1MQ_RECEIVE(3P) POSIX Programmer's Manual MQ_RECEIVE(3P)
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6 This manual page is part of the POSIX Programmer's Manual. The Linux
7 implementation of this interface may differ (consult the corresponding
8 Linux manual page for details of Linux behavior), or the interface may
9 not be implemented on Linux.
10
13 mq_receive, mq_timedreceive — receive a message from a message queue
14 (REALTIME)
15
17 #include <mqueue.h>
18 ssize_t mq_receive(mqd_t mqdes, char *msg_ptr, size_t msg_len,
20 unsigned *msg_prio);
21 #include <mqueue.h>
23 #include <time.h>
24 ssize_t mq_timedreceive(mqd_t mqdes, char *restrict msg_ptr,
26 size_t msg_len, unsigned *restrict msg_prio,
27 const struct timespec *restrict abstime);
28
30 The mq_receive() function shall receive the oldest of the highest pri‐
31 ority message(s) from the message queue specified by mqdes. If the
32 size of the buffer in bytes, specified by the msg_len argument, is less
33 than the mq_msgsize attribute of the message queue, the function shall
34 fail and return an error. Otherwise, the selected message shall be
35 removed from the queue and copied to the buffer pointed to by the
36 msg_ptr argument.
37 If the value of msg_len is greater than {SSIZE_MAX}, the result is
39 implementation-defined.
40 If the argument msg_prio is not NULL, the priority of the selected mes‐
42 sage shall be stored in the location referenced by msg_prio.
43 If the specified message queue is empty and O_NONBLOCK is not set in
45 the message queue description associated with mqdes, mq_receive() shall
46 block until a message is enqueued on the message queue or until
47 mq_receive() is interrupted by a signal. If more than one thread is
48 waiting to receive a message when a message arrives at an empty queue
49 and the Priority Scheduling option is supported, then the thread of
50 highest priority that has been waiting the longest shall be selected to
51 receive the message. Otherwise, it is unspecified which waiting thread
52 receives the message. If the specified message queue is empty and
53 O_NONBLOCK is set in the message queue description associated with
54 mqdes, no message shall be removed from the queue, and mq_receive()
55 shall return an error.
56 The mq_timedreceive() function shall receive the oldest of the highest
58 priority messages from the message queue specified by mqdes as
59 described for the mq_receive() function. However, if O_NONBLOCK was not
60 specified when the message queue was opened via the mq_open() function,
61 and no message exists on the queue to satisfy the receive, the wait for
62 such a message shall be terminated when the specified timeout expires.
63 If O_NONBLOCK is set, this function is equivalent to mq_receive().
64 The timeout expires when the absolute time specified by abstime passes,
66 as measured by the clock on which timeouts are based (that is, when the
67 value of that clock equals or exceeds abstime), or if the absolute time
68 specified by abstime has already been passed at the time of the call.
69 The timeout shall be based on the CLOCK_REALTIME clock. The resolution
71 of the timeout shall be the resolution of the clock on which it is
72 based. The timespec argument is defined in the <time.h> header.
73 Under no circumstance shall the operation fail with a timeout if a mes‐
75 sage can be removed from the message queue immediately. The validity of
76 the abstime parameter need not be checked if a message can be removed
77 from the message queue immediately.
78
80 Upon successful completion, the mq_receive() and mq_timedreceive()
81 functions shall return the length of the selected message in bytes and
82 the message shall be removed from the queue. Otherwise, no message
83 shall be removed from the queue, the functions shall return a value of
84 −1, and set errno to indicate the error.
85
87 These functions shall fail if:
88 EAGAIN O_NONBLOCK was set in the message description associated with
90 mqdes, and the specified message queue is empty.
91 EBADF The mqdes argument is not a valid message queue descriptor open
93 for reading.
94 EMSGSIZE
96 The specified message buffer size, msg_len, is less than the
97 message size attribute of the message queue.
98 EINTR The mq_receive() or mq_timedreceive() operation was interrupted
100 by a signal.
101 EINVAL The process or thread would have blocked, and the abstime param‐
103 eter specified a nanoseconds field value less than zero or
104 greater than or equal to 1000 million.
105 ETIMEDOUT
107 The O_NONBLOCK flag was not set when the message queue was
108 opened, but no message arrived on the queue before the specified
109 timeout expired.
110 These functions may fail if:
112 EBADMSG
114 The implementation has detected a data corruption problem with
115 the message.
116 The following sections are informative.
118
120 None.
121
123 None.
124
126 None.
127
129 None.
130
132 mq_open(), mq_send(), msgctl(), msgget(), msgrcv(), msgsnd(), time()
133 The Base Definitions volume of POSIX.1‐2008, <mqueue.h>, <time.h>
135
137 Portions of this text are reprinted and reproduced in electronic form
138 from IEEE Std 1003.1, 2013 Edition, Standard for Information Technology
139 -- Portable Operating System Interface (POSIX), The Open Group Base
140 Specifications Issue 7, Copyright (C) 2013 by the Institute of Electri‐
141 cal and Electronics Engineers, Inc and The Open Group. (This is
142 POSIX.1-2008 with the 2013 Technical Corrigendum 1 applied.) In the
143 event of any discrepancy between this version and the original IEEE and
144 The Open Group Standard, the original IEEE and The Open Group Standard
145 is the referee document. The original Standard can be obtained online
146 at http://www.unix.org/online.html .
147 Any typographical or formatting errors that appear in this page are
149 most likely to have been introduced during the conversion of the source
150 files to man page format. To report such errors, see https://www.ker‐
151 nel.org/doc/man-pages/reporting_bugs.html .
152IEEE/The Open Group 2013 MQ_RECEIVE(3P)