The Single UNIX ® Specification, Version 2
Copyright © 1997 The Open Group


msgsnd - message send operation


#include <sys/msg.h>

int msgsnd(int msqid, const void *msgp, size_t msgsz, int msgflg);


The msgsnd() function is used to send a message to the queue associated with the message queue identifier specified by msqid.

The argument msgp points to a user-defined buffer that must contain first a field of type long int that will specify the type of the message, and then a data portion that will hold the data bytes of the message. The structure below is an example of what this user-defined buffer might look like:

struct mymsg {
    long int    mtype;       /* message type */
    char        mtext[1];    /* message text */

The structure member mtype is a non-zero positive type long int 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 are true:

These actions are as follows:

Upon successful completion, the following actions are taken with respect to the data structure associated with msqid, see <sys/msg.h>:


Upon successful completion, msgsnd() returns 0. Otherwise, no message will be sent, msgsnd() will return -1 and errno will be set to indicate the error.


The msgsnd() function will fail if:
Operation permission is denied to the calling process. See IPC.
The message cannot be sent for one of the reasons cited above and (msgflg & IPC_NOWAIT) is non-zero.
The message queue identifier msgid is removed from the system.
The msgsnd() function was interrupted by a signal.
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 POSIX Realtime Extension defines alternative interfaces for interprocess communication. Application developers who need to use IPC should design their applications so that modules using the IPC routines described in IPC can be easily modified to use the alternative interfaces.




mq_close(), mq_getattr(), mq_notify(), mq_open(), mq_receive(), mq_send(), mq_setattr(), mq_unlink(), msgctl(), msgget(), msgrcv(), sigaction(), <sys/msg.h>, IPC.


Derived from Issue 2 of the SVID.

UNIX ® is a registered Trademark of The Open Group.
Copyright © 1997 The Open Group
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