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MSGSND(2) System Calls Manual MSGSND(2)

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:
[]
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.
[]
The calling process does not have write access to the message queue.
[]
There was no space for this message either on the queue, or in the whole system, and IPC_NOWAIT was set in msgflg.
[]
The msgp argument points to an invalid address.
[]
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.
July 9, 2009 Debian