man3/mq_receive.3

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  26 .TH MQ_RECEIVE 3 2017-09-15 "Linux" "Linux Programmer's Manual"
  27 .SH NAME
  28 mq_receive, mq_timedreceive \- receive a message from a message queue
  29 .SH SYNOPSIS
  30 .nf
  31 .B #include <mqueue.h>
  32 .PP
  33 .BI "ssize_t mq_receive(mqd_t " mqdes ", char *" msg_ptr ,
  34 .BI "                   size_t " msg_len ", unsigned int *" msg_prio );
  35 .PP
  36 .B #include <time.h>
  37 .B #include <mqueue.h>
  38 .PP
  39 .BI "ssize_t mq_timedreceive(mqd_t " mqdes ", char *" msg_ptr ,
  40 .BI "                   size_t " msg_len ", unsigned int *" msg_prio ,
  41 .BI "                   const struct timespec *" abs_timeout );
  42 .fi
  43 .PP
  44 Link with \fI\-lrt\fP.
  45 .PP
  46 .ad l
  47 .in -4n
  48 Feature Test Macro Requirements for glibc (see
  49 .BR feature_test_macros (7)):
  50 .in
  51 .PP
  52 .BR mq_timedreceive ():
  53 .RS 4
  54 _POSIX_C_SOURCE\ >=\ 200112L
  55 .RE
  56 .ad
  57 .SH DESCRIPTION
  58 .BR mq_receive ()
  59 removes the oldest message with the highest priority from
  60 the message queue referred to by the message queue descriptor
  61 .IR mqdes ,
  62 and places it in the buffer pointed to by
  63 .IR msg_ptr .
  64 The
  65 .I msg_len
  66 argument specifies the size of the buffer pointed to by
  67 .IR msg_ptr ;
  68 this must be greater than or equal to the
  69 .I mq_msgsize
  70 attribute of the queue (see
  71 .BR mq_getattr (3)).
  72 If
  73 .I msg_prio
  74 is not NULL, then the buffer to which it points is used
  75 to return the priority associated with the received message.
  76 .PP
  77 If the queue is empty, then, by default,
  78 .BR mq_receive ()
  79 blocks until a message becomes available,
  80 or the call is interrupted by a signal handler.
  81 If the
  82 .B O_NONBLOCK
  83 flag is enabled for the message queue description,
  84 then the call instead fails immediately with the error
  85 .BR EAGAIN .
  86 .PP
  87 .BR mq_timedreceive ()
  88 behaves just like
  89 .BR mq_receive (),
  90 except that if the queue is empty and the
  91 .B O_NONBLOCK
  92 flag is not enabled for the message queue description, then
  93 .I abs_timeout
  94 points to a structure which specifies how long the call will block.
  95 This value is an absolute timeout in seconds and nanoseconds
  96 since the Epoch, 1970-01-01 00:00:00 +0000 (UTC),
  97 specified in the following structure:
  98 .PP
  99 .in +4n
 100 .EX
 101 struct timespec {
 102     time_t tv_sec;        /* seconds */
 103     long   tv_nsec;       /* nanoseconds */
 104 };
 105 .EE
 106 .in
 107 .PP
 108 If no message is available,
 109 and the timeout has already expired by the time of the call,
 110 .BR mq_timedreceive ()
 111 returns immediately.
 112 .SH RETURN VALUE
 113 On success,
 114 .BR mq_receive ()
 115 and
 116 .BR mq_timedreceive ()
 117 return the number of bytes in the received message;
 118 on error, \-1 is returned, with
 119 .I errno
 120 set to indicate the error.
 121 .SH ERRORS
 122 .TP
 123 .B EAGAIN
 124 The queue was empty, and the
 125 .B O_NONBLOCK
 126 flag was set for the message queue description referred to by
 127 .IR mqdes .
 128 .TP
 129 .B EBADF
 130 The descriptor specified in
 131 .I mqdes
 132 was invalid or not opened for reading.
 133 .TP
 134 .B EINTR
 135 The call was interrupted by a signal handler; see
 136 .BR signal (7).
 137 .TP
 138 .B EINVAL
 139 The call would have blocked, and
 140 .I abs_timeout
 141 was invalid, either because
 142 .I tv_sec
 143 was less than zero, or because
 144 .I tv_nsec
 145 was less than zero or greater than 1000 million.
 146 .TP
 147 .B EMSGSIZE
 148 .I msg_len
 149 was less than the
 150 .I mq_msgsize
 151 attribute of the message queue.
 152 .TP
 153 .B ETIMEDOUT
 154 The call timed out before a message could be transferred.
 155 .SH ATTRIBUTES
 156 For an explanation of the terms used in this section, see
 157 .BR attributes (7).
 158 .TS
 159 allbox;
 160 lbw31 lb lb
 161 l l l.
 162 Interface       Attribute       Value
 163 T{
 164 .BR mq_receive (),
 165 .BR mq_timedreceive ()
 166 T}      Thread safety   MT-Safe
 167 .TE
 168 .SH CONFORMING TO
 169 POSIX.1-2001, POSIX.1-2008.
 170 .SH NOTES
 171 On Linux,
 172 .BR mq_timedreceive ()
 173 is a system call, and
 174 .BR mq_receive ()
 175 is a library function layered on top of that system call.
 176 .SH SEE ALSO
 177 .BR mq_close (3),
 178 .BR mq_getattr (3),
 179 .BR mq_notify (3),
 180 .BR mq_open (3),
 181 .BR mq_send (3),
 182 .BR mq_unlink (3),
 183 .BR mq_overview (7),
 184 .BR time (7)