(standards.info) ANSI C

 
 ANSI C and pre-ANSI C
 =====================
 
    Do not ever use the "trigraph" feature of ANSI C.
 
    ANSI C is widespread enough now that it is ok to write new programs
 that use ANSI C features (and therefore will not work in non-ANSI
 compilers).  And if a program is already written in ANSI C, there's no
 need to convert it to support non-ANSI compilers.
 
    However, it is easy to support non-ANSI compilers in most programs,
 so you might still consider doing so when you write a program.  Instead
 of writing function definitions in ANSI prototype form,
 
      int
      foo (int x, int y)
      ...
 
 write the definition in pre-ANSI style like this,
 
      int
      foo (x, y)
           int x, y;
      ...
 
 and use a separate declaration to specify the argument prototype:
 
      int foo (int, int);
 
    You need such a declaration anyway, in a header file, to get the
 benefit of ANSI C prototypes in all the files where the function is
 called.  And once you have it, you lose nothing by writing the function
 definition in the pre-ANSI style.
 
    If you don't know non-ANSI C, there's no need to learn it; just
 write in ANSI C.

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