File: libtool.info,  Node: C header files,  Up: Library tips

8.1 Writing C header files
==========================

Writing portable C header files can be difficult, since they may be read
by different types of compilers:

C++ compilers
     C++ compilers require that functions be declared with full
     prototypes, since C++ is more strongly typed than C.  C functions
     and variables also need to be declared with the ‘extern "C"’
     directive, so that the names aren't mangled.  *Note C++
     libraries::, for other issues relevant to using C++ with libtool.

ANSI C compilers
     ANSI C compilers are not as strict as C++ compilers, but functions
     should be prototyped to avoid unnecessary warnings when the header
     file is ‘#include’d.

non-ANSI C compilers
     Non-ANSI compilers will report errors if functions are prototyped.

   These complications mean that your library interface headers must use
some C preprocessor magic to be usable by each of the above compilers.

   ‘foo.h’, defined in the ‘tests/demo.at’ Autotest of the libtool
distribution, serves as an example for how to write a header file that
can be safely installed in a system directory.

   Here are the relevant portions of that file:

     #ifndef FOO_H
     #define FOO_H

     #ifdef __cplusplus
     extern "C" {
     #endif

     int foo (void);
     int hello (void);

     #ifdef __cplusplus
     }
     #endif

     #endif /* !FOO_H */

   This can also be achieved by utilizing macros:

     /* BEGIN_C_DECLS should be used at the beginning of your declarations,
        so that C++ compilers don't mangle their names.  Use END_C_DECLS at
        the end of C declarations. */
     #undef BEGIN_C_DECLS
     #undef END_C_DECLS
     #ifdef __cplusplus
     # define BEGIN_C_DECLS extern "C" {
     # define END_C_DECLS }
     #else
     # define BEGIN_C_DECLS /* empty */
     # define END_C_DECLS /* empty */
     #endif

     /* PARAMS is a macro used to wrap function prototypes, so that
        compilers that don't understand ANSI C prototypes still work,
        and ANSI C compilers can issue warnings about type mismatches. */
     #undef PARAMS
     #if defined __STDC__ || defined _AIX \
             || (defined __mips && defined _SYSTYPE_SVR4) \
             || defined WIN32 || defined __cplusplus
     # define PARAMS(protos) protos
     #else
     # define PARAMS(protos) ()
     #endif

   These macros can be used in ‘foo.h’ as follows:

     #ifndef FOO_H
     #define FOO_H

     /* The above macro definitions. */
     #include "..."

     BEGIN_C_DECLS

     int foo PARAMS((void));
     int hello PARAMS((void));

     END_C_DECLS

     #endif /* !FOO_H */

   Note that the ‘#ifndef FOO_H’ prevents the body of ‘foo.h’ from being
read more than once in a given compilation.

   Also the only thing that must go outside the
‘BEGIN_C_DECLS’/‘END_C_DECLS’ pair are ‘#include’ lines.  Strictly
speaking it is only C symbol names that need to be protected, but your
header files will be more maintainable if you have a single pair of
these macros around the majority of the header contents.

   You should use these definitions of ‘PARAMS’, ‘BEGIN_C_DECLS’, and
‘END_C_DECLS’ into your own headers.  Then, you may use them to create
header files that are valid for C++, ANSI, and non-ANSI compilers(1).

   Do not be naive about writing portable code.  Following the tips
given above will help you miss the most obvious problems, but there are
definitely other subtle portability issues.  You may need to cope with
some of the following issues:

   • Pre-ANSI compilers do not always support the ‘void *’ generic
     pointer type, and so need to use ‘char *’ in its place.

   • The ‘const’, ‘inline’ and ‘signed’ keywords are not supported by
     some compilers, especially pre-ANSI compilers.

   • The ‘long double’ type is not supported by many compilers.

   ---------- Footnotes ----------

   (1) We used to recommend ‘__P’, ‘__BEGIN_DECLS’ and ‘__END_DECLS’.
This was bad advice since symbols (even preprocessor macro names) that
begin with an underscore are reserved for the use of the compiler.