NAME

msgsnd — XSI message send operation

SYNOPSIS

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

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

DESCRIPTION

The msgsnd() function operates on XSI message queues (see XBD 3.206 Message Queue ). It is unspecified whether this function interoperates with the realtime interprocess communication facilities defined in 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 .

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 2.7.1 IPC General Description .

RETURN VALUE

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.

ERRORS

The msgsnd() function shall fail if:

[EACCES]

Operation permission is denied to the calling process; see 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.

EXAMPLES

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);

APPLICATION USAGE

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 2.7 XSI Interprocess Communication can be easily modified to use the alternative interfaces.

RATIONALE

None.

FUTURE DIRECTIONS

None.

SEE ALSO

2.7 XSI Interprocess Communication , 2.8 Realtime , mq_close , mq_getattr , mq_notify , mq_open , mq_receive , mq_send , mq_setattr , mq_unlink , msgctl , msgget , msgrcv , sigaction

XBD 3.206 Message Queue , <sys/msg.h>

CHANGE HISTORY

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.