#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
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[1]; /* 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
msgrcv ();
depends on whether the
IPC_NOWAIT
flag is set in
Fa msgflg
or not.
If
IPC_NOWAIT
is set,
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 VALUES
Upon successful completion,
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.
ERRORS
The
msgrcv ();
function
will fail if:
Bq Er EINVAL
The
Fa msqid
argument
is not a valid message queue identifier.
The message queue was removed while
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 .
