The
msgsnd() function sends a message from the message queue specified in
msqid. The
msgp argument is a pointer to a user-defined structure containing the message. This structure must contain a first field of type
long that will indicate the user-defined type of the message. The remaining fields will contain the contents of the message. The following is an example of what this user-defined structure might look like:
struct mymsg {
long mtype; /* message type */
char mtext[1]; /* body of message */
};
The
mtype field is an integer greater than 0 that can be used for selecting messages (see
msgrcv(2)). The
mtext field is an array of bytes, with size up to the system limit
MSGMAX.
If the number of bytes already on the message queue plus
msgsz is greater than the maximum number of bytes in the message queue (
msg_qbytes, see
msgctl(2)), or if 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 no longer exists. The message was sent.
•
The message queue is removed, in which case -1 will be returned and errno set to EINVAL.
•
The caller catches a signal. The call returns with errno set to EINTR.
After a successful call, the data structure associated with the message queue is updated in the following way:
•
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.