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PROLOG | NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | EXAMPLES | APPLICATION USAGE | RATIONALE | FUTURE DIRECTIONS | SEE ALSO | COPYRIGHT |
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MSGSND(3P) POSIX Programmer's Manual MSGSND(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.
msgsnd — XSI message send operation
#include <sys/msg.h>
int msgsnd(int msqid, const void *msgp, size_t msgsz, int msgflg);
The msgsnd() 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 msgsnd() function shall send a message to the queue associated
with the message queue identifier specified by msqid.
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 a non-zero positive type long that
can be used by the receiving process for message selection.
The structure member mtext is any text of length msgsz bytes. The
argument msgsz can range from 0 to a system-imposed maximum.
The argument msgflg specifies the action to be taken if one or
more of the following is true:
* The number of bytes already on the queue is equal to
msg_qbytes; see <sys/msg.h>.
* The total number of messages on all queues system-wide is
equal to the system-imposed limit.
These actions are as follows:
* If (msgflg & IPC_NOWAIT) is non-zero, the message shall not be
sent and the calling thread shall return immediately.
* If (msgflg & IPC_NOWAIT) is 0, the calling thread shall
suspend execution until one of the following occurs:
-- The condition responsible for the suspension no longer
exists, in which case the message is sent.
-- 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 the message is not sent 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; see
<sys/msg.h>:
* msg_qnum shall be incremented by 1.
* msg_lspid shall be set to the process ID of the calling
process.
* msg_stime shall be set to the current time, as described in
Section 2.7.1, IPC General Description.
Upon successful completion, msgsnd() shall return 0; otherwise, no
message shall be sent, msgsnd() shall return -1, and errno shall
be set to indicate the error.
The msgsnd() function shall fail if:
EACCES Operation permission is denied to the calling process; see
Section 2.7, XSI Interprocess Communication.
EAGAIN The message cannot be sent for one of the reasons cited
above and (msgflg & IPC_NOWAIT) is non-zero.
EIDRM The message queue identifier msqid is removed from the
system.
EINTR The msgsnd() function was interrupted by a signal.
EINVAL The value of msqid is not a valid message queue identifier,
or the value of mtype is less than 1; or the value of msgsz
is greater than the system-imposed limit.
The following sections are informative.
Sending a Message
The following example sends a message to the queue identified by
the msqid argument (assuming that value has previously been set).
This call specifies that an error should be reported if no message
is available. 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;
msg.type = 1;
strcpy(msg.text, "This is message 1");
...
result = msgsnd(msqid, (void *) &msg, sizeof(msg.text), 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), msgrcv(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 MSGSND(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), msgrcv(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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