Receives a message from a message queue (REALTIME).
#include <mqueue.h>
ssize_t mq_receive(mqd_t mqdes, char *msg_ptr,
size_t msg_len, unsigned *msg_prio,
#include <mqueue.h>
#include <time.h>
ssize_t mq_timedreceive(mqd_t mqdes, char *restrict msg_ptr,
size_t msg_len, unsigned *restrict msg_prio,
const struct timespec *restrict abs_timeout); The mq_receive() function receives the oldest of the highest priority messages from the message queue specified by mqdes. If the size of the buffer, in bytes, specified by the msg_len argument is less than the mq_msgsize attribute of the message queue, the function fails and returns an error. Otherwise, the selected message is removed from the queue and copied to the buffer pointed to by the msg_ptr argument.
If the value of msg_len is greater than {SSIZE_MAX}, the result is implementation-defined.
If the msg_prio argument is not NULL, the priority of the selected message is stored in the location referenced by msg_prio.
If the specified message queue is empty and O_NONBLOCK is not set in the message queue description associated with mqdes, mq_receive() blocks until a message is enqueued on the message queue or until mq_receive() is interrupted by a signal. If more than one thread is waiting to receive a message when a message arrives at an empty queue and the Priority Scheduling option is supported, then the thread of highest priority that has been waiting the longest is selected to receive the message. Otherwise, it is unspecified which waiting thread receives the message. If the specified message queue is empty and O_NONBLOCK is set in the message queue description associated with mqdes, no message is removed from the queue, and mq_receive() returns an error.
The mq_timedreceive() function receives the oldest of the highest priority messages from the message queue specified by mqdes as described for the mq_receive() function. However, if O_NONBLOCK was not specified when the message queue was opened by the mq_open() function, and no message exists on the queue to satisfy the receive, the wait for such a message is terminated when the specified timeout expires. If O_NONBLOCK is set, this function matches mq_receive().
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 resolution of the timeout matches the resolution of the clock on which it is based. The timespec argument is defined in the <time.h> header.
The operation never fails with a timeout if a message can be removed from the message queue immediately. The validity of the abs_timeout parameter does not need to be checked if a message can be removed from the message queue immediately.
Upon successful completion, the mq_receive() and mq_timedreceive() functions return the length of the selected message in bytes and the message is removed from the queue. Otherwise, no message shall be removed from the queue, the functions return a value of -1, and errno is set to indicate the error.
The mq_receive() and mq_timedreceive() functions fail if:
| Item | Description |
|---|---|
| [EAGAIN] | O_NONBLOCK was set in the message description associated with mqdes, and the specified message queue is empty. |
| [EBADF] | The mqdes argument is not a valid message queue descriptor open for reading. |
| [EFAULT] | abs_timeout references invalid memory. |
| [EIDRM] | Specified message queue was removed during required operation. |
| [EINTR] | The mq_receive() or mq_timedreceive() operation was interrupted by a signal. |
| [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. |
| [EINVAL] | msg_ptr value was null. |
| [EMSGSIZE] | The specified message buffer size, msg_len, is less than 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 no message arrived on the queue before the specified timeout expired. |
The mq_receive() and mq_timedreceive() functions might fail if:
| Item | Description |
|---|---|
| [EBADMSG] | The implementation has detected a data corruption problem with the message. |