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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)
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