mq_receive(3)

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
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SYNOPSIS

9       #include <mqueue.h>
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11       ssize_t mq_receive(mqd_t mqdes, char *msg_ptr,
12                          size_t msg_len, unsigned *msg_prio);
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14       #define _XOPEN_SOURCE 600
15       #include <time.h>
16       #include <mqueue.h>
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18       ssize_t mq_timedreceive(mqd_t mqdes, char *msg_ptr,
19                          size_t msg_len, unsigned *msg_prio,
20                          const struct timespec *abs_timeout);
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22       Link with -lrt.
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DESCRIPTION

25       mq_receive()  removes the oldest message with the highest priority from
26       the message queue referred to by the descriptor mqdes, and places it in
27       the  buffer  pointed to by msg_ptr.  The msg_len argument specifies the
28       size of the buffer pointed to by msg_ptr; this must be greater than the
29       mq_msgsize  attribute of the queue (see mq_getattr(3)).  If prio is not
30       NULL, then the buffer to which it points is used to return the priority
31       associated with the received message.
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33       If  the  queue  is empty, then, by default, mq_receive() blocks until a
34       message becomes available, or the call is interrupted by a signal  han‐
35       dler.  If the O_NONBLOCK flag is enabled for the message queue descrip‐
36       tion, then the call instead fails immediately with the error EAGAIN.
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38       mq_timedreceive() behaves just like mq_receive(), except  that  if  the
39       queue  is  empty and the O_NONBLOCK flag is not enabled for the message
40       queue description, then abs_timeout points to a structure which  speci‐
41       fies a ceiling on the time for which the call will block.  This ceiling
42       is an absolute timeout in seconds  and  nanoseconds  since  the  Epoch,
43       1970-01-01  00:00:00  +0000 (UTC), and it is specified in the following
44       structure:
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46           struct timespec {
47               time_t tv_sec;        /* seconds */
48               long   tv_nsec;       /* nanoseconds */
49           };
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51       If no message is available, and the timeout has already expired by  the
52       time of the call, mq_timedreceive() returns immediately.
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RETURN VALUE

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

60       EAGAIN The  queue  was  empty,  and the O_NONBLOCK flag was set for the
61              message queue description referred to by mqdes.
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63       EBADF  The descriptor specified in mqdes was invalid.
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65       EINTR  The call was interrupted by a signal handler; see signal(7).
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67       EINVAL The call would have blocked, and abs_timeout was invalid, either
68              because  tv_sec  was less than zero, or because tv_nsec was less
69              than zero or greater than 1000 million.
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71       EMSGSIZE
72              msg_len was less than the mq_msgsize attribute  of  the  message
73              queue.
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75       ETIMEDOUT
76              The call timed out before a message could be transferred.
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CONFORMING TO

79       POSIX.1-2001.
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NOTES

82       On  Linux,  mq_timedreceive()  is  a system call, and mq_receive() is a
83       library function layered on top of that system call.
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COLOPHON

90       This  page  is  part of release 3.25 of the Linux man-pages project.  A
91       description of the project, and information about reporting  bugs,  can
92       be found at http://www.kernel.org/doc/man-pages/.
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96Linux                             2010-02-25                     MQ_RECEIVE(3)