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PROLOG | NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | EXAMPLES | APPLICATION USAGE | RATIONALE | FUTURE DIRECTIONS | SEE ALSO | COPYRIGHT |
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MSGRCV(3P) POSIX Programmer's Manual MSGRCV(3P)
This manual page is part of the POSIX Programmer's Manual. The
Linux implementation of this interface may differ (consult the
corresponding Linux manual page for details of Linux behavior), or
the interface may not be implemented on Linux.
msgrcv — XSI message receive operation
#include <sys/msg.h>
ssize_t msgrcv(int msqid, void *msgp, size_t msgsz, long msgtyp,
int msgflg);
The msgrcv() function operates on XSI message queues (see the Base
Definitions volume of POSIX.1‐2017, Section 3.226, Message Queue).
It is unspecified whether this function interoperates with the
realtime interprocess communication facilities defined in Section
2.8, Realtime.
The msgrcv() function shall read a message from the queue
associated with the message queue identifier specified by msqid
and place it in the user-defined buffer pointed to by msgp.
The application shall ensure that the argument msgp points to a
user-defined buffer that contains first a field of type long
specifying the type of the message, and then a data portion that
holds the data bytes of the message. The structure below is an
example of what this user-defined buffer might look like:
struct mymsg {
long mtype; /* Message type. */
char mtext[1]; /* Message text. */
}
The structure member mtype is the received message's type as
specified by the sending process.
The structure member mtext is the text of the message.
The argument msgsz specifies the size in bytes of mtext. The
received message shall be truncated to msgsz bytes if it is larger
than msgsz and (msgflg & MSG_NOERROR) is non-zero. The truncated
part of the message shall be lost and no indication of the
truncation shall be given to the calling process.
If the value of msgsz is greater than {SSIZE_MAX}, the result is
implementation-defined.
The argument msgtyp specifies the type of message requested as
follows:
* If msgtyp is 0, the first message on the queue shall be
received.
* If msgtyp is greater than 0, the first message of type msgtyp
shall be received.
* If msgtyp is less than 0, the first message of the lowest type
that is less than or equal to the absolute value of msgtyp
shall be received.
The argument msgflg specifies the action to be taken if a message
of the desired type is not on the queue. These are as follows:
* If (msgflg & IPC_NOWAIT) is non-zero, the calling thread shall
return immediately with a return value of -1 and errno set to
[ENOMSG].
* If (msgflg & IPC_NOWAIT) is 0, the calling thread shall
suspend execution until one of the following occurs:
-- A message of the desired type is placed on the queue.
-- The message queue identifier msqid is removed from the
system; when this occurs, errno shall be set to [EIDRM]
and -1 shall be returned.
-- The calling thread receives a signal that is to be caught;
in this case a message is not received and the calling
thread resumes execution in the manner prescribed in
sigaction(3p).
Upon successful completion, the following actions are taken with
respect to the data structure associated with msqid:
* msg_qnum shall be decremented by 1.
* msg_lrpid shall be set to the process ID of the calling
process.
* msg_rtime shall be set to the current time, as described in
Section 2.7.1, IPC General Description.
Upon successful completion, msgrcv() shall return a value equal to
the number of bytes actually placed into the buffer mtext.
Otherwise, no message shall be received, msgrcv() shall return -1,
and errno shall be set to indicate the error.
The msgrcv() function shall fail if:
E2BIG The value of mtext is greater than msgsz and (msgflg &
MSG_NOERROR) is 0.
EACCES Operation permission is denied to the calling process; see
Section 2.7, XSI Interprocess Communication.
EIDRM The message queue identifier msqid is removed from the
system.
EINTR The msgrcv() function was interrupted by a signal.
EINVAL msqid is not a valid message queue identifier.
ENOMSG The queue does not contain a message of the desired type
and (msgflg & IPC_NOWAIT) is non-zero.
The following sections are informative.
Receiving a Message
The following example receives the first message on the queue
(based on the value of the msgtyp argument, 0). The queue is
identified by the msqid argument (assuming that the value has
previously been set). This call specifies that an error should be
reported if no message is available, but not if the message is too
large. The message size is calculated directly using the sizeof
operator.
#include <sys/msg.h>
...
int result;
int msqid;
struct message {
long type;
char text[20];
} msg;
long msgtyp = 0;
...
result = msgrcv(msqid, (void *) &msg, sizeof(msg.text),
msgtyp, MSG_NOERROR | IPC_NOWAIT);
The POSIX Realtime Extension defines alternative interfaces for
interprocess communication (IPC). Application developers who need
to use IPC should design their applications so that modules using
the IPC routines described in Section 2.7, XSI Interprocess
Communication can be easily modified to use the alternative
interfaces.
None.
None.
Section 2.7, XSI Interprocess Communication, Section 2.8,
Realtime, mq_close(3p), mq_getattr(3p), mq_notify(3p),
mq_open(3p), mq_receive(3p), mq_send(3p), mq_setattr(3p),
mq_unlink(3p), msgctl(3p), msgget(3p), msgsnd(3p), sigaction(3p)
The Base Definitions volume of POSIX.1‐2017, Section 3.226,
Message Queue, sys_msg.h(0p)
Portions of this text are reprinted and reproduced in electronic
form from IEEE Std 1003.1-2017, Standard for Information
Technology -- Portable Operating System Interface (POSIX), The
Open Group Base Specifications Issue 7, 2018 Edition, Copyright
(C) 2018 by the Institute of Electrical and Electronics Engineers,
Inc and The Open Group. In the event of any discrepancy between
this version and the original IEEE and The Open Group Standard,
the original IEEE and The Open Group Standard is the referee
document. The original Standard can be obtained online at
http://www.opengroup.org/unix/online.html .
Any typographical or formatting errors that appear in this page
are most likely to have been introduced during the conversion of
the source files to man page format. To report such errors, see
https://www.kernel.org/doc/man-pages/reporting_bugs.html .
IEEE/The Open Group 2017 MSGRCV(3P)
Pages that refer to this page: sys_msg.h(0p), ipcs(1p), mq_close(3p), mq_getattr(3p), mq_notify(3p), mq_open(3p), mq_receive(3p), mq_setattr(3p), mq_unlink(3p), msgctl(3p), msgget(3p), msgsnd(3p)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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