msgrcv - receive a message from a message queue
msgrcv(int msqid, void *msgp, size_t msgsz, long msgtyp, int
The msgrcv() function receives a message from the message
in msqid, and places it into the structure pointed to by
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 value of msgtyp has one of the following meanings:
+o msgtyp is greater than 0. The first message of type
msgtyp will be
+o msgtyp is equal to 0. The first message on the queue
will be received.
+o msgtyp is less than 0. The first message of the lowest
that is less than or equal to the absolute value of
msgtyp will be
msgsz specifies the maximum length of the requested message.
If the received
message has a length greater than msgsz it will be
if the MSG_NOERROR flag is set in msgflg, otherwise an
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
return a value of -1, and set errno to EAGAIN. If
IPC_NOWAIT is not set,
the calling process will be blocked until:
+o A message of the requested type becomes available on the
+o The message queue is removed, in which case -1 will be
errno set to EINVAL.
+o A signal is received and caught. -1 is returned, and
errno set to
If a message is successfully received, the data structure
msqid is updated as follows:
+o msg_cbytes is decremented by the size of the message.
+o msg_lrpid is set to the pid of the caller.
+o msg_lrtime is set to the current time.
+o msg_qnum is decremented by 1.
Upon successful completion, msgrcv() returns the number of
into the mtext field of the structure pointed to by msgp.
is returned, and errno set to indicate the error.
msgrcv() will fail if:
[EINVAL] msqid is not a valid message queue identifier.
The message queue was removed while msgrcv()
for a message of the requested type to become
msgsz is less than 0.
[E2BIG] A matching message was received, but its size
than msgsz and the MSG_NOERROR flag was not
set in msgflg.
[EACCES] The calling process does not have read access
to the message
[EFAULT] msgp points to an invalid address.
[EINTR] The system call was interrupted by the delivery of a signal.
[EAGAIN] There is no message of the requested type
available on the
message queue, and IPC_NOWAIT is set in
msgctl(2), msgget(2), msgsnd(2)
Message queues appeared in the first release of AT&T Unix
OpenBSD does not define the EIDRM error value, which should
be used in
the case of a removed message queue, nor the ENOMSG value,
be used when no suitable message is available and IPC_NOWAIT
OpenBSD 3.6 August 17, 1995
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