. |
| [EINVAL] | The process or thread would have blocked, and the abs_timeout parameter specified a nanoseconds field value less than 0 or greater than or equal to 1000 million. |
| [EMSGSIZE] | The specified message length, msg_len, exceeds the message size attribute of the message queue. |
| [ENOTSUP] | Function is not supported with checkpoint-restart'ed processes. |
| [ETIMEDOUT] | The O_NONBLOCK flag was not set when the message queue was opened, but the timeout expired before the message could be added to the queue. |
The mq_send() and mq_timedsend() functions might fail if:
| Item | Description |
|---|---|
| [EBADMSG] | The implementation has detected a data corruption problem with the message. |