I was trying to find out how to tell, from within a C program under GCC, what OS the code was being compiled under. I was porting RPCemu to MacOSX, and wanted to switch between big and little endian code without doing something clunky in autoconf.

I Googled for lists of pre-defined C pre-processor macros, but found didn't find any. Eventually, I discovered that you can get a list of the macros under GCC on a particular platform by creating an empty c file (empty.c) and running this command:

 gcc -std=c99 -E -dM empty.c

Linux - gcc (GCC) 3.2.3 20030502 (Red Hat Linux 3.2.3-56)

 #define HAVE_BUILTIN_SETJMP 1
 #define unix 1
 #define i386 1
 #define SIZE_TYPE unsigned int
 #define ELF 1
 #define GNUC_PATCHLEVEL 3
 #define GNUC_RH_RELEASE 56
 #define __linux 1
 #define __unix 1
 #define linux 1
 #define STDC_VERSION 199901L
 #define USER_LABEL_PREFIX 
 #define STDC_HOSTED 1
 #define WCHAR_TYPE long int
 #define gnu_linux 1
 #define WINT_TYPE unsigned int
 #define GNUC 3
 #define STRICT_ANSI 1
 #define __GXX_ABI_VERSION 102
 #define GNUC_MINOR 2
 #define STDC 1
 #define PTRDIFF_TYPE int
 #define tune_i386 1
 #define REGISTER_PREFIX 
 #define NO_INLINE 1
 #define __i386 1
 #define VERSION "3.2.3 20030502 (Red Hat Linux 3.2.3-56)"

Mac - powerpc-apple-darwin8-gcc-4.0.1 (GCC) 4.0.1 (Apple Computer, Inc. build 5341)

 #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 GNUC_PATCHLEVEL 1
 #define SHRT_MAX 32767
 #define LDBL_MAX 1.79769313486231580793728971405301e+308L
 #define APPLE_CC 5341
 #define UINTMAX_TYPE long long unsigned int
 #define LDBL_MAX_EXP 1024
 #define SCHAR_MAX 127
 #define USER_LABEL_PREFIX _
 #define STDC_HOSTED 1
 #define LDBL_HAS_INFINITY 1
 #define DBL_DIG 15
 #define FLT_EPSILON 1.19209290e-7F
 #define LDBL_MIN 2.00416836000897277799610805135016e-292L
 #define ppc 1
 #define __strong 
 #define APPLE 1
 #define DECIMAL_DIG 33
 #define LDBL_HAS_QUIET_NAN 1
 #define DYNAMIC 1
 #define GNUC 4
 #define DBL_MAX 1.7976931348623157e+308
 #define DBL_HAS_INFINITY 1
 #define STRICT_ANSI 1
 #define __weak 
 #define DBL_MAX_EXP 1024
 #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 _ARCH_PPC 1
 #define FLT_MANT_DIG 24
 #define VERSION "4.0.1 (Apple Computer, Inc. build 5341)"
 #define BIG_ENDIAN 1
 #define SIZE_TYPE long unsigned int
 #define FLT_RADIX 2
 #define LDBL_EPSILON 4.94065645841246544176568792868221e-324L
 #define NATURAL_ALIGNMENT 1
 #define FLT_HAS_QUIET_NAN 1
 #define FLT_MAX_10_EXP 38
 #define LONG_MAX 2147483647L
 #define FLT_HAS_INFINITY 1
 #define STDC_VERSION 199901L
 #define _BIG_ENDIAN 1
 #define LDBL_MANT_DIG 106
 #define WCHAR_TYPE int
 #define FLT_DIG 6
 #define INT_MAX 2147483647
 #define LONG_DOUBLE_128 1
 #define FLT_MAX_EXP 128
 #define DBL_MANT_DIG 53
 #define WINT_TYPE int
 #define LDBL_MIN_EXP (-968)
 #define MACH 1
 #define LDBL_MAX_10_EXP 308
 #define DBL_EPSILON 2.2204460492503131e-16
 #define INTMAX_MAX 9223372036854775807LL
 #define FLT_DENORM_MIN 1.40129846e-45F
 #define PIC 1
 #define FLT_MAX 3.40282347e+38F
 #define FLT_MIN_10_EXP (-37)
 #define INTMAX_TYPE long long int
 #define GNUC_MINOR 0
 #define DBL_MAX_10_EXP 308
 #define LDBL_DENORM_MIN 4.94065645841246544176568792868221e-324L
 #define PTRDIFF_TYPE int
 #define LDBL_MIN_10_EXP (-291)
 #define LDBL_DIG 31
 #define POWERPC 1