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.TH MQ_RECEIVE "3P" 2013 "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

\(mi1, 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\(hy2008,

.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, 2013 Edition, Standard for Information Technology

-- Portable Operating System Interface (POSIX), The Open Group Base

Specifications Issue 7, Copyright (C) 2013 by the Institute of

Electrical and Electronics Engineers, Inc and The Open Group.

(This is POSIX.1-2008 with the 2013 Technical Corrigendum 1 applied.) 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.unix.org/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 .