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;
      {
          /* ... */
      }
      /* ... */