NAME

msgsnd — send a message to a message queue

SYNOPSIS

#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 by msqid. msgp 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, with a size between 0 and that of the system limit (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 EIDRM.
• 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

Upon successful completion, the value 0 is returned; otherwise the value -1 is returned and the global variable errno is set to indicate the error.

ERRORS

msgsnd() will fail if:

[EINVAL]

msqid is not a valid message queue identifier.

mtype is less than 1.

msgsz is greater than msg_qbytes.

[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]

msgp points to an invalid address.

[EINTR]

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

[EIDRM]

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

SEE ALSO

msgctl(2), msgget(2), msgrcv(2)

STANDARDS

The msgsnd() function conforms to the X/Open System Interfaces option of IEEE Std 1003.1-2008 (“POSIX.1”).

HISTORY

Message queues first appeared in AT&T System V Release 1 UNIX and have been available since NetBSD 1.0.