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MSGRCV(3P) POSIX Programmer's Manual MSGRCV(3P)
This manual page is part of the POSIX Programmer's Manual. The Linux
implementation of this interface may differ (consult the
corresponding Linux manual page for details of Linux behavior), or
the interface may not be implemented on Linux.
msgrcv — XSI message receive operation
#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 POSIX.1‐2008, Section 3.225, Message Queue).
It is unspecified whether this function interoperates with the
realtime interprocess communication facilities defined in Section
2.8, 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 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(3p).
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 to the process ID of the calling process.
* msg_rtime shall be set to the current time, as described in
Section 2.7.1, IPC General Description.
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 −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
Section 2.7, 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.
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);
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 Section 2.7, XSI Interprocess Communication
can be easily modified to use the alternative interfaces.
None.
None.
Section 2.7, XSI Interprocess Communication, Section 2.8, Realtime,
mq_close(3p), mq_getattr(3p), mq_notify(3p), mq_open(3p),
mq_receive(3p), mq_send(3p), mq_setattr(3p), mq_unlink(3p),
msgctl(3p), msgget(3p), msgsnd(3p), sigaction(3p)
The Base Definitions volume of POSIX.1‐2008, Section 3.225, Message
Queue, sys_msg.h(0p)
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2013 Edition, Standard for Information
Technology -- Portable Operating System Interface (POSIX), The Open
Group Base Specifications Issue 7, Copyright (C) 2013 by the
Institute of Electrical and Electronics Engineers, Inc and The Open
Group. (This is POSIX.1-2008 with the 2013 Technical Corrigendum 1
applied.) 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.unix.org/online.html .
Any typographical or formatting errors that appear in this page are
most likely to have been introduced during the conversion of the
source files to man page format. To report such errors, see
https://www.kernel.org/doc/man-pages/reporting_bugs.html .
IEEE/The Open Group 2013 MSGRCV(3P)
Pages that refer to this page: sys_msg.h(0p), ipcs(1p), mq_close(3p), mq_getattr(3p), mq_notify(3p), mq_open(3p), mq_receive(3p), mq_setattr(3p), mq_unlink(3p), msgctl(3p), msgget(3p), msgsnd(3p)