Routines in the ILOG CPLEX Callable Library obey the C programming convention of call by value (as opposed to call by reference, for example, in FORTRAN and BASIC). If a routine in the Callable Library needs the address of a variable in order to change the value of the variable, then that fact is documented in the ILOG CPLEX Reference Manual by the suffix _p in the parameter name in the synopsis of the routine. In C, you create such values by means of the & operator to take the address of a variable and to pass this address to the Callable Library routine.

In that routine, the third parameter is a pointer to a variable of type double. To call this routine from C, declare:

double objval;

You call it by creating a double-precision array by means of either one of two methods. The first method dynamically allocates the array, like this:

double *x = NULL;
x = (double *) malloc (100*sizeof(double));

The second method declares the array as a local variable, like this:

double x[100];

Then to see the optimal values for columns 5 through 104, for example, you could write this:

status = CPXgetx (env, lp, x, 5, 104);