1MQ_RECEIVE(3P) POSIX Programmer's Manual MQ_RECEIVE(3P)
2
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
12 mq_receive, mq_timedreceive — receive a message from a message queue
13 (REALTIME)
14
16 #include <mqueue.h>
17 ssize_t mq_receive(mqd_t mqdes, char *msg_ptr, size_t msg_len,
19 unsigned *msg_prio);
20 #include <mqueue.h>
22 #include <time.h>
23 ssize_t mq_timedreceive(mqd_t mqdes, char *restrict msg_ptr,
25 size_t msg_len, unsigned *restrict msg_prio,
26 const struct timespec *restrict abstime);
27
29 The mq_receive() function shall receive the oldest of the highest pri‐
30 ority message(s) from the message queue specified by mqdes. If the
31 size of the buffer in bytes, specified by the msg_len argument, is less
32 than the mq_msgsize attribute of the message queue, the function shall
33 fail and return an error. Otherwise, the selected message shall be
34 removed from the queue and copied to the buffer pointed to by the
35 msg_ptr argument.
36 If the value of msg_len is greater than {SSIZE_MAX}, the result is
38 implementation-defined.
39 If the argument msg_prio is not NULL, the priority of the selected mes‐
41 sage shall be stored in the location referenced by msg_prio.
42 If the specified message queue is empty and O_NONBLOCK is not set in
44 the message queue description associated with mqdes, mq_receive() shall
45 block until a message is enqueued on the message queue or until
46 mq_receive() is interrupted by a signal. If more than one thread is
47 waiting to receive a message when a message arrives at an empty queue
48 and the Priority Scheduling option is supported, then the thread of
49 highest priority that has been waiting the longest shall be selected to
50 receive the message. Otherwise, it is unspecified which waiting thread
51 receives the message. If the specified message queue is empty and
52 O_NONBLOCK is set in the message queue description associated with
53 mqdes, no message shall be removed from the queue, and mq_receive()
54 shall return an error.
55 The mq_timedreceive() function shall receive the oldest of the highest
57 priority messages from the message queue specified by mqdes as
58 described for the mq_receive() function. However, if O_NONBLOCK was not
59 specified when the message queue was opened via the mq_open() function,
60 and no message exists on the queue to satisfy the receive, the wait for
61 such a message shall be terminated when the specified timeout expires.
62 If O_NONBLOCK is set, this function is equivalent to mq_receive().
63 The timeout expires when the absolute time specified by abstime passes,
65 as measured by the clock on which timeouts are based (that is, when the
66 value of that clock equals or exceeds abstime), or if the absolute time
67 specified by abstime has already been passed at the time of the call.
68 The timeout shall be based on the CLOCK_REALTIME clock. The resolution
70 of the timeout shall be the resolution of the clock on which it is
71 based. The timespec argument is defined in the <time.h> header.
72 Under no circumstance shall the operation fail with a timeout if a mes‐
74 sage can be removed from the message queue immediately. The validity of
75 the abstime parameter need not be checked if a message can be removed
76 from the message queue immediately.
77
79 Upon successful completion, the mq_receive() and mq_timedreceive()
80 functions shall return the length of the selected message in bytes and
81 the message shall be removed from the queue. Otherwise, no message
82 shall be removed from the queue, the functions shall return a value of
83 -1, and set errno to indicate the error.
84
86 These functions shall fail if:
87 EAGAIN O_NONBLOCK was set in the message description associated with
89 mqdes, and the specified message queue is empty.
90 EBADF The mqdes argument is not a valid message queue descriptor open
92 for reading.
93 EMSGSIZE
95 The specified message buffer size, msg_len, is less than the
96 message size attribute of the message queue.
97 EINTR The mq_receive() or mq_timedreceive() operation was interrupted
99 by a signal.
100 EINVAL The process or thread would have blocked, and the abstime param‐
102 eter specified a nanoseconds field value less than zero or
103 greater than or equal to 1000 million.
104 ETIMEDOUT
106 The O_NONBLOCK flag was not set when the message queue was
107 opened, but no message arrived on the queue before the specified
108 timeout expired.
109 These functions may fail if:
111 EBADMSG
113 The implementation has detected a data corruption problem with
114 the message.
115 The following sections are informative.
117
119 None.
120
122 None.
123
125 None.
126
128 None.
129
131 mq_open(), mq_send(), msgctl(), msgget(), msgrcv(), msgsnd(), time()
132 The Base Definitions volume of POSIX.1‐2017, <mqueue.h>, <time.h>
134
136 Portions of this text are reprinted and reproduced in electronic form
137 from IEEE Std 1003.1-2017, Standard for Information Technology -- Por‐
138 table Operating System Interface (POSIX), The Open Group Base Specifi‐
139 cations Issue 7, 2018 Edition, Copyright (C) 2018 by the Institute of
140 Electrical and Electronics Engineers, Inc and The Open Group. In the
141 event of any discrepancy between this version and the original IEEE and
142 The Open Group Standard, the original IEEE and The Open Group Standard
143 is the referee document. The original Standard can be obtained online
144 at http://www.opengroup.org/unix/online.html .
145 Any typographical or formatting errors that appear in this page are
147 most likely to have been introduced during the conversion of the source
148 files to man page format. To report such errors, see https://www.ker‐
149 nel.org/doc/man-pages/reporting_bugs.html .
150IEEE/The Open Group 2017 MQ_RECEIVE(3P)