compile_value

The AC_C_COMPILE_VALUE macro determines a compile time value by generating the object code and reading the value from the code. Static data initializers like sizeof(int) are unavailable to preprocessor. The macro calculates the values known to compiler's static initializer.

Assumptions: The sought value should not exceed 65535. The shell interpreter and the sed utility are expected to exist and work similarly across possible build platforms.

Result: The resulting configure script will generate the preprocessor symbol definition:

   #define COMPILE_VALUE_<ALIAS> <NUMBER>

It was important that the value was embedded into the object file in a predefined byte order during the test. This ensured that the result was independent from the target platform's byte order.

The existing AC_CHECK_SIZEOF macro also computes the size of the given type without running the test program. However, the existing macro will produce a piece of configure script that will take the time proportional to the logarithm of the sought value.

Example of use in configure.in:

   AC_C_COMPILE_VALUE(sizeof(int), sizeof_int)
   AC_C_COMPILE_VALUE([sizeof(int[[543]])], sizeof_int543)

As a result of runnfing the generated configure script, the following definition will appear in config.h:

   #define COMPILE_VALUE_SIZEOF_INT 4
   #define COMPILE_VALUE_SIZEOF_INT543 2172

## Portability defines that help interoperate with classic and modern autoconfs
ifdef([AC_TR_SH],[
define([AC_TR_SH_REUSE],[AC_TR_SH([$1])])
define([AC_TR_CPP_REUSE],[AC_TR_CPP([$1])])
], [
define([AC_TR_SH_REUSE],
       [patsubst(translit([[$1]], [*+], [pp]), [[^a-zA-Z0-9_]], [_])])
define([AC_TR_CPP_REUSE],
       [patsubst(translit([[$1]],
                          [*abcdefghijklmnopqrstuvwxyz],
                          [PABCDEFGHIJKLMNOPQRSTUVWXYZ]),
                 [[^A-Z0-9_]], [_])])
])

AC_DEFUN([AC_C_COMPILE_VALUE], [
  pushdef([ac_c_compile_value],
    AC_TR_SH_REUSE([ac_cv_c_compile_value_$2]))dnl
  ac_c_compile_value_expand="$1"
  AC_CACHE_CHECK([value of $1 by analyzing object code],
                 ac_c_compile_value, [
    save_CFLAGS="$CFLAGS"
    CFLAGS="$CFLAGS -c -o conftest.o"
    AC_TRY_COMPILE([$3
      #include <stddef.h>
      #include <stdint.h>
      #include <stdlib.h>
      #define COMPILE_VALUE $ac_c_compile_value_expand
      #define HEX_DIGIT(n)      ((n) >= 10 ? 'a' + (n) - 10 : '0' + (n))
      char object_code_block[] = {
        '\n', 'e', '4', 'V', 'A',
        '0', 'x',
        (char) HEX_DIGIT((((COMPILE_VALUE / 16) / 16) / 16) % 16),
        (char) HEX_DIGIT(((COMPILE_VALUE / 16) / 16) % 16),
        (char) HEX_DIGIT((COMPILE_VALUE / 16) % 16),
        (char) HEX_DIGIT(COMPILE_VALUE % 16),
        'Y', '3', 'p', 'M', '\n'
      };],
      [],
      [ac_c_compile_value=`
        typeset -i n=\`sed -ne 's/^e4VA0x\(.*\)Y3pM$/0x\1/p' < conftest.o\`;
        echo $n`],
      [ac_c_compile_value=0])
    CFLAGS="$save_CFLAGS"])
  AC_DEFINE_UNQUOTED(AC_TR_CPP_REUSE(compile_value_$2),
                     [$[]ac_c_compile_value],
                     [$1])
  popdef([ac_c_compile_value])dnl
])