1MQ_RECEIVE(3)              Linux Programmer's Manual             MQ_RECEIVE(3)
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NAME

6       mq_receive, mq_timedreceive - receive a message from a message queue
7

SYNOPSIS

9       #include <mqueue.h>
10
11       ssize_t mq_receive(mqd_t mqdes, char *msg_ptr,
12                          size_t msg_len, unsigned int *msg_prio);
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14       #include <time.h>
15       #include <mqueue.h>
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17       ssize_t mq_timedreceive(mqd_t mqdes, char *restrict msg_ptr,
18                          size_t msg_len, unsigned int *restrict msg_prio,
19                          const struct timespec *restrict abs_timeout);
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21       Link with -lrt.
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23   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
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25       mq_timedreceive():
26           _POSIX_C_SOURCE >= 200112L
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DESCRIPTION

29       mq_receive() removes the oldest message with the highest priority from
30       the message queue referred to by the message queue descriptor mqdes,
31       and places it in the buffer pointed to by msg_ptr.  The msg_len argu‐
32       ment specifies the size of the buffer pointed to by msg_ptr; this must
33       be greater than or equal to the mq_msgsize attribute of the queue (see
34       mq_getattr(3)).  If msg_prio is not NULL, then the buffer to which it
35       points is used to return the priority associated with the received mes‐
36       sage.
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38       If the queue is empty, then, by default, mq_receive() blocks until a
39       message becomes available, or the call is interrupted by a signal han‐
40       dler.  If the O_NONBLOCK flag is enabled for the message queue descrip‐
41       tion, then the call instead fails immediately with the error EAGAIN.
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43       mq_timedreceive() behaves just like mq_receive(), except that if the
44       queue is empty and the O_NONBLOCK flag is not enabled for the message
45       queue description, then abs_timeout points to a structure which speci‐
46       fies how long the call will block.  This value is an absolute timeout
47       in seconds and nanoseconds since the Epoch, 1970-01-01 00:00:00 +0000
48       (UTC), specified in the following structure:
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50           struct timespec {
51               time_t tv_sec;        /* seconds */
52               long   tv_nsec;       /* nanoseconds */
53           };
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55       If no message is available, and the timeout has already expired by the
56       time of the call, mq_timedreceive() returns immediately.
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RETURN VALUE

59       On success, mq_receive() and mq_timedreceive() return the number of
60       bytes in the received message; on error, -1 is returned, with errno set
61       to indicate the error.
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ERRORS

64       EAGAIN The queue was empty, and the O_NONBLOCK flag was set for the
65              message queue description referred to by mqdes.
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67       EBADF  The descriptor specified in mqdes was invalid or not opened for
68              reading.
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70       EINTR  The call was interrupted by a signal handler; see signal(7).
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72       EINVAL The call would have blocked, and abs_timeout was invalid, either
73              because tv_sec was less than zero, or because tv_nsec was less
74              than zero or greater than 1000 million.
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76       EMSGSIZE
77              msg_len was less than the mq_msgsize attribute of the message
78              queue.
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80       ETIMEDOUT
81              The call timed out before a message could be transferred.
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ATTRIBUTES

84       For an explanation of the terms used in this section, see at‐
85       tributes(7).
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87       ┌────────────────────────────────────────────┬───────────────┬─────────┐
88Interface                                   Attribute     Value   
89       ├────────────────────────────────────────────┼───────────────┼─────────┤
90mq_receive(), mq_timedreceive()             │ Thread safety │ MT-Safe │
91       └────────────────────────────────────────────┴───────────────┴─────────┘
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CONFORMING TO

94       POSIX.1-2001, POSIX.1-2008.
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NOTES

97       On Linux, mq_timedreceive() is a system call, and mq_receive() is a li‐
98       brary function layered on top of that system call.
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SEE ALSO

101       mq_close(3),  mq_getattr(3),  mq_notify(3),   mq_open(3),   mq_send(3),
102       mq_unlink(3), mq_overview(7), time(7)
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COLOPHON

105       This  page  is  part of release 5.12 of the Linux man-pages project.  A
106       description of the project, information about reporting bugs,  and  the
107       latest     version     of     this    page,    can    be    found    at
108       https://www.kernel.org/doc/man-pages/.
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112Linux                             2021-03-22                     MQ_RECEIVE(3)
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