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MSGSND(2) System Calls Manual MSGSND(2)

msgsndsend a message to a message queue

#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 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 (). 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:

After a successful call, the data structure associated with the message queue is updated in the following way:

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.

msgsnd() will fail if:

msqid is not a valid message queue identifier.

mtype is less than 1.

msgsz is greater than msg_qbytes.

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.
msgp points to an invalid address.
The system call was interrupted by the delivery of a signal.
The message queue was removed while msgsnd() was waiting for a resource to become available in order to deliver the message.

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

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

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

July 18, 2019 OpenBSD-7.2