The msgrcv function receives a message from the message queue specified in msqid, and places it into the structure pointed to by msgp. 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, mtext is an array of bytes, with a size up to that of the system limit ( MSGMAX).
The value of msgtyp has one of the following meanings:
- The msgtyp argument is greater than 0. The first message of type msgtyp will be received.
- The msgtyp argument is equal to 0. The first message on the queue will be received.
- The msgtyp argument is less than 0. The first message of the lowest message type that is less than or equal to the absolute value of msgtyp will be received.
The msgsz argument specifies the maximum length of the requested message. If the received message has a length greater than msgsz it will be silently truncated if the MSG_NOERROR flag is set in msgflg, otherwise an error will be returned.
If no matching message is present on the message queue specified by msqid, the behavior of msgrcv depends on whether the IPC_NOWAIT flag is set in msgflg or not. If IPC_NOWAIT is set, msgrcv will immediately return a value of -1, and set errno to ENOMSG. If IPC_NOWAIT is not set, the calling process will be blocked until:
- A message of the requested type becomes available on the message queue.
- The message queue is removed, in which case -1 will be returned, and errno set to EINVAL.
- A signal is received and caught. -1 is returned, and errno set to EINTR.
If a message is successfully received, the data structure associated with msqid is updated as follows:
- msg_cbytes is decremented by the size of the message.
- msg_lrpid is set to the pid of the caller.
- msg_lrtime is set to the current time.
- msg_qnum is decremented by 1.