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 variable 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.

For example, let's look at the synopses for two routines, CPXgetobjval() and CPXgetx(), as they are documented in the ILOG CPLEX Reference Manual to clarify this calling convention. Here is the synopsis of the routine CPXgetobjval():

int CPXgetobjval (CPXENVptr env, CPXLPptr lp, double *objval_p)

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

double objval;

Then call CPXgetobjval() in this way:

status = CPXgetobjval (env, lp, &objval);

In contrast, here is the synopsis of the routine CPXgetx():

int CPXgetx (CPXENV env, CPXLPptr lp, double *x, int begin, int end)

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);

The variable objval_p in the synopsis of CPXgetobjval() and the variable x in the synopsis of CPXgetx() are both of type (double *). However, the suffix _p in the parameter objval_p indicates that you should use an address of a single variable in one call, while the lack of _p in x indicates that you should pass an array in the other.

For guidance about how to pass values to ILOG CPLEX routines from application languages such as FORTRAN or BASIC that conventionally call by reference, see Call by Reference in this manual, and consult the documentation for those languages.