MQ_RECEIVE(3POSIX)

💌 mq_receive(3POSIX) — bestie's guide to grabbing messages off a queue

ok so this is mq_receive(3POSIX) from the POSIX Programmer's Manual, and honestly? she's giving main character energy in the inter-process communication department. come sit with me. 🫶

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🌸 PROLOG

ok so first things first, real talk: this manual page is part of the POSIX Programmer's Manual. the Linux implementation of this interface may differ (so like, go check the corresponding Linux manual page for the deets on Linux behavior), or the interface may just... not be implemented on Linux at all. 🙈 iykyk.

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✨ NAME

mq_receive, mq_timedreceive — receive a message from a message queue (REALTIME)

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🎀 SYNOPSIS

#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 abstime);

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💖 DESCRIPTION

so mq_receive() receives the oldest of the highest priority message(s) from the message queue specified by mqdes. periodt. highest priority first, and if there's a tie, the oldest one wins. 💅

if the size of the buffer in bytes — that's 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. slay.

if the value of msg_len is greater than {SSIZE_MAX}, the result is implementation-defined. no cap, that's just what the standard says, don't come for me.

if the argument msg_prio is not NULL, the priority of the selected message shall be stored in the location referenced by msg_prio. cute, right?

🌈 here's the blocking tea: 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.

and 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.

now mq_timedreceive() — she receives the oldest of the highest priority messages from the message queue specified by mqdes, exactly 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(). same girl, different outfit. 👑

the timeout expires when the absolute time specified by abstime passes, as measured by the clock on which timeouts are based (that is, when the value of that clock equals or exceeds abstime), or if the absolute time specified by abstime has already been passed at the time of the call. ⏰

the timeout shall be based on the CLOCK_REALTIME clock. 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. and the validity of the abstime parameter need not be checked if a message can be removed from the message queue immediately. she said what she said.

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🦋 RETURN VALUE

upon successful completion, the mq_receive() 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. it's giving sad girl hours. 😭

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💅 ERRORS

these 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() or mq_timedreceive() operation was interrupted by a signal.
• EINVAL — the process or thread would have blocked, and the abstime parameter specified a nanoseconds field value less than zero or greater than or equal to 1000 million.
• 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.

these functions may fail if:

• EBADMSG — the implementation has detected a data corruption problem with the message.

and bestie, the following sections are informative — so like, they're the vibes, not the rules. 💜

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🍓 EXAMPLES

None. (yeah. nothing. she's mysterious like that. 🙈)

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🌈 APPLICATION USAGE

None. (again. the girl said nothing and left. iconic honestly.)

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🧚 RATIONALE

None.

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⭐ FUTURE DIRECTIONS

None.

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🔥 SEE ALSO

• mq_open()
• mq_send()
• msgctl()
• msgget()
• msgrcv()
• msgsnd()
• time()

and also the Base Definitions volume of POSIX.1‐2017: <mqueue.h>, <time.h> 📚

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👑 COPYRIGHT

portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1-2017, Standard for Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base Specifications Issue 7, 2018 Edition, Copyright (C) 2018 by the Institute of Electrical and Electronics Engineers, Inc and The Open Group.

in the event of any discrepancy between this version and the original IEEE and The Open Group Standard, the original IEEE and The Open Group Standard is the referee document. (she's the final boss, no debating. 💅)

the original Standard can be obtained online at http://www.opengroup.org/unix/online.html .

any typographical or formatting errors that appear in this page are most likely to have been introduced during the conversion of the source files to man page format. to report such errors, see https://www.kernel.org/doc/man-pages/reporting_bugs.html .

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and that's the whole page, bestie!! 💖✨🎀🦋🌸

this iconic piece of documentation was brought to you by IEEE/The Open Group, dated 2017, under the name MQ_RECEIVE(3POSIX). go forth and receive your messages, highest priority first, always. mwah. 💋