mirror of https://github.com/mkerrisk/man-pages
184 lines
4.8 KiB
Groff
184 lines
4.8 KiB
Groff
.\" Copyright (C) 2006 Michael Kerrisk <mtk.manpages@gmail.com>
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.\"
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.\" %%%LICENSE_START(VERBATIM)
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.\" Permission is granted to make and distribute verbatim copies of this
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.\" manual provided the copyright notice and this permission notice are
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.\" preserved on all copies.
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.\"
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.\" Permission is granted to copy and distribute modified versions of this
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.\" manual under the conditions for verbatim copying, provided that the
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.\" entire resulting derived work is distributed under the terms of a
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.\" permission notice identical to this one.
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.\"
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.\" Since the Linux kernel and libraries are constantly changing, this
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.\" manual page may be incorrect or out-of-date. The author(s) assume no
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.\" responsibility for errors or omissions, or for damages resulting from
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.\" the use of the information contained herein. The author(s) may not
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.\" have taken the same level of care in the production of this manual,
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.\" which is licensed free of charge, as they might when working
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.\" professionally.
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.\"
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.\" Formatted or processed versions of this manual, if unaccompanied by
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.\" the source, must acknowledge the copyright and authors of this work.
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.\" %%%LICENSE_END
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.\"
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.TH MQ_RECEIVE 3 2020-08-13 "Linux" "Linux Programmer's Manual"
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.SH NAME
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mq_receive, mq_timedreceive \- receive a message from a message queue
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.SH SYNOPSIS
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.nf
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.B #include <mqueue.h>
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.PP
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.BI "ssize_t mq_receive(mqd_t " mqdes ", char *" msg_ptr ,
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.BI " size_t " msg_len ", unsigned int *" msg_prio );
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.PP
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.B #include <time.h>
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.B #include <mqueue.h>
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.PP
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.BI "ssize_t mq_timedreceive(mqd_t " mqdes ", char *" msg_ptr ,
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.BI " size_t " msg_len ", unsigned int *" msg_prio ,
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.BI " const struct timespec *" abs_timeout );
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.fi
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.PP
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Link with \fI\-lrt\fP.
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.PP
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.ad l
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.RS -4
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Feature Test Macro Requirements for glibc (see
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.BR feature_test_macros (7)):
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.RE
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.PP
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.BR mq_timedreceive ():
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.RS 4
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_POSIX_C_SOURCE\ >=\ 200112L
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.RE
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.ad
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.SH DESCRIPTION
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.BR mq_receive ()
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removes the oldest message with the highest priority from
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the message queue referred to by the message queue descriptor
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.IR mqdes ,
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and places it in the buffer pointed to by
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.IR msg_ptr .
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The
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.I msg_len
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argument specifies the size of the buffer pointed to by
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.IR msg_ptr ;
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this must be greater than or equal to the
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.I mq_msgsize
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attribute of the queue (see
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.BR mq_getattr (3)).
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If
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.I msg_prio
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is not NULL, then the buffer to which it points is used
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to return the priority associated with the received message.
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.PP
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If the queue is empty, then, by default,
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.BR mq_receive ()
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blocks until a message becomes available,
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or the call is interrupted by a signal handler.
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If the
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.B O_NONBLOCK
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flag is enabled for the message queue description,
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then the call instead fails immediately with the error
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.BR EAGAIN .
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.PP
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.BR mq_timedreceive ()
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behaves just like
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.BR mq_receive (),
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except that if the queue is empty and the
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.B O_NONBLOCK
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flag is not enabled for the message queue description, then
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.I abs_timeout
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points to a structure which specifies how long the call will block.
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This value is an absolute timeout in seconds and nanoseconds
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since the Epoch, 1970-01-01 00:00:00 +0000 (UTC),
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specified in the following structure:
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.PP
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.in +4n
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.EX
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struct timespec {
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time_t tv_sec; /* seconds */
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long tv_nsec; /* nanoseconds */
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};
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.EE
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.in
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.PP
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If no message is available,
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and the timeout has already expired by the time of the call,
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.BR mq_timedreceive ()
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returns immediately.
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.SH RETURN VALUE
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On success,
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.BR mq_receive ()
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and
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.BR mq_timedreceive ()
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return the number of bytes in the received message;
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on error, \-1 is returned, with
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.I errno
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set to indicate the error.
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.SH ERRORS
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.TP
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.B EAGAIN
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The queue was empty, and the
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.B O_NONBLOCK
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flag was set for the message queue description referred to by
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.IR mqdes .
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.TP
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.B EBADF
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The descriptor specified in
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.I mqdes
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was invalid or not opened for reading.
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.TP
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.B EINTR
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The call was interrupted by a signal handler; see
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.BR signal (7).
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.TP
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.B EINVAL
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The call would have blocked, and
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.I abs_timeout
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was invalid, either because
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.I tv_sec
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was less than zero, or because
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.I tv_nsec
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was less than zero or greater than 1000 million.
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.TP
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.B EMSGSIZE
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.I msg_len
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was less than the
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.I mq_msgsize
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attribute of the message queue.
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.TP
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.B ETIMEDOUT
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The call timed out before a message could be transferred.
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.SH ATTRIBUTES
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For an explanation of the terms used in this section, see
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.BR attributes (7).
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.TS
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allbox;
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lbw31 lb lb
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l l l.
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Interface Attribute Value
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T{
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.BR mq_receive (),
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.BR mq_timedreceive ()
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T} Thread safety MT-Safe
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.TE
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.SH CONFORMING TO
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POSIX.1-2001, POSIX.1-2008.
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.SH NOTES
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On Linux,
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.BR mq_timedreceive ()
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is a system call, and
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.BR mq_receive ()
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is a library function layered on top of that system call.
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.SH SEE ALSO
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.BR mq_close (3),
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.BR mq_getattr (3),
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.BR mq_notify (3),
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.BR mq_open (3),
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.BR mq_send (3),
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.BR mq_unlink (3),
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.BR mq_overview (7),
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.BR time (7)
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