The Open Group Base Specifications Issue 6
IEEE Std 1003.1, 2004 Edition
Copyright © 2001-2004 The IEEE and The Open Group, All Rights reserved.

#### NAME

> mq_receive, mq_timedreceive - receive a message from a message queue (REALTIME)

#### SYNOPSIS

> ``[MSG] #include <mqueue.h>
> ssize_t mq_receive(mqd_t` mqdes`, char *`msg_ptr`, size_t` msg_len`,
> unsigned *`msg_prio`);``
> `
> `
> ``[MSG TMO]
> #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`);``
> `
> `

#### DESCRIPTION

> The mq_receive() function shall receive the oldest of the highest priority message(s) 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 shall fail and return an error. Otherwise, the selected message shall be 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 argument msg_prio is not NULL, the priority of the selected message shall be 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() shall block 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 shall be 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 shall be removed from the queue, and mq_receive() shall return an error.

> [TMO] The mq_timedreceive() function shall receive 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 via the mq_open() function, and no message exists on the queue to satisfy the receive, the wait for such a message shall be terminated when the specified timeout expires. If O_NONBLOCK is set, this function is equivalent to 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 if the absolute time specified by abs_timeout has already been passed at the time of the call.

> [TMO TMR] If the Timers option is supported, the timeout shall be based on the CLOCK_REALTIME clock; if the Timers option is not supported, the timeout shall be based on the system clock as returned by the time() function.

> [TMO] The resolution of the timeout shall be the resolution of the clock on which it is based. The timespec argument is defined in the <time.h> header.

> Under no circumstance shall the operation fail with a timeout if a message can be removed from the message queue immediately. The validity of the abs_timeout parameter need not be checked if a message can be removed from the message queue immediately.

#### RETURN VALUE

> Upon successful completion, the mq_receive() [TMO] and mq_timedreceive() functions shall return the length of the selected message in bytes and the message shall be removed from the queue. Otherwise, no message shall be removed from the queue, the functions shall return a value of -1, and set errno to indicate the error.

#### ERRORS

> The mq_receive() [TMO] and mq_timedreceive() functions shall fail if:

> [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.
> [EMSGSIZE]
>   The specified message buffer size, msg_len, is less than the message size attribute of the message queue.
> [EINTR]
>   The mq_receive() [TMO] or mq_timedreceive() operation was interrupted by a signal.
> [EINVAL]
>   [TMO] The process or thread would have blocked, and the abs_timeout parameter specified a nanoseconds field value less than zero or greater than or equal to 1000 million.
> [ETIMEDOUT]
>   [TMO] 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() [TMO] and mq_timedreceive() functions may fail if:

> [EBADMSG]
>   The implementation has detected a data corruption problem with the message.

The following sections are informative.

#### EXAMPLES

> None.

#### APPLICATION USAGE

> None.

#### RATIONALE

> None.

#### FUTURE DIRECTIONS

> None.

#### SEE ALSO

> mq_open(), mq_send(), mq_timedsend(), msgctl(), msgget(), msgrcv(), msgsnd(), time(), the Base Definitions volume of IEEE Std 1003.1-2001, <mqueue.h>, <time.h>

#### CHANGE HISTORY

> First released in Issue 5. Included for alignment with the POSIX Realtime Extension.

#### Issue 6

> The mq_receive() function is marked as part of the Message Passing option.

> The Open Group Corrigendum U021/4 is applied. The DESCRIPTION is changed to refer to msg_len rather than maxsize.

> The [ENOSYS] error condition has been removed as stubs need not be provided if an implementation does not support the Message Passing option.

> The following new requirements on POSIX implementations derive from alignment with the Single UNIX Specification:

> - In this function it is possible for the return value to exceed the range of the type ssize_t (since size_t has a larger range of positive values than ssize_t). A sentence restricting the size of the size_t object is added to the description to resolve this conflict.

> The mq_timedreceive() function is added for alignment with IEEE Std 1003.1d-1999.

> The restrict keyword is added to the mq_timedreceive() prototype for alignment with the ISO/IEC 9899:1999 standard.

> IEEE PASC Interpretation 1003.1 #109 is applied, correcting the return type for mq_timedreceive() from int to ssize_t.

End of informative text.

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