I keep forgetting how to find predefined macros, and end up scanning cpp docs for the answer. So if anyone else has the same problem, here is the solution:
$ touch foo.h; cpp -dM foo.h
On OSX 10.4.9 running on the Intel platform, this gives:
#define __DBL_MIN_EXP__ (-1021) #define __FLT_MIN__ 1.17549435e-38F #define __CHAR_BIT__ 8 #define __WCHAR_MAX__ 2147483647 #define __DBL_DENORM_MIN__ 4.9406564584124654e-324 #define __FLT_EVAL_METHOD__ 0 #define __DBL_MIN_10_EXP__ (-307) #define __FINITE_MATH_ONLY__ 0 #define __SHRT_MAX__ 32767 #define __LDBL_MAX__ 1.18973149535723176502e+4932L #define __APPLE_CC__ 5363 #define __UINTMAX_TYPE__ long long unsigned int #define __SCHAR_MAX__ 127 #define __USER_LABEL_PREFIX__ _ #define __STDC_HOSTED__ 1 #define __DBL_DIG__ 15 #define __FLT_EPSILON__ 1.19209290e-7F #define __LDBL_MIN__ 3.36210314311209350626e-4932L #define __strong #define __APPLE__ 1 #define __DECIMAL_DIG__ 21 #define __LDBL_HAS_QUIET_NAN__ 1 #define __DYNAMIC__ 1 #define __GNUC__ 4 #define __MMX__ 1 #define __DBL_MAX__ 1.7976931348623157e+308 #define __DBL_HAS_INFINITY__ 1 #define __weak #define __DBL_MAX_EXP__ 1024 #define __SSE2_MATH__ 1 #define __LONG_LONG_MAX__ 9223372036854775807LL #define __GXX_ABI_VERSION 1002 #define __FLT_MIN_EXP__ (-125) #define __DBL_MIN__ 2.2250738585072014e-308 #define __DBL_HAS_QUIET_NAN__ 1 #define __REGISTER_PREFIX__ #define __NO_INLINE__ 1 #define __i386 1 #define __FLT_MANT_DIG__ 24 #define __VERSION__ "4.0.1 (Apple Computer, Inc. build 5363)" #define __tune_nocona__ 1 #define i386 1 #define __i386__ 1 #define __SIZE_TYPE__ long unsigned int #define __FLT_RADIX__ 2 #define __LDBL_EPSILON__ 1.08420217248550443401e-19L #define __SSE_MATH__ 1 #define __FLT_HAS_QUIET_NAN__ 1 #define __FLT_MAX_10_EXP__ 38 #define __LONG_MAX__ 2147483647L #define __FLT_HAS_INFINITY__ 1 #define __LITTLE_ENDIAN__ 1 #define __LDBL_MANT_DIG__ 64 #define __CONSTANT_CFSTRINGS__ 1 #define __WCHAR_TYPE__ int #define __FLT_DIG__ 6 #define __INT_MAX__ 2147483647 #define __nocona 1 #define __FLT_MAX_EXP__ 128 #define __DBL_MANT_DIG__ 53 #define __WINT_TYPE__ int #define __SSE__ 1 #define __LDBL_MIN_EXP__ (-16381) #define __MACH__ 1 #define __LDBL_MAX_EXP__ 16384 #define __LDBL_MAX_10_EXP__ 4932 #define __DBL_EPSILON__ 2.2204460492503131e-16 #define __GNUC_PATCHLEVEL__ 1 #define __LDBL_HAS_INFINITY__ 1 #define __INTMAX_MAX__ 9223372036854775807LL #define __FLT_DENORM_MIN__ 1.40129846e-45F #define __PIC__ 1 #define __FLT_MAX__ 3.40282347e+38F #define __SSE2__ 1 #define __FLT_MIN_10_EXP__ (-37) #define __INTMAX_TYPE__ long long int #define __nocona__ 1 #define __GNUC_MINOR__ 0 #define __DBL_MAX_10_EXP__ 308 #define __LDBL_DENORM_MIN__ 3.64519953188247460253e-4951L #define __PTRDIFF_TYPE__ int #define __LDBL_MIN_10_EXP__ (-4931) #define __LDBL_DIG__ 18