msgsnd - send a message to a message queue
msgsnd(int msqid, const void *msgp, size_t msgsz, int
The msgsnd() function sends a message from the message queue
msqid. msgp points to a structure containing the message.
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 bytes, with a
size up to 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
msgctl(2)), or the number of messages on all queues systemwide is already
equal to the system limit, msgflg determines the action of
msgsnd(). If msgflg has IPC_NOWAIT mask set in it, the call
immediately. If msgflg does not have IPC_NOWAIT set in it,
the call will
+o The condition which caused the call to block does no
The message will be sent.
+o The message queue is removed, in which case -1 will be
errno is set to EINVAL.
+o The caller catches a signal. The call returns with
errno set to
After a successful call, the data structure associated with
queue is updated in the following way:
+o msg_cbytes is incremented by the size of the message.
+o msg_qnum is incremented by 1.
+o msg_lspid is set to the pid of the calling process.
+o msg_stime is set to the current time.
Upon successful completion, 0 is returned. Otherwise, -1 is
errno is set to indicate the error.
msgsnd() will fail if:
[EINVAL] msqid is not a valid message queue identifier.
The message queue was removed while msgsnd()
for a resource to become available in order to
msgsz is less than 0, or greater than
[EACCES] The calling process does not have write access
to the message
[EAGAIN] There was no space for this message either on
the queue, or
in the whole system, and IPC_NOWAIT was set in
[EFAULT] msgp points to an invalid address.
[EINTR] The system call was interrupted by the delivery of a signal.
msgctl(2), msgget(2), msgrcv(2)
Message queues appeared in the first release of AT&T Unix
OpenBSD does not define the EIDRM error value, which should
be used in
the case of a removed message queue.
OpenBSD 3.6 August 17, 1995
[ Back ]