gcc defines some macros based on the platform, architecture, etc that it is running on. I always forget the gcc arguments that makes it display all these macro definitions, so here's a reminder for myself.

gcc -E -dM foo.c

foo.c can be anything, even an empty file (gcc only pre-processes the file).

Here's an ultra-simple mini script that takes care of the temp file creation.

gcc_macros.sh:

#!/bin/sh
tmpfile=/var/tmp/foo.c
touch $tmpfile
gcc -E -dM $tmpfile
rm $tmpfile

If I run this script on my Mac I get a large list of macro definitions, i.e.:

$ ./gcc_macros.sh
#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__ 5465
#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

and so on.