File: autoconf.info,  Node: Standard Symbols,  Next: Default Includes,  Up: Common Behavior

5.1.1 Standard Symbols
----------------------

All the generic macros that ‘AC_DEFINE’ a symbol as a result of their
test transform their ARGUMENT values to a standard alphabet.  First,
ARGUMENT is converted to upper case and any asterisks (‘*’) are each
converted to ‘P’.  Any remaining characters that are not alphanumeric
are converted to underscores.

   For instance,

     AC_CHECK_TYPES([struct $Expensive*])

defines the symbol ‘HAVE_STRUCT__EXPENSIVEP’ if the check succeeds.