NAME
msgrcv - XSI message receive operation
SYNOPSIS
#include <sys/msg.h>
ssize_t msgrcv(int msqid, void *msgp, size_t msgsz, long msgtyp,
int msgflg);
DESCRIPTION
The msgrcv() function operates on XSI message queues (see the Base
Definitions volume of IEEE Std 1003.1-2001, Section 3.224, Message
Queue). It is unspecified whether this function interoperates with the
realtime interprocess communication facilities defined in Realtime .
The msgrcv() function shall read a message from the queue associated
with the message queue identifier specified by msqid and place it in
the user-defined buffer pointed to by msgp.
The application shall ensure that the argument msgp points to a user-
defined buffer that contains first a field of type long specifying the
type of the message, and then a data portion that holds the data bytes
of the message. The structure below is an example of what this user-
defined buffer might look like:
struct mymsg {
long mtype; /* Message type. */
char mtext[1]; /* Message text. */
}
The structure member mtype is the received message’s type as specified
by the sending process.
The structure member mtext is the text of the message.
The argument msgsz specifies the size in bytes of mtext. The received
message shall be truncated to msgsz bytes if it is larger than msgsz
and (msgflg & MSG_NOERROR) is non-zero. The truncated part of the
message shall be lost and no indication of the truncation shall be
given to the calling process.
If the value of msgsz is greater than {SSIZE_MAX}, the result is
implementation-defined.
The argument msgtyp specifies the type of message requested as follows:
* If msgtyp is 0, the first message on the queue shall be received.
* If msgtyp is greater than 0, the first message of type msgtyp shall
be received.
* If msgtyp is less than 0, the first message of the lowest type that
is less than or equal to the absolute value of msgtyp shall be
received.
The argument msgflg specifies the action to be taken if a message of
the desired type is not on the queue. These are as follows:
* If (msgflg & IPC_NOWAIT) is non-zero, the calling thread shall
return immediately with a return value of -1 and errno set to
[ENOMSG].
* If (msgflg & IPC_NOWAIT) is 0, the calling thread shall suspend
execution until one of the following occurs:
* A message of the desired type is placed on the queue.
* The message queue identifier msqid is removed from the system;
when this occurs, errno shall be set equal to [EIDRM] and -1
shall be returned.
* The calling thread receives a signal that is to be caught; in
this case a message is not received and the calling thread
resumes execution in the manner prescribed in sigaction() .
Upon successful completion, the following actions are taken with
respect to the data structure associated with msqid:
* msg_qnum shall be decremented by 1.
* msg_lrpid shall be set equal to the process ID of the calling
process.
* msg_rtime shall be set equal to the current time.
RETURN VALUE
Upon successful completion, msgrcv() shall return a value equal to the
number of bytes actually placed into the buffer mtext. Otherwise, no
message shall be received, msgrcv() shall return (ssize_t)-1, and errno
shall be set to indicate the error.
ERRORS
The msgrcv() function shall fail if:
E2BIG The value of mtext is greater than msgsz and (msgflg &
MSG_NOERROR) is 0.
EACCES Operation permission is denied to the calling process; see XSI
Interprocess Communication .
EIDRM The message queue identifier msqid is removed from the system.
EINTR The msgrcv() function was interrupted by a signal.
EINVAL msqid is not a valid message queue identifier.
ENOMSG The queue does not contain a message of the desired type and
(msgflg & IPC_NOWAIT) is non-zero.
The following sections are informative.
EXAMPLES
Receiving a Message
The following example receives the first message on the queue (based on
the value of the msgtyp argument, 0). The queue is identified by the
msqid argument (assuming that the value has previously been set). This
call specifies that an error should be reported if no message is
available, but not if the message is too large. The message size is
calculated directly using the sizeof operator.
#include <sys/msg.h>
...
int result;
int msqid;
struct message {
long type;
char text[20];
} msg;
long msgtyp = 0;
...
result = msgrcv(msqid, (void *) &msg, sizeof(msg.text),
msgtyp, MSG_NOERROR | IPC_NOWAIT);
APPLICATION USAGE
The POSIX Realtime Extension defines alternative interfaces for
interprocess communication (IPC). Application developers who need to
use IPC should design their applications so that modules using the IPC
routines described in XSI Interprocess Communication can be easily
modified to use the alternative interfaces.
RATIONALE
None.
FUTURE DIRECTIONS
None.
SEE ALSO
XSI Interprocess Communication , Realtime , mq_close() , mq_getattr() ,
mq_notify() , mq_open() , mq_receive() , mq_send() , mq_setattr() ,
mq_unlink() , msgctl() , msgget() , msgsnd() , sigaction() , the Base
Definitions volume of IEEE Std 1003.1-2001, <sys/msg.h>
COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 6, Copyright (C) 2001-2003 by the Institute of
Electrical and Electronics Engineers, Inc and The Open Group. In the
event of any discrepancy between this version and the original IEEE and
The Open Group Standard, the original IEEE and The Open Group Standard
is the referee document. The original Standard can be obtained online
at http://www.opengroup.org/unix/online.html .