Originální popis anglicky:
msgsnd - XSI message send operation
Návod, kniha: POSIX Programmer's Manual
#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 IEEE Std 1003.1-2001, Section 3.224,
Message Queue). It is unspecified whether this function interoperates with the
realtime interprocess communication facilities defined in
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 equal 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() .
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 equal to the process ID of
the calling process.
- *
- msg_stime shall be set equal to the current
time.
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
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 less than 0 or greater than the system-imposed limit.
The following sections are informative.
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
XSI
Interprocess Communication can be easily modified to use the alternative
interfaces.
None.
None.
XSI Interprocess Communication ,
Realtime ,
mq_close() ,
mq_getattr() ,
mq_notify() ,
mq_open() ,
mq_receive() ,
mq_send() ,
mq_setattr() ,
mq_unlink() ,
msgctl() ,
msgget() ,
msgrcv() ,
sigaction() , the Base Definitions volume of
IEEE Std 1003.1-2001,
<sys/msg.h>
Portions of this text are reprinted and reproduced in electronic form from IEEE
Std 1003.1, 2003 Edition, Standard for Information Technology -- Portable
Operating System Interface (POSIX), The Open Group Base Specifications Issue
6, Copyright (C) 2001-2003 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
.
