table of contents
|MSGRCV(2)||System Calls Manual||MSGRCV(2)|
msgrcv — receive a
message from a message queue
Standard C Library (libc, -lc)
msqid, void *msgp,
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
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
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
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
depends on whether the
IPC_NOWAIT flag is set in
msgflg or not. If
msgrcv() will immediately return a value of
-1, and set errno to
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
- A signal is received and caught. -1 is returned, and
errno set to
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.
Upon successful completion,
returns the number of bytes received into the mtext
field of the structure pointed to by msgp. Otherwise,
-1 is returned, and errno set to indicate the
msgrcv() function will fail if:
- The msqid argument is not a valid message queue
The message queue was removed while
msgrcv() was waiting for a message of the requested type to become available on it.
The msgsz argument is less than 0.
- A matching message was received, but its size was greater than
msgsz and the
MSG_NOERRORflag was not set in msgflg.
- The calling process does not have read access to the message queue.
- The msgp argument points to an invalid address.
- The system call was interrupted by the delivery of a signal.
- There is no message of the requested type available on the message queue,
IPC_NOWAITis set in msgflg.
Message queues appeared in the first release of AT&T System V UNIX.
|July 28, 2016||Debian|