mq_receive.3p (6277B)
- '\" et
- .TH MQ_RECEIVE "3P" 2017 "IEEE/The Open Group" "POSIX Programmer's Manual"
- .\"
- .SH PROLOG
- This manual page is part of the POSIX Programmer's Manual.
- The Linux implementation of this interface may differ (consult
- the corresponding Linux manual page for details of Linux behavior),
- or the interface may not be implemented on Linux.
- .\"
- .SH NAME
- mq_receive,
- mq_timedreceive
- \(em receive a message from a message queue (\fBREALTIME\fP)
- .SH SYNOPSIS
- .LP
- .nf
- #include <mqueue.h>
- .P
- ssize_t mq_receive(mqd_t \fImqdes\fP, char *\fImsg_ptr\fP, size_t \fImsg_len\fP,
- unsigned *\fImsg_prio\fP);
- .P
- #include <mqueue.h>
- #include <time.h>
- .P
- ssize_t mq_timedreceive(mqd_t \fImqdes\fP, char *restrict \fImsg_ptr\fP,
- size_t \fImsg_len\fP, unsigned *restrict \fImsg_prio\fP,
- const struct timespec *restrict \fIabstime\fP);
- .fi
- .SH DESCRIPTION
- The
- \fImq_receive\fR()
- function shall receive the oldest of the highest priority
- message(s) from the message queue specified by
- .IR mqdes .
- If the size of the buffer in bytes, specified by the
- .IR msg_len
- argument, is less than the
- .IR 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
- .IR msg_ptr
- argument.
- .P
- If the value of
- .IR msg_len
- is greater than
- {SSIZE_MAX},
- the result is implementation-defined.
- .P
- If the argument
- .IR msg_prio
- is not NULL, the priority of the selected message shall be stored in the
- location referenced by
- .IR msg_prio .
- .P
- If the specified message queue is empty and O_NONBLOCK
- is not set in the message queue description associated with
- .IR mqdes ,
- \fImq_receive\fR()
- shall block until a message is enqueued on the message queue or until
- \fImq_receive\fR()
- 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
- .IR mqdes ,
- no message shall be removed from the queue, and
- \fImq_receive\fR()
- shall return an error.
- .P
- The
- \fImq_timedreceive\fR()
- function shall receive the oldest of the highest priority messages
- from the message queue specified by
- .IR mqdes
- as described for the
- \fImq_receive\fR()
- function. However, if O_NONBLOCK was not specified when the message
- queue was opened via the
- \fImq_open\fR()
- 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
- \fImq_receive\fR().
- .P
- The timeout expires when the absolute time specified by
- .IR abstime
- passes, as measured by the clock on which timeouts are based (that is,
- when the value of that clock equals or exceeds
- .IR abstime ),
- or if the absolute time specified by
- .IR abstime
- has already been passed at the time of the call.
- .P
- 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
- .IR timespec
- argument is defined in the
- .IR <time.h>
- header.
- .P
- 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
- .IR abstime
- parameter need not be checked if a message can be removed from the
- message queue immediately.
- .SH "RETURN VALUE"
- Upon successful completion, the
- \fImq_receive\fR()
- and
- \fImq_timedreceive\fR()
- 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
- .IR errno
- to indicate the error.
- .SH ERRORS
- These functions shall fail if:
- .TP
- .BR EAGAIN
- O_NONBLOCK was set in the message description associated with
- .IR mqdes ,
- and the specified message queue is empty.
- .TP
- .BR EBADF
- The
- .IR mqdes
- argument is not a valid message queue descriptor open for reading.
- .TP
- .BR EMSGSIZE
- The specified message buffer size,
- .IR msg_len ,
- is less than the message size attribute of the message queue.
- .TP
- .BR EINTR
- The
- \fImq_receive\fR()
- or
- \fImq_timedreceive\fR()
- operation was interrupted by a signal.
- .TP
- .BR EINVAL
- The process or thread would have blocked, and the
- .IR abstime
- parameter specified a nanoseconds field value less than zero or greater
- than or equal to 1\|000 million.
- .TP
- .BR 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.
- .P
- These functions may fail if:
- .TP
- .BR EBADMSG
- The implementation has detected a data corruption problem with the
- message.
- .LP
- .IR "The following sections are informative."
- .SH EXAMPLES
- None.
- .SH "APPLICATION USAGE"
- None.
- .SH RATIONALE
- None.
- .SH "FUTURE DIRECTIONS"
- None.
- .SH "SEE ALSO"
- .IR "\fImq_open\fR\^(\|)",
- .IR "\fImq_send\fR\^(\|)",
- .IR "\fImsgctl\fR\^(\|)",
- .IR "\fImsgget\fR\^(\|)",
- .IR "\fImsgrcv\fR\^(\|)",
- .IR "\fImsgsnd\fR\^(\|)",
- .IR "\fItime\fR\^(\|)"
- .P
- The Base Definitions volume of POSIX.1\(hy2017,
- .IR "\fB<mqueue.h>\fP",
- .IR "\fB<time.h>\fP"
- .\"
- .SH 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. The original Standard can be obtained online at
- http://www.opengroup.org/unix/online.html .
- .PP
- 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 .