Can you define which header file to include at compile time? Ans. Yes. This can be done by using the #if, #else, and #endif preprocessor directives. For example, certain compilers use different names for header files. One such case is between Borland C , which uses the header file alloc.h, and Microsoft C , which uses the header file malloc.h. Both of these headers serve the same purpose, and each contains roughly the same definitions. If, however, you are writing a program that is to support Borland C and Microsoft C , you must define which header to include at compile time. The following example shows how this can be done: #ifdef __BORLANDC__ #include #else #include #endif When you compile your program with Borland C , the __BORLANDC__ symbolic name is automatically defined by the compiler. You can use this predefined symbolic name to determine whether your program is being compiled with Borland C . If it is, you must include the alloc.h file rather than the malloc.h file.