---
name: "xpamainloop"
section: "3"
platform: "linux"
package: "libxpa-dev"
version: "2.1.20-3"
release: "Debian 13 (Trixie)"
platforms: ["linux"]
url: "https://new.carta.tech/xpamainloop"
---

# xpamainloop(3)

optional main loop for XPA

## NAME

**XPAMainLoop - optional main loop for XPA**

## SYNOPSIS

```
  #include <xpa.h>
  void XPAMainLoop();
```

## DESCRIPTION

Once XPA 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 XPA events.  In cases where there are no non-XPA events to be processed, the program can simply call the *XPAMainLoop()* event loop.  This loop is implemented essentially as follows (error checking is simplified in this example):

```
  FD_ZERO(&readfds);
  while( XPAAddSelect(NULL, &readfds) ){
    if( sgot = select(swidth, &readfds, NULL, NULL, NULL) >0 )
      XPAProcessSelect(&readfds, 0);
    else
      break;
    FD_ZERO(&readfds);
  }
```

The *XPAAddSelect()* routine sets up the *select()* readfds variable so that *select()* will wait for I/O on all the active XPA channels.  It returns the number of XPAs that are active; the loop will end when there are no active XPAs. The standard *select()* routine is called to wait for an external I/O request.  Since no timeout struct is passed in argument 5, the *select()* call hangs until there is an external request.  When an external I/O request is made, the *XPAProcessSelect()* 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.)

If a program has its own Unix *select()* loop, then XPA access points can be added to it by using a variation of the standard XPAMainLoop:

```
  XPAAddSelect(xpa, &readfds);
  [app-specific ...]
  if( select(width, &readfds, ...) ){
    XPAProcessSelect(&readfds, maxreq);
    [app-specific ...]
    FD_ZERO(&readfds);
  }
```

*XPAAddSelect()* is called before *select()* to add the access points. If the first argument is NULL, then all active XPA access points are added. Otherwise only the specified access point is added. After *select()* is called, the *XPAProcessSelect()* routine can be called to process XPA 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.

XPA access points can be added to Xt event loops (using *XtAppMainLoop()*) and Tcl/Tk event loops (using vwait and the Tk loop). When using XPA with these event loops, you only need to call:

int XPAXtAddInput(XtAppContext app, XPA xpa)

or

```
  int XPATclAddInput(XPA xpa)
```

respectively before entering the loop.

## SEE ALSO

See xpa(n) for a list of XPA help pages