MSGSND(2) BSD System Calls Manual MSGSND(2)
msgsnd -- send a message to a message queue
msgsnd(int msqid, const void *msgp, size_t msgsz, int msgflg);
The msgsnd() function sends a message to the message queue specified by
msqid. msgp points to a structure containing the message. This struc-
ture 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 mes-
sages (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
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
o The condition which caused the call to block does no longer exist.
The message will be sent.
o The message queue is removed, in which case -1 will be returned, and
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 the message
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, the value 0 is returned; otherwise the
value -1 is returned and the global variable errno is set to indicate the
msgsnd() will fail if:
[EINVAL] msqid is not a valid message queue identifier.
msgsz is greater than msg_qbytes.
[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] msgp points to an invalid address.
[EINTR] The system call was interrupted by the delivery of a
[EIDRM] The message queue was removed while msgsnd() was wait-
ing for a resource to become available in order to
deliver the message.
msgctl(2), msgget(2), msgrcv(2)
Message queues appeared in AT&T System V Release 1 UNIX.
BSD March 27, 2017 BSD