ILOG CPLEX 11.0 User's Manual

For example, let’s look at the synopses for two routines, CPXgetobjval and CPXgetx, asthey are documented in the ILOG CPLEX Reference Manual to clarify this callingconvention. Here is the synopsis of the routine CPXgetobjval:int CPXgetobjval (CPXCENVptr env, CPXCLPptr lp, double *objval_p);In that routine, the third parameter is a pointer to a variable of type double. To call thisroutine 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. Thefirst 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 parameter objval_p in the synopsis of CPXgetobjval and the parameter x in thesynopsis of CPXgetx are both of type (double *). However, the suffix _p in the parameterobjval_p indicates that you should use an address of a single variable in one call, while thelack 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 languagessuch as FORTRAN or BASIC that conventionally call by reference, see Call by Referenceon page 123 in this manual, and consult the documentation for those languages.ILOG CPLEX 11.0 — USER’ S MANUAL 115