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(autoconf.info)Generic Compiler Characteristics

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Generic Compiler Characteristics
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 - Macro: AC_CHECK_SIZEOF (TYPE, [UNUSED], [INCLUDES =
          `default-includes'])
     Define `SIZEOF_TYPE' (Note: Standard Symbols) to be the size in
     bytes of TYPE.  If `type' is unknown, it gets a size of 0.  If no
     INCLUDES are specified, the default includes are used (Note:
     Default Includes).  If you provide INCLUDE, make sure to include
     `stdio.h' which is required for this macro to run.

     This macro now works even when cross-compiling.  The UNUSED
     argument was used when cross-compiling.

     For example, the call

          AC_CHECK_SIZEOF(int *)

     defines `SIZEOF_INT_P' to be 8 on DEC Alpha AXP systems.