Hidden Variables

Ever wonder what preprocessor symbols your GNU C and C++ compiler automatically includes in your program? I'm talking about symbols above and beyond those specified in the ANSI C and ISO C++ standards, like __FILE__, __LINE__, __func__, and __cplusplus. Having gotten bit by this once or twice, I've learned it pays to know these things. Try

gcc -dM -E - < /dev/null

or

g++ -dM -E - < /dev/null

which produce the same set of #define lines for me.

#define __DBL_MIN_EXP__ (-1021)

#define __FLT_MIN__ 1.17549435e-38F

#define __CHAR_BIT__ 8

#define __WCHAR_MAX__ 2147483647

#define __GCC_HAVE_SYNC_COMPARE_AND_SWAP_1 1

#define __GCC_HAVE_SYNC_COMPARE_AND_SWAP_2 1

#define __GCC_HAVE_SYNC_COMPARE_AND_SWAP_4 1

#define __DBL_DENORM_MIN__ 4.9406564584124654e-324

#define __FLT_EVAL_METHOD__ 2

#define __unix__ 1

#define __DBL_MIN_10_EXP__ (-307)

#define __FINITE_MATH_ONLY__ 0

#define __GNUC_PATCHLEVEL__ 3

#define __DEC64_MAX_EXP__ 385

#define __SHRT_MAX__ 32767

#define __LDBL_MAX__ 1.18973149535723176502e+4932L

#define __UINTMAX_TYPE__ long long unsigned int

#define __linux 1

#define __DEC32_EPSILON__ 1E-6DF

#define __unix 1

#define __LDBL_MAX_EXP__ 16384

#define __linux__ 1

#define __SCHAR_MAX__ 127

#define __DBL_DIG__ 15

#define _FORTIFY_SOURCE 2

#define __SIZEOF_INT__ 4

#define __SIZEOF_POINTER__ 4

#define __USER_LABEL_PREFIX__

#define __STDC_HOSTED__ 1

#define __LDBL_HAS_INFINITY__ 1

#define __FLT_EPSILON__ 1.19209290e-7F

#define __LDBL_MIN__ 3.36210314311209350626e-4932L

#define __DEC32_MAX__ 9.999999E96DF

#define __SIZEOF_LONG__ 4

#define __DECIMAL_DIG__ 21

#define __gnu_linux__ 1

#define __LDBL_HAS_QUIET_NAN__ 1

#define __GNUC__ 4

#define __FLT_HAS_DENORM__ 1

#define __SIZEOF_LONG_DOUBLE__ 12

#define __BIGGEST_ALIGNMENT__ 16

#define __DBL_MAX__ 1.7976931348623157e+308

#define __DBL_HAS_INFINITY__ 1

#define __DEC32_MIN_EXP__ (-94)

#define __LDBL_HAS_DENORM__ 1

#define __DEC128_MAX__ 9.999999999999999999999999999999999E6144DL

#define __DEC32_MIN__ 1E-95DF

#define __DBL_MAX_EXP__ 1024

#define __DEC128_EPSILON__ 1E-33DL

#define __LONG_LONG_MAX__ 9223372036854775807LL

#define __SIZEOF_SIZE_T__ 4

#define __SIZEOF_WINT_T__ 4

#define __GXX_ABI_VERSION 1002

#define __FLT_MIN_EXP__ (-125)

#define __DBL_MIN__ 2.2250738585072014e-308

#define __DECIMAL_BID_FORMAT__ 1

#define __DEC128_MIN__ 1E-6143DL

#define __REGISTER_PREFIX__

#define __DBL_HAS_DENORM__ 1

#define __NO_INLINE__ 1

#define __i386 1

#define __FLT_MANT_DIG__ 24

#define __VERSION__ "4.4.3"

#define __DEC64_EPSILON__ 1E-15DD

#define __DEC128_MIN_EXP__ (-6142)

#define __i486__ 1

#define unix 1

#define __i386__ 1

#define __SIZE_TYPE__ unsigned int

#define __ELF__ 1

#define __FLT_RADIX__ 2

#define __LDBL_EPSILON__ 1.08420217248550443401e-19L

#define __SIZEOF_PTRDIFF_T__ 4

#define __DEC32_SUBNORMAL_MIN__ 0.000001E-95DF

#define __FLT_HAS_QUIET_NAN__ 1

#define __FLT_MAX_10_EXP__ 38

#define __LONG_MAX__ 2147483647L

#define __DEC128_SUBNORMAL_MIN__ 0.000000000000000000000000000000001E-6143DL

#define __FLT_HAS_INFINITY__ 1

#define __DEC64_MAX__ 9.999999999999999E384DD

#define __CHAR16_TYPE__ short unsigned int

#define __DEC64_MANT_DIG__ 16

#define __DEC32_MAX_EXP__ 97

#define linux 1

#define __LDBL_MANT_DIG__ 64

#define __DBL_HAS_QUIET_NAN__ 1

#define __WCHAR_TYPE__ int

#define __SIZEOF_FLOAT__ 4

#define __DEC64_MIN_EXP__ (-382)

#define __FLT_DIG__ 6

#define __INT_MAX__ 2147483647

#define __i486 1

#define __FLT_MAX_EXP__ 128

#define __DBL_MANT_DIG__ 53

#define __DEC64_MIN__ 1E-383DD

#define __WINT_TYPE__ unsigned int

#define __SIZEOF_SHORT__ 2

#define __LDBL_MIN_EXP__ (-16381)

#define __SSP__ 1

#define __LDBL_MAX_10_EXP__ 4932

#define __DBL_EPSILON__ 2.2204460492503131e-16

#define __SIZEOF_WCHAR_T__ 4

#define __DEC_EVAL_METHOD__ 2

#define __INTMAX_MAX__ 9223372036854775807LL

#define __FLT_DENORM_MIN__ 1.40129846e-45F

#define __CHAR32_TYPE__ unsigned int

#define __FLT_MAX__ 3.40282347e+38F

#define __SIZEOF_DOUBLE__ 8

#define __FLT_MIN_10_EXP__ (-37)

#define __INTMAX_TYPE__ long long int

#define i386 1

#define __DEC128_MAX_EXP__ 6145

#define __GNUC_MINOR__ 4

#define __DEC32_MANT_DIG__ 7

#define __DBL_MAX_10_EXP__ 308

#define __LDBL_DENORM_MIN__ 3.64519953188247460253e-4951L

#define __STDC__ 1

#define __PTRDIFF_TYPE__ int

#define __DEC64_SUBNORMAL_MIN__ 0.000000000000001E-383DD

#define __DEC128_MANT_DIG__ 34

#define __LDBL_MIN_10_EXP__ (-4931)

#define __SIZEOF_LONG_LONG__ 8

#define __LDBL_DIG__ 18

#define __GNUC_GNU_INLINE__ 1