Complying with standard C

Examples

Appropriate use of __STDC__ produces a header file that can be used for both old and new compilers:

   header.h:
   
     struct s { / ... / };
   

     #ifdef __STDC__
         void errmsg(int, ...);
         struct s f(const char );
         int g(void);
     #else
         void errmsg();
         struct s f();
         int g();
     #endif

The following function uses prototypes and can still be compiled on an older system:

   struct s 
   
   #ifdef __STDC__
   f(const char p)
   #else
   f(p) char p;
   #endif
   {
       / ... /
   }

The following is an updated source file (as with choice 3 above). The local function still uses an old style definition, but a prototype is included for newer compilers:

   source.c:
   
     #include <header.h>
     typedef / ... / MyType;
     #ifdef __STDC__
         static void del(MyType );
         / ... /
     #endif
     static void
     del(p)
         MyType p;
     {
         / ... /
     }
     / ... /