The Open Group Base Specifications Issue 7, 2018 edition
IEEE Std 1003.1-2017 (Revision of IEEE Std 1003.1-2008)
Copyright © 2001-2018 IEEE and The Open Group


msgsnd - XSI message send operation


[XSI] [Option Start] #include <sys/msg.h>

int msgsnd(int
msqid, const void *msgp, size_t msgsz, int msgflg); [Option End]


The msgsnd() function operates on XSI message queues (see XBD 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:

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() 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:

Operation permission is denied to the calling process; see XSI Interprocess Communication.
The message cannot be sent for one of the reasons cited above and (msgflg & IPC_NOWAIT) is non-zero.
The message queue identifier msqid 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 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 XSI Interprocess Communication can be easily modified to use the alternative interfaces.






XSI Interprocess Communication, Realtime, mq_close, mq_getattr, mq_notify, mq_open, mq_receive, mq_send, mq_setattr, mq_unlink, msgctl, msgget, msgrcv, sigaction

XBD Message Queue, <sys/msg.h>


First released in Issue 2. Derived from Issue 2 of the SVID.

Issue 5

The note about use of POSIX Realtime Extension IPC routines has been moved from FUTURE DIRECTIONS to a new APPLICATION USAGE section.

Issue 6

The normative text is updated to avoid use of the term "must" for application requirements.

Issue 7

POSIX.1-2008, Technical Corrigendum 1, XSH/TC1-2008/0400 [345] and XSH/TC1-2008/0401 [359] are applied.

End of informative text.


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