SYNOPSISIn sys/types.h In sys/ipc.h In sys/msg.h Ft int Fn msgrcv int msqid void *msgp size_t msgsz long msgtyp int msgflg
DESCRIPTIONThe Fn msgrcv function receives a message from the message queue specified in Fa msqid , and places it into the structure pointed to by Fa 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 Fa msgtyp has one of the following meanings:
- The Fa msgtyp argument is greater than 0. The first message of type Fa msgtyp will be received.
- The Fa msgtyp argument is equal to 0. The first message on the queue will be received.
- The Fa 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 Fa msgtyp will be received.
The Fa msgsz argument specifies the maximum length of the requested message. If the received message has a length greater than Fa msgsz it will be silently truncated if the MSG_NOERROR flag is set in Fa msgflg , otherwise an error will be returned.
If no matching message is present on the message queue specified by Fa msqid , the behavior of Fn msgrcv depends on whether the IPC_NOWAIT flag is set in Fa msgflg or not. If IPC_NOWAIT is set, Fn msgrcv will immediately return a value of -1, and set errno to Er 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 Er EINVAL .
- A signal is received and caught. -1 is returned, and errno set to Er EINTR .
If a message is successfully received, the data structure associated with Fa 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.
RETURN VALUESUpon successful completion, Fn msgrcv returns the number of bytes received into the mtext field of the structure pointed to by Fa msgp . Otherwise, -1 is returned, and errno set to indicate the error.
ERRORSThe Fn msgrcv function will fail if:
- Bq Er EINVAL
is not a valid message queue identifier.
The message queue was removed while Fn msgrcv was waiting for a message of the requested type to become available on it.
The Fa msgsz argument is less than 0.
- Bq Er E2BIG
- A matching message was received, but its size was greater than Fa msgsz and the MSG_NOERROR flag was not set in Fa msgflg .
- Bq Er EACCES
- The calling process does not have read access to the message queue.
- Bq Er EFAULT
- The Fa msgp argument points to an invalid address.
- Bq Er EINTR
- The system call was interrupted by the delivery of a signal.
- Bq Er ENOMSG
- There is no message of the requested type available on the message queue, and IPC_NOWAIT is set in Fa msgflg .
HISTORYMessage queues appeared in the first release of AT&T System V .