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MSGSND(2) System Calls Manual MSGSND(2)


msgsndsend a message to a message queue


Standard C Library (libc, -lc)


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

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


The () 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 (). 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.


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.


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.


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


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