Originální popis anglicky:
msgrcv - XSI message receive operation
Návod, kniha: POSIX Programmer's Manual
#include <sys/msg.h>
ssize_t msgrcv(int
msqid, void
*msgp, size_t
msgsz , long
msgtyp,
int
msgflg );
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.
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.
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.
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);
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.
None.
None.
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>
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
.
