(autoconf.info.gz) Generic Declarations

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 Generic Declaration Checks
 --------------------------
 
    These macros are used to find declarations not covered by the
 "particular" test macros.
 
  - Macro: AC_CHECK_DECL (SYMBOL, [ACTION-IF-FOUND],
           [ACTION-IF-NOT-FOUND], [INCLUDES = `default-includes'])
      If SYMBOL (a function or a variable) is not declared in INCLUDES
      and a declaration is needed, run the shell commands
      ACTION-IF-NOT-FOUND, otherwise ACTION-IF-FOUND.  If no INCLUDES
      are specified, the default includes are used ( Default
      Includes).
 
      This macro actually tests whether it is valid to use SYMBOL as an
      r-value, not if it is really declared, because it is much safer to
      avoid introducing extra declarations when they are not needed.
 
  - Macro: AC_CHECK_DECLS (SYMBOLS, [ACTION-IF-FOUND],
           [ACTION-IF-NOT-FOUND], [INCLUDES = `default-includes'])
      For each of the SYMBOLS (_comma_-separated list), define
      `HAVE_DECL_SYMBOL' (in all capitals) to `1' if SYMBOL is declared,
      otherwise to `0'.  If ACTION-IF-NOT-FOUND is given, it is
      additional shell code to execute when one of the function
      declarations is needed, otherwise ACTION-IF-FOUND is executed.
 
      This macro uses an m4 list as first argument:
           AC_CHECK_DECLS(strdup)
           AC_CHECK_DECLS([strlen])
           AC_CHECK_DECLS([malloc, realloc, calloc, free])
 
      Unlike the other `AC_CHECK_*S' macros, when a SYMBOL is not
      declared, `HAVE_DECL_SYMBOL' is defined to `0' instead of leaving
      `HAVE_DECL_SYMBOL' undeclared.  When you are _sure_ that the check
      was performed, use `HAVE_DECL_SYMBOL' just like any other result
      of Autoconf:
 
           #if !HAVE_DECL_SYMBOL
           extern char *symbol;
           #endif
 
      If the test may have not been performed, however, because it is
      safer _not_ to declare a symbol than to use a declaration that
      conflicts with the system's one, you should use:
 
           #if defined HAVE_DECL_MALLOC && !HAVE_DECL_MALLOC
           char *malloc (size_t *s);
           #endif
 
      You fall into the second category only in extreme situations:
      either your files may be used without being configured, or they
      are used during the configuration.  In most cases the traditional
      approach is enough.
 

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