July 9, 2009
msgsnd - send a message to a message queue
.In sys/msg.h int
msgsnd int msqid const void *msgp size_t msgsz int msgflg
msgsndfunction 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,
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_NOWAITmask set in it, the call will return immediately. If msgflg does not have
IPC_NOWAITset 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.
.Rv -std msgsnd
msgsndfunction will fail if:
|The msqid argument is not a valid message queue identifier.
The message queue was removed while
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 system, and
|The msgp argument points to an invalid address.|
|The system call was interrupted by the delivery of a signal.|
Message queues appeared in the first release of AT&T Unix System V.
.Fx do not define the
EIDRMerror value, which should be used in the case of a removed message queue.