— Macro: AC_CHECK_DECL (symbol, [action-if-found], [action-if-not-found], [includes = ‘AC_INCLUDES_DEFAULT’])

If symbol (a function, variable, or constant) is not declared in includes and a declaration is needed, run the shell commands action-if-not-found, otherwise action-if-found. includes is a series of include directives, defaulting to AC_INCLUDES_DEFAULT (see Default Includes), which are used prior to the declaration under test.

This macro actually tests whether symbol is defined as a macro or can be used as an r-value, not whether 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 = ‘AC_INCLUDES_DEFAULT’])

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.

includes is a series of include directives, defaulting to AC_INCLUDES_DEFAULT (see Default Includes), which are used prior to the declarations under test.

This macro uses an M4 list as first argument:

          AC_CHECK_DECLS([strdup])
          AC_CHECK_DECLS([strlen])
          AC_CHECK_DECLS([malloc, realloc, calloc, free])
          AC_CHECK_DECLS([j0], [], [], [[#include <math.h>]])

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 in #if:

          #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
          void *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.

— Macro: AC_CHECK_DECLS_ONCE (symbols)

For each of the symbols (comma-separated list), define HAVE_DECL_symbol (in all capitals) to ‘1’ if symbol is declared in the default include files, otherwise to ‘0’. This is a once-only variant of AC_CHECK_DECLS. It generates the checking code at most once, so that configure is smaller and faster; but the checks cannot be conditionalized and are always done once, early during the configure run.