GETMSG(3P) POSIX Programmer's Manual GETMSG(3P)
PROLOG
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.
NAME
getmsg, getpmsg - receive next message from a STREAMS file (STREAMS)
SYNOPSIS
#include <stropts.h>
int getmsg(int fildes, struct strbuf *restrict ctlptr,
struct strbuf *restrict dataptr, int *restrict flagsp);
int getpmsg(int fildes, struct strbuf *restrict ctlptr,
struct strbuf *restrict dataptr, int *restrict bandp,
int *restrict flagsp);
DESCRIPTION
The getmsg() function shall retrieve the contents of a message located
at the head of the STREAM head read queue associated with a STREAMS
file and place the contents into one or more buffers. The message con-
tains either a data part, a control part, or both. The data and control
parts of the message shall be placed into separate buffers, as
described below. The semantics of each part are defined by the origina-
tor of the message.
The getpmsg() function shall be equivalent to getmsg(), except that it
provides finer control over the priority of the messages received.
Except where noted, all requirements on getmsg() also pertain to
getpmsg().
The fildes argument specifies a file descriptor referencing a STREAMS-
based file.
The ctlptr and dataptr arguments each point to a strbuf structure, in
which the buf member points to a buffer in which the data or control
information is to be placed, and the maxlen member indicates the maxi-
mum number of bytes this buffer can hold. On return, the len member
shall contain the number of bytes of data or control information actu-
ally received. The len member shall be set to 0 if there is a zero-
length control or data part and len shall be set to -1 if no data or
control information is present in the message.
When getmsg() is called, flagsp should point to an integer that indi-
cates the type of message the process is able to receive. This is
described further below.
The ctlptr argument is used to hold the control part of the message,
and dataptr is used to hold the data part of the message. If ctlptr (or
dataptr) is a null pointer or the maxlen member is -1, the control (or
data) part of the message shall not be processed and shall be left on
the STREAM head read queue, and if the ctlptr (or dataptr) is not a
null pointer, len shall be set to -1. If the maxlen member is set to 0
and there is a zero-length control (or data) part, that zero-length
part shall be removed from the read queue and len shall be set to 0. If
the maxlen member is set to 0 and there are more than 0 bytes of con-
trol (or data) information, that information shall be left on the read
queue and len shall be set to 0. If the maxlen member in ctlptr (or
dataptr) is less than the control (or data) part of the message, maxlen
bytes shall be retrieved. In this case, the remainder of the message
shall be left on the STREAM head read queue and a non-zero return value
shall be provided.
By default, getmsg() shall process the first available message on the
STREAM head read queue. However, a process may choose to retrieve only
high-priority messages by setting the integer pointed to by flagsp to
RS_HIPRI. In this case, getmsg() shall only process the next message if
it is a high-priority message. When the integer pointed to by flagsp is
0, any available message shall be retrieved. In this case, on return,
the integer pointed to by flagsp shall be set to RS_HIPRI if a high-
priority message was retrieved, or 0 otherwise.
For getpmsg(), the flags are different. The flagsp argument points to a
bitmask with the following mutually-exclusive flags defined: MSG_HIPRI,
MSG_BAND, and MSG_ANY. Like getmsg(), getpmsg() shall process the
first available message on the STREAM head read queue. A process may
choose to retrieve only high-priority messages by setting the integer
pointed to by flagsp to MSG_HIPRI and the integer pointed to by bandp
to 0. In this case, getpmsg() shall only process the next message if it
is a high-priority message. In a similar manner, a process may choose
to retrieve a message from a particular priority band by setting the
integer pointed to by flagsp to MSG_BAND and the integer pointed to by
bandp to the priority band of interest. In this case, getpmsg() shall
only process the next message if it is in a priority band equal to, or
greater than, the integer pointed to by bandp, or if it is a high-pri-
ority message. If a process wants to get the first message off the
queue, the integer pointed to by flagsp should be set to MSG_ANY and
the integer pointed to by bandp should be set to 0. On return, if the
message retrieved was a high-priority message, the integer pointed to
by flagsp shall be set to MSG_HIPRI and the integer pointed to by bandp
shall be set to 0. Otherwise, the integer pointed to by flagsp shall be
set to MSG_BAND and the integer pointed to by bandp shall be set to the
priority band of the message.
If O_NONBLOCK is not set, getmsg() and getpmsg() shall block until a
message of the type specified by flagsp is available at the front of
the STREAM head read queue. If O_NONBLOCK is set and a message of the
specified type is not present at the front of the read queue, getmsg()
and getpmsg() shall fail and set errno to [EAGAIN].
If a hangup occurs on the STREAM from which messages are retrieved,
getmsg() and getpmsg() shall continue to operate normally, as described
above, until the STREAM head read queue is empty. Thereafter, they
shall return 0 in the len members of ctlptr and dataptr.
RETURN VALUE
Upon successful completion, getmsg() and getpmsg() shall return a non-
negative value. A value of 0 indicates that a full message was read
successfully. A return value of MORECTL indicates that more control
information is waiting for retrieval. A return value of MOREDATA indi-
cates that more data is waiting for retrieval. A return value of the
bitwise-logical OR of MORECTL and MOREDATA indicates that both types of
information remain. Subsequent getmsg() and getpmsg() calls shall
retrieve the remainder of the message. However, if a message of higher
priority has come in on the STREAM head read queue, the next call to
getmsg() or getpmsg() shall retrieve that higher-priority message
before retrieving the remainder of the previous message.
If the high priority control part of the message is consumed, the mes-
sage shall be placed back on the queue as a normal message of band 0.
Subsequent getmsg() and getpmsg() calls shall retrieve the remainder of
the message. If, however, a priority message arrives or already exists
on the STREAM head, the subsequent call to getmsg() or getpmsg() shall
retrieve the higher-priority message before retrieving the remainder of
the message that was put back.
Upon failure, getmsg() and getpmsg() shall return -1 and set errno to
indicate the error.
ERRORS
The getmsg() and getpmsg() functions shall fail if:
EAGAIN The O_NONBLOCK flag is set and no messages are available.
EBADF The fildes argument is not a valid file descriptor open for
reading.
EBADMSG
The queued message to be read is not valid for getmsg() or
getpmsg() or a pending file descriptor is at the STREAM head.
EINTR A signal was caught during getmsg() or getpmsg().
EINVAL An illegal value was specified by flagsp, or the STREAM or mul-
tiplexer referenced by fildes is linked (directly or indirectly)
downstream from a multiplexer.
ENOSTR A STREAM is not associated with fildes.
In addition, getmsg() and getpmsg() shall fail if the STREAM head had
processed an asynchronous error before the call. In this case, the
value of errno does not reflect the result of getmsg() or getpmsg() but
reflects the prior error.
The following sections are informative.
EXAMPLES
Getting Any Message
In the following example, the value of fd is assumed to refer to an
open STREAMS file. The call to getmsg() retrieves any available message
on the associated STREAM-head read queue, returning control and data
information to the buffers pointed to by ctrlbuf and databuf, respec-
tively.
#include <stropts.h>
...
int fd;
char ctrlbuf[128];
char databuf[512];
struct strbuf ctrl;
struct strbuf data;
int flags = 0;
int ret;
ctrl.buf = ctrlbuf;
ctrl.maxlen = sizeof(ctrlbuf);
data.buf = databuf;
data.maxlen = sizeof(databuf);
ret = getmsg (fd, &ctrl, &data, &flags);
Getting the First Message off the Queue
In the following example, the call to getpmsg() retrieves the first
available message on the associated STREAM-head read queue.
#include <stropts.h>
...
int fd;
char ctrlbuf[128];
char databuf[512];
struct strbuf ctrl;
struct strbuf data;
int band = 0;
int flags = MSG_ANY;
int ret;
ctrl.buf = ctrlbuf;
ctrl.maxlen = sizeof(ctrlbuf);
data.buf = databuf;
data.maxlen = sizeof(databuf);
ret = getpmsg (fd, &ctrl, &data, &band, &flags);
APPLICATION USAGE
None.
RATIONALE
None.
FUTURE DIRECTIONS
None.
SEE ALSO
STREAMS, poll(), putmsg(), read(), write(), the Base Definitions volume
of IEEE Std 1003.1-2001, <stropts.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 .
IEEE/The Open Group 2003 GETMSG(3P)
Linux-Distributionen
Huschi als Linux-Admin