build: find FILE_OFFSET_BITS via array

This makes cross-compiling happy, use a trick similar to autoconf's
AC_CHECK_SIZEOF macro.
Basically we make an array:

static int array[1 - 2 * !(((long int)(sizeof(off_t))) < 8)];

This gives -1 multiplied by the negation of the condition
(sizeof(off_t) < 8) cast to a long int.
So if the condition is true it gives array[(-1 * 0)] (remember the
condition is cast and negated) thus passing a build test with a 0-sized
array.
If it's false it gives array[(-1 * 1)] thus failing with a
negative-sized array.

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>
Reviewed-by: Andreas Schneider <asn@samba.org>
Autobuild-User(master): Andrew Bartlett <abartlet@samba.org>
Autobuild-Date(master): Tue Feb 18 23:18:32 CET 2014 on sn-devel-104

diff --git a/lib/ccan/wscript b/lib/ccan/wscript
index 59b82059c90dcdc721d6098b4d2bd59a4a34487e..81039d006e58763a3479b709536cc902becf1b94 100644 (file)
--- a/lib/ccan/wscript
+++ b/lib/ccan/wscript
@@ -127,15 +127,18 @@ def configure(conf):
     # Only check for FILE_OFFSET_BITS=64 if off_t is normally small:
     # use raw routines because wrappers include previous _GNU_SOURCE
     # or _FILE_OFFSET_BITS defines.
+    # The math for these tests is:
+    # array[-1 * !((int)(condition)) ] (condition is true) = array[0] = builds
+    # array[-1 * !((int)(condition)) ] (condition is false) = array[-1] = fails
     conf.check(fragment="""#include <sys/types.h>
-               int main(void) { return !(sizeof(off_t) < 8); }""",
-               execute=True, msg='Checking for small off_t',
+               int main(void) { static int test_array[1 - 2 * !(((long int)(sizeof(off_t))) < 8)]; }""",
+               msg='Checking for small off_t',
                define_name='SMALL_OFF_T')
     # Unreliable return value above, hence use define.
     if conf.CONFIG_SET('SMALL_OFF_T'):
         conf.check(fragment="""#include <sys/types.h>
-                   int main(void) { return !(sizeof(off_t) >= 8); }""",
-                   execute=True, msg='Checking for -D_FILE_OFFSET_BITS=64',
+                  int main(void) { static int test_array[1 - 2 * !(((long int)(sizeof(off_t))) >= 8)]; }""",
+                   msg='Checking for -D_FILE_OFFSET_BITS=64',
                    ccflags='-D_FILE_OFFSET_BITS=64',
                    define_name='HAVE_FILE_OFFSET_BITS')