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(autoconf.info)Specific Compiler Characteristics

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Specific Compiler Characteristics
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   Some compilers exhibit different behaviors.

Static/Dynamic Expressions
     Autoconf relies on a trick to extract one bit of information from
     the C compiler: using negative array sizes.  For instance the
     following excerpt of a C source demonstrates how to test whether
     `int's are 4 bytes long:

          int
          main (void)
          {
            static int test_array [sizeof (int) == 4 ? 1 : -1];
            test_array [0] = 0
            return 0;
          }

     To our knowledge, there is a single compiler that does not support
     this trick: the HP C compilers (the real one, not only the
     "bundled") on HP-UX 11.00:

          $ cc -c -Ae +O2 +Onolimit conftest.c
          cc: "conftest.c": error 1879: Variable-length arrays cannot \
              have static storage.

     Autoconf works around this problem by casting `sizeof (int)' to
     `long' before comparing it.