NAME

msgsnd — send a message to a message queue

LIBRARY

Standard C Library (libc, -lc)

SYNOPSIS

#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/msg.h>

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

DESCRIPTION

The msgsnd() function sends a message to the message queue specified in msqid. The msgp argument points to a structure containing the message. This structure should consist of the following members:

    long mtype;    /* message type */
    char mtext[1]; /* body of message */

mtype is an integer greater than 0 that can be used for selecting messages (see msgrcv(2)), mtext is an array of msgsz bytes. The argument msgsz can range from 0 to a system-imposed maximum, MSGMAX.

If the number of bytes already on the message queue plus msgsz is bigger than the maximum number of bytes on the message queue (msg_qbytes, see msgctl(2)), or the number of messages on all queues system-wide is already equal to the system limit, msgflg determines the action of msgsnd(). If msgflg has IPC_NOWAIT mask set in it, the call will return immediately. If msgflg does not have IPC_NOWAIT set in it, the call will block until:

• The condition which caused the call to block does no longer exist. The message will be sent.
• The message queue is removed, in which case -1 will be returned, and errno is set to EINVAL.
• The caller catches a signal. The call returns with errno set to EINTR.

After a successful call, the data structure associated with the message queue is updated in the following way:

• msg_cbytes is incremented by the size of the message.
• msg_qnum is incremented by 1.
• msg_lspid is set to the pid of the calling process.
• msg_stime is set to the current time.

RETURN VALUES

The msgsnd() function returns the value 0 if successful; otherwise the value -1 is returned and the global variable errno is set to indicate the error.

ERRORS

The msgsnd() function will fail if:

[EINVAL]

The msqid argument is not a valid message queue identifier.

The message queue was removed while msgsnd() was waiting for a resource to become available in order to deliver the message.

The msgsz argument is greater than msg_qbytes.

The mtype argument is not greater than 0.

[EACCES]

The calling process does not have write access to the message queue.

[EAGAIN]

There was no space for this message either on the queue, or in the whole system, and IPC_NOWAIT was set in msgflg.

[EFAULT]

The msgp argument points to an invalid address.

[EINTR]

The system call was interrupted by the delivery of a signal.

HISTORY

Message queues appeared in the first release of AT&T Unix System V.

BUGS

NetBSD and FreeBSD do not define the EIDRM error value, which should be used in the case of a removed message queue.