table of contents
|MSGSND(2)||System Calls Manual||MSGSND(2)|
LIBRARY¶Standard C Library (libc, -lc)
msqid, const void
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
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 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
- 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.
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.
msgsnd() function will fail if:
- 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.
- 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
IPC_NOWAITwas set in msgflg.
- The msgp argument points to an invalid address.
- The system call was interrupted by the delivery of a signal.
HISTORY¶Message queues appeared in the first release of AT&T Unix System V.
BUGS¶NetBSD and FreeBSD do not define the
EIDRMerror value, which should be used in the case of a removed message queue.
|July 9, 2009||Linux 4.9.0-9-amd64|