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.\" ========================================================================
.\"
.IX Title "xpamainloop 3"
.TH xpamainloop 3 "July 23, 2013" "version 2.1.15" "SAORD Documentation"
.\" For nroff, turn off justification.  Always turn off hyphenation; it makes
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.nh
.SH "NAME"
\&\fBXPAMainLoop: optional main loop for \s-1XPA\s0\fR
.SH "SYNOPSIS"
.IX Header "SYNOPSIS"
.Vb 1
\&  #include <xpa.h>
\&
\&  void XPAMainLoop();
.Ve
.SH "DESCRIPTION"
.IX Header "DESCRIPTION"
Once \s-1XPA\s0 access points have been defined, a program must enter an
event loop to watch for requests from external programs. This can be
done in a variety of ways, depending on whether the event loop is
processing events other than \s-1XPA\s0 events.  In cases where there are no
non-XPA events to be processed, the program can simply call the
\&\fIXPAMainLoop()\fR event loop.  This loop is implemented essentially as
follows (error checking is simplified in this example):
.PP
.Vb 8
\&  FD_ZERO(&readfds);
\&  while( XPAAddSelect(NULL, &readfds) ){
\&    if( sgot = select(swidth, &readfds, NULL, NULL, NULL) >0 )
\&      XPAProcessSelect(&readfds, 0);
\&    else
\&      break;
\&    FD_ZERO(&readfds);
\&  }
.Ve
.PP
The \fIXPAAddSelect()\fR routine sets up the \fIselect()\fR readfds variable so
that \fIselect()\fR will wait for I/O on all the active \s-1XPA\s0 channels.  It
returns the number of XPAs that are active; the loop will end when
there are no active XPAs. The standard \fIselect()\fR routine is called to
wait for an external I/O request.  Since no timeout struct is passed
in argument 5, the \fIselect()\fR call hangs until there is an external
request.  When an external I/O request is made, the \fIXPAProcessSelect()\fR
routine is executed to process the pending requests.  In this routine,
the maxreq value determines how many requests will be processed: if
maxreq <=0, then all currently pending requests will be processed.
Otherwise, up to maxreq requests will be processed.  (The most usual
values for maxreq is 0 to process all requests.)
.PP
If a program has its own Unix \fIselect()\fR loop, then \s-1XPA\s0 access points can
be added to it by using a variation of the standard XPAMainLoop:
.PP
.Vb 7
\&  XPAAddSelect(xpa, &readfds);
\&  [app\-specific ...]
\&  if( select(width, &readfds, ...) ){
\&    XPAProcessSelect(&readfds, maxreq);
\&    [app\-specific ...]
\&    FD_ZERO(&readfds);
\&  }
.Ve
.PP
\&\fIXPAAddSelect()\fR is called before \fIselect()\fR to add the access points.
If the first argument is \s-1NULL\s0, then all active \s-1XPA\s0 access points
are added. Otherwise only the specified access point is added.
After \fIselect()\fR is called, the \fIXPAProcessSelect()\fR routine can be called
to process \s-1XPA\s0 requests.  Once again, the maxreq value determines how
many requests will be processed: if maxreq <=0, then all currently
pending requests will be processed.  Otherwise, up to maxreq requests
will be processed.
.PP
\&\s-1XPA\s0 access points can be added to
Xt event loops (using \fIXtAppMainLoop()\fR)
and
Tcl/Tk event loops (using vwait and the Tk loop).
When using \s-1XPA\s0 with these event loops, you only need to call:
.PP
int XPAXtAddInput(XtAppContext app, \s-1XPA\s0 xpa)
.PP
or
.PP
.Vb 1
\&  int XPATclAddInput(XPA xpa)
.Ve
.PP
respectively before entering the loop.
.SH "SEE ALSO"
.IX Header "SEE ALSO"
See xpa(n) for a list of \s-1XPA\s0 help pages