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| .TH MQ_RECEIVE 3 2016-03-15 "Linux" "Linux Programmer's Manual" |
| .SH NAME |
| mq_receive, mq_timedreceive \- receive a message from a message queue |
| .SH SYNOPSIS |
| .nf |
| .B #include <mqueue.h> |
| .sp |
| .BI "ssize_t mq_receive(mqd_t " mqdes ", char *" msg_ptr , |
| .BI " size_t " msg_len ", unsigned int *" msg_prio ); |
| .sp |
| .B #include <time.h> |
| .B #include <mqueue.h> |
| .sp |
| .BI "ssize_t mq_timedreceive(mqd_t " mqdes ", char *" msg_ptr , |
| .BI " size_t " msg_len ", unsigned int *" msg_prio , |
| .BI " const struct timespec *" abs_timeout ); |
| .fi |
| .sp |
| Link with \fI\-lrt\fP. |
| .sp |
| .ad l |
| .in -4n |
| Feature Test Macro Requirements for glibc (see |
| .BR feature_test_macros (7)): |
| .in |
| .sp |
| .BR mq_timedreceive (): |
| .RS 4 |
| _POSIX_C_SOURCE\ >=\ 200112L |
| .RE |
| .ad |
| .SH DESCRIPTION |
| .BR mq_receive () |
| removes the oldest message with the highest priority from |
| the message queue referred to by the message queue descriptor |
| .IR mqdes , |
| and places it in the buffer pointed to by |
| .IR msg_ptr . |
| The |
| .I msg_len |
| argument specifies the size of the buffer pointed to by |
| .IR msg_ptr ; |
| this must be greater than or equal to the |
| .I mq_msgsize |
| attribute of the queue (see |
| .BR mq_getattr (3)). |
| If |
| .I msg_prio |
| is not NULL, then the buffer to which it points is used |
| to return the priority associated with the received message. |
| |
| If the queue is empty, then, by default, |
| .BR mq_receive () |
| blocks until a message becomes available, |
| or the call is interrupted by a signal handler. |
| If the |
| .B O_NONBLOCK |
| flag is enabled for the message queue description, |
| then the call instead fails immediately with the error |
| .BR EAGAIN . |
| |
| .BR mq_timedreceive () |
| behaves just like |
| .BR mq_receive (), |
| except that if the queue is empty and the |
| .B O_NONBLOCK |
| flag is not enabled for the message queue description, then |
| .I abs_timeout |
| points to a structure which specifies how long the call will block. |
| This value is an absolute timeout in seconds and nanoseconds |
| since the Epoch, 1970-01-01 00:00:00 +0000 (UTC), |
| specified in the following structure: |
| .sp |
| .in +4n |
| .nf |
| struct timespec { |
| time_t tv_sec; /* seconds */ |
| long tv_nsec; /* nanoseconds */ |
| }; |
| |
| .fi |
| .in |
| If no message is available, |
| and the timeout has already expired by the time of the call, |
| .BR mq_timedreceive () |
| returns immediately. |
| .SH RETURN VALUE |
| On success, |
| .BR mq_receive () |
| and |
| .BR mq_timedreceive () |
| return the number of bytes in the received message; |
| on error, \-1 is returned, with |
| .I errno |
| set to indicate the error. |
| .SH ERRORS |
| .TP |
| .B EAGAIN |
| The queue was empty, and the |
| .B O_NONBLOCK |
| flag was set for the message queue description referred to by |
| .IR mqdes . |
| .TP |
| .B EBADF |
| The file descriptor specified in |
| .I mqdes |
| was invalid. |
| .TP |
| .B EINTR |
| The call was interrupted by a signal handler; see |
| .BR signal (7). |
| .TP |
| .B EINVAL |
| The call would have blocked, and |
| .I abs_timeout |
| was invalid, either because |
| .I tv_sec |
| was less than zero, or because |
| .I tv_nsec |
| was less than zero or greater than 1000 million. |
| .TP |
| .B EMSGSIZE |
| .I msg_len |
| was less than the |
| .I mq_msgsize |
| attribute of the message queue. |
| .TP |
| .B ETIMEDOUT |
| The call timed out before a message could be transferred. |
| .SH ATTRIBUTES |
| For an explanation of the terms used in this section, see |
| .BR attributes (7). |
| .TS |
| allbox; |
| lbw31 lb lb |
| l l l. |
| Interface Attribute Value |
| T{ |
| .BR mq_receive (), |
| .BR mq_timedreceive () |
| T} Thread safety MT-Safe |
| .TE |
| .SH CONFORMING TO |
| POSIX.1-2001, POSIX.1-2008. |
| .SH NOTES |
| On Linux, |
| .BR mq_timedreceive () |
| is a system call, and |
| .BR mq_receive () |
| is a library function layered on top of that system call. |
| .SH SEE ALSO |
| .BR mq_close (3), |
| .BR mq_getattr (3), |
| .BR mq_notify (3), |
| .BR mq_open (3), |
| .BR mq_send (3), |
| .BR mq_unlink (3), |
| .BR mq_overview (7), |
| .BR time (7) |