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msgsnd — send a message to a message queue
Standard C Library (libc, -lc)
msgsnd(int msqid, const void *msgp, size_t msgsz, int msgflg);
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; /* 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
· 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
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
The msgsnd() function will fail if:
[EINVAL] The msqid argument is not a valid message queue
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
[EAGAIN] There was no space for this message either on the
queue, or in the whole system, and IPC_NOWAIT was set
[EFAULT] The msgp argument points to an invalid address.
[EINTR] The system call was interrupted by the delivery of a
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.