Skip site navigation (1)Skip section navigation (2)

FreeBSD Manual Pages

  
 
  

home | help
getmsg(2)			 System	Calls			     getmsg(2)

NAME
       getmsg, getpmsg - get next message off a	stream

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	retrieves  the contents	of a message (see  in-
       tro(2)) located at  the stream head read	queue from a STREAMS file, and
       places  the  contents  into  user specified buffer(s). The message must
       contain either a	data part, a control part, or both. The	data and  con-
       trol  parts  of	the  message are placed	into separate buffers,	as de-
       scribed below. The semantics of each part is  defined  by  the  STREAMS
       module that generated  the message.

       The   getpmsg() function	behaved	like getmsg(), but provides finer con-
       trol over the priority of the messages received.	 Except	 where	noted,
       all information pertaining to getmsg() also pertains to getpmsg().

       The  fildes  argument  specifies	 a file	descriptor referencing an open
       stream.	The ctlptr and dataptr arguments each point to a strbuf	struc-
       ture, which contains the	following members:

       int    maxlen;	   /* maximum buffer length */
       int    len;	   /* length of	data */
       char   *buf;	   /* ptr to buffer */

       The buf member points to	a buffer into which the	data or	control	infor-
       mation is to be placed, and the maxlen  member  indicates  the  maximum
       number  of  bytes  this buffer can hold.	On return, the len member con-
       tains the number	of bytes of data or control information	 actually  re-
       ceived;	0  if there is a zero-length control or	data part; or -1 if no
       data or control information is present in the message. The flagsp argu-
       ment  should point to an	integer	that indicates the type	of message the
       user is able to receive,	as described below.

       The ctlptr argument holds the control part from	the  message  and  the
       dataptr	argument   holds the data part from the	message. If ctlptr (or
       dataptr)	is NULL	or the maxlen member is	-1,   the  control  (or	 data)
       part  of	 the  message  is not processed	and is left on the stream head
       read queue. If ctlptr (or dataptr) is not NULL and there	is  no	corre-
       sponding	control	(or data) part of the messages on the stream head read
       queue, len is 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 is removed
       from the	read queue and len is set to 0.	If the maxlen member is	set to
       0  and there are	more than zero bytes of	control	(or data) information,
       that information	is left	on the read queue and len is set to 0. If  the
       maxlen member in	ctlptr or dataptr is less than,
	respectively,  the  control  or	data part of the message, maxlen bytes
       are  retrieved. In this case, the remainder of the message is  left  on
       the stream head read  queue and a non-zero return value is provided, as
       described below under  RETURN VALUES.

       By default, getmsg() processes  the  first  available  message  on  the
       stream  head  read  queue. A user may, however, choose to retrieve only
       high priority messages by setting  the integer pointed to by flagsp  to
       RS_HIPRI.  In this case,	getmsg() processes the next message only if it
       is a high priority message.

       If the integer pointed to by flagsp is 0, getmsg() retrieves  any  mes-
       sage  available on the stream head read queue. In this case, on return,
       the integer pointed to by flagsp	will be	set to	 RS_HIPRI  if  a  high
       priority	message	was retrieved, or to 0 otherwise.

       For getpmsg(), the flagsp argument points to a bitmask with the follow-
       ing mutually-exclusive flags defined: MSG_HIPRI,	MSG_BAND, and MSG_ANY.
       Like  getmsg(),	getpmsg() processes the	first available	message	on the
       stream head read	queue. A user may choose to retrieve only  high-prior-
       ity  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()  will
       only  process  the  next	message	if it is a high-priority message. In a
       similar manner, a user may choose to retrieve a message from a particu-
       lar  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() will 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-priority message. If a user just 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-pri-
       ority  message,	the  integer  pointed  to  by  flagsp  will  be	set to
       MSG_HIPRI and the integer pointed to by bandp will be set to 0.	Other-
       wise,  the integer pointed to by	flagsp will be set to MSG_BAND and the
       integer pointed to by bandp will	be set to the  priority	 band  of  the
       message.

       If  O_NDELAY  and O_NONBLOCK are	clear, getmsg()	blocks until a message
       of the type specified by	flagsp is available on the  stream  head  read
       queue.  If  O_NDELAY  or	 O_NONBLOCK  has been set and a	message	of the
       specified type is not present on	the read  queue,  getmsg()  fails  and
       sets errno to EAGAIN.

       If  a  hangup  occurs  on  the stream from which	messages are to	be re-
       trieved,	getmsg() continues to operate normally,	 as  described	above,
       until the  stream head read queue is empty. Thereafter, it returns 0 in
       the len member of ctlptr	and
	dataptr.

RETURN VALUES
       Upon successful completion, a non-negative value	is returned. A	return
       value  of  0 indicates that a full message was read successfully. A re-
       turn value of MORECTL indicates that more control information is	 wait-
       ing  for	retrieval. A return value of MOREDATA indicates	that more data
       are waiting for retrieval. A return value of MORECTL |  MOREDATA	 indi-
       cates  that both	types of information remain. Subsequent	getmsg() calls
       retrieve	the remainder of the message. However, if a message of	higher
       priority	has been received by the stream	head read queue, the next call
       to getmsg() will	retrieve that higher priority message before  retriev-
       ing the remainder of the	previously received partial message.

ERRORS
       The getmsg() and	getpmsg() functions will fail if:

       EAGAIN	       The  O_NDELAY or	O_NONBLOCK flag	is set and no messages
		       are available.

       EBADF	       The fildes argument is not a valid file descriptor open
		       for reading.

       EBADMSG	       Queued message to be read is not	valid for getmsg.

       EFAULT	       The  ctlptr,  dataptr, bandp, or	flagsp argument	points
		       to an illegal address.

       EINTR	       A signal	was caught during the execution	of the	getmsg
		       function.

       EINVAL	       An illegal value	was specified in flagsp, or the	stream
		       referenced by fildes is linked under a multiplexor.

       ENOSTR	       A stream	is not associated with fildes.

       The getmsg() function can also fail if a	STREAMS	error message had been
       received	 at the	stream head before the call to getmsg(). The error re-
       turned is the value contained in	the STREAMS error message.

ATTRIBUTES
       See attributes(5) for descriptions of the following attributes:

       +-----------------------------+-----------------------------+
       |      ATTRIBUTE	TYPE	     |	    ATTRIBUTE VALUE	   |
       +-----------------------------+-----------------------------+
       |Interface Stability	     |Standard			   |
       +-----------------------------+-----------------------------+

SEE ALSO
       intro(2), poll(2), putmsg(2), read(2), write(2),	 attributes(5),	 stan-
       dards(5)

       STREAMS Programming Guide

SunOS 5.10			  1 Nov	2001			     getmsg(2)

NAME | SYNOPSIS | DESCRIPTION | RETURN VALUES | ERRORS | ATTRIBUTES | SEE ALSO

Want to link to this manual page? Use this URL:
<https://www.freebsd.org/cgi/man.cgi?query=getmsg&sektion=2&manpath=SunOS+5.10>

home | help