XtAppAddWorkProc, XtRemoveWorkProc - Add and remove background processing
  procedures
#include <X11/Intrinsic.h>
XtWorkProcId XtAppAddWorkProc(XtAppContext app_context,
    XtWorkProc proc, XtPointer client_data);
void XtRemoveWorkProc(XtWorkProcId id);
  - app_context
- Specifies the application context that identifies the application.
- client_data
- Specifies the argument that is to be passed to the specified procedure
      when it is called.
- proc
- Specifies the procedure that is to be called.
- id
- Specifies which work procedure to remove.
The XtAppAddWorkProc function adds the specified work procedure for the
  application identified by app_context.The XtRemoveWorkProc function explicitly removes the
    specified background work procedure.
XtAppNextEvent(3)
X Toolkit Intrinsics - C Language Interface
Xlib - C Language X Interface