|
PROLOG | NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | EXAMPLES | APPLICATION USAGE | RATIONALE | FUTURE DIRECTIONS | SEE ALSO | COPYRIGHT |
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.1‐2008, Section 3.225, 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.1‐2008, Section 3.225, Message
Queue, sys_msg.h(0p)
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2013 Edition, Standard for Information
Technology -- Portable Operating System Interface (POSIX), The Open
Group Base Specifications Issue 7, Copyright (C) 2013 by the
Institute of Electrical and Electronics Engineers, Inc and The Open
Group. (This is POSIX.1-2008 with the 2013 Technical Corrigendum 1
applied.) 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.unix.org/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 2013 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)