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5.7.2 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 (see section 5.1.2 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([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:

extern char *symbol;

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:

void *malloc (size_t *s);

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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