NAME

msgsnd - message send operation

 SYNOPSIS

#include <sys/msg.h>

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

 DESCRIPTION

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:

• 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 will not be sent and the calling thread will return immediately.

• If (msgflg & IPC_NOWAIT) is 0, the calling thread will 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 is set equal to [EIDRM] and -1 is 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 is incremented by 1.

• msg_lspid is set equal to the process ID of the calling process.

• msg_stime is set equal to the current time.

 RETURN VALUE

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.

 ERRORS

The msgsnd() function will fail if:

[EACCES]

Operation permission is denied to the calling process. See IPC.

[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 msgid 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.

 EXAMPLES

None.

 APPLICATION USAGE

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.

 FUTURE DIRECTIONS

None.

 SEE ALSO

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

DERIVATION

Derived from Issue 2 of the SVID.