MSGRCV(2) BSD Programmer's Manual MSGRCV(2)NAME
msgrcv - receive a message from a message queue
SYNOPSIS
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/msg.h>
int
msgrcv(int msqid, void *msgp, size_t msgsz, long msgtyp, int msgflg);
DESCRIPTION
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[1]; /* body of message */
The mtype field is an integer greater than 0 that can be used for select-
ing 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:
o The msgtyp is greater than 0. The first message of type msgtyp will
be received.
o The msgtyp is equal to 0. The first message on the queue will be re-
ceived.
o The msgtyp 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 function argument msgsz 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 re-
turn 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 message
queue.
o The message queue is removed, in which case -1 will be returned, and
errno set to EIDRM.
o 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:
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.
RETURN VALUES
Upon successful completion, msgrcv() 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 error.
ERRORSmsgrcv() will fail if:
[EINVAL] The msqid is not a valid message queue identifier
The function argument msgsz is less than 0.
[EIDRM] The message queue was removed while msgrcv() was waiting
for a message of the requested type to become available on
it.
[E2BIG] A matching message was received, but its size was greater
than msgsz and the MSG_NOERROR flag was not set in msgflg.
[EACCESS] The calling process does not have read access to the mes-
sage queue.
[EFAULT] msgp points to an invalid address.
[EINTR] The system call was interrupted by the delivery of a sig-
nal.
[ENOMSG] There is no message of the requested type available on the
message queue, and IPC_NOWAIT is set in msgflg.
SEE ALSOmsgsnd(2), msgctl(2), msgget2
4th Berkeley Distribution July 3, 1996 2