Example: using callbacks lpex4.c

Illustrates callbacks in the C API.

This example shows you how to use callbacks effectively with routines from the Callable Library. It is based on lpex1.c, a program from the manual Getting Started. This example about callbacks differs from that simpler one in several ways:

• To make the output more interesting, this example optimizes a slightly different linear program.

• The CPLEX screen indicator (that is, the messages to screen switch CPX_PARAM_SCRIND) is not turned on. Only the callback function produces output. Consequently, this program calls CPXgeterrorstring to retrieve any error messages and then prints them. After the TERMINATE: label, the program uses separate status variables so that if an error occurred earlier, its error status will not be lost or destroyed by freeing the problem object and closing the CPLEX environment. Table 1 summarizes those status variables.

  Table 1. Status variables in lpex4.c

  Variable	Represents status returned by this routine
  frstatus	CPXfreeprob
  clstatus	CPXcloseCPLEX

• The function mycallback at the end of the program is called by the optimizer. This function tests whether the primal simplex optimizer has been called. If so, then a call to CPXgetcallbackinfo gets the following information:

  • iteration count;

  • feasibility indicator;

  • sum of infeasibilities (if infeasible);

  • objective value (if feasible).

    The function then prints these values to indicate progress.

• Before the program calls CPXlpopt, the default optimizer from the Callable Library, it sets the callback function by calling CPXsetlpcallbackfunc. It unsets the callback immediately after optimization.

This callback function offers a model for graphic user interfaces that display optimization progress as well as those GUIs that allow a user to interrupt and stop optimization. If you want to provide your end-user a facility like that to interrupt and stop optimization, then you should make mycallback return a nonzero value to indicate the end-user interrupt.

The complete program lpex4.c appears online in the standard distribution at yourCPLEXinstallation /examples/src.