Why is _GNU_SOURCE defined by default and how to turn it off?

Question

I'm using g++ and it told me that I defined _GNU_SOURCE on the command line, but I didn't.

I now that defining _GNU_SOURCE can be used to enable the gnu extension, but now I don't want these extensions. I added -std=c++0x -pedantic, but _GNU_SOURCE is still define.

The out put of g++ -v is as follows:

$ g++ -v
Using built-in specs.
COLLECT_GCC=/usr/bin/g++
COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-linux-gnu/4.6/lto-wrapper
Target: x86_64-linux-gnu
Configured with: ../src/configure -v --with-pkgversion='Ubuntu/Linaro 4.6.3-1ubuntu5' --with-bugurl=file:///usr/share/doc/gcc-4.6/README.Bugs --enable-languages=c,c++,fortran,objc,obj-c++ --prefix=/usr --program-suffix=-4.6 --enable-shared --enable-linker-build-id --with-system-zlib --libexecdir=/usr/lib --without-included-gettext --enable-threads=posix --with-gxx-include-dir=/usr/include/c++/4.6 --libdir=/usr/lib --enable-nls --with-sysroot=/ --enable-clocale=gnu --enable-libstdcxx-debug --enable-libstdcxx-time=yes --enable-gnu-unique-object --enable-plugin --enable-objc-gc --disable-werror --with-arch-32=i686 --with-tune=generic --enable-checking=release --build=x86_64-linux-gnu --host=x86_64-linux-gnu --target=x86_64-linux-gnu
Thread model: posix
gcc version 4.6.3 (Ubuntu/Linaro 4.6.3-1ubuntu5)

When I give it an empty file and compile with g++ -E -dD -std=c++98 -pedantic empty.cpp, I get this:

$ g++ -E -dD -std=c++98 -pedantic empty.cpp
# 1 "empty.cpp"
# 1 "<built-in>"
#define __STDC__ 1
#define __cplusplus 1
#define __STDC_HOSTED__ 1
#define __GNUC__ 4
#define __GNUC_MINOR__ 6
#define __GNUC_PATCHLEVEL__ 3
#define __VERSION__ "4.6.3"
#define __FINITE_MATH_ONLY__ 0
#define _LP64 1
#define __LP64__ 1
#define __SIZEOF_INT__ 4
#define __SIZEOF_LONG__ 8
#define __SIZEOF_LONG_LONG__ 8
#define __SIZEOF_SHORT__ 2
#define __SIZEOF_FLOAT__ 4
#define __SIZEOF_DOUBLE__ 8
#define __SIZEOF_LONG_DOUBLE__ 16
#define __SIZEOF_SIZE_T__ 8
#define __CHAR_BIT__ 8
#define __BIGGEST_ALIGNMENT__ 16
#define __ORDER_LITTLE_ENDIAN__ 1234
#define __ORDER_BIG_ENDIAN__ 4321
#define __ORDER_PDP_ENDIAN__ 3412
#define __BYTE_ORDER__ __ORDER_LITTLE_ENDIAN__
#define __FLOAT_WORD_ORDER__ __ORDER_LITTLE_ENDIAN__
#define __SIZEOF_POINTER__ 8
#define __GNUG__ 4
#define __SIZE_TYPE__ long unsigned int
#define __PTRDIFF_TYPE__ long int
#define __WCHAR_TYPE__ int
#define __WINT_TYPE__ unsigned int
#define __INTMAX_TYPE__ long int
#define __UINTMAX_TYPE__ long unsigned int
#define __CHAR16_TYPE__ short unsigned int
#define __CHAR32_TYPE__ unsigned int
#define __SIG_ATOMIC_TYPE__ int
#define __INT8_TYPE__ signed char
#define __INT16_TYPE__ short int
#define __INT32_TYPE__ int
#define __INT64_TYPE__ long int
#define __UINT8_TYPE__ unsigned char
#define __UINT16_TYPE__ short unsigned int
#define __UINT32_TYPE__ unsigned int
#define __UINT64_TYPE__ long unsigned int
#define __INT_LEAST8_TYPE__ signed char
#define __INT_LEAST16_TYPE__ short int
#define __INT_LEAST32_TYPE__ int
#define __INT_LEAST64_TYPE__ long int
#define __UINT_LEAST8_TYPE__ unsigned char
#define __UINT_LEAST16_TYPE__ short unsigned int
#define __UINT_LEAST32_TYPE__ unsigned int
#define __UINT_LEAST64_TYPE__ long unsigned int
#define __INT_FAST8_TYPE__ signed char
#define __INT_FAST16_TYPE__ long int
#define __INT_FAST32_TYPE__ long int
#define __INT_FAST64_TYPE__ long int
#define __UINT_FAST8_TYPE__ unsigned char
#define __UINT_FAST16_TYPE__ long unsigned int
#define __UINT_FAST32_TYPE__ long unsigned int
#define __UINT_FAST64_TYPE__ long unsigned int
#define __INTPTR_TYPE__ long int
#define __UINTPTR_TYPE__ long unsigned int
#define __GXX_WEAK__ 1
#define __DEPRECATED 1
#define __GXX_RTTI 1
#define __EXCEPTIONS 1
#define __GXX_ABI_VERSION 1002
#define __SCHAR_MAX__ 127
#define __SHRT_MAX__ 32767
#define __INT_MAX__ 2147483647
#define __LONG_MAX__ 9223372036854775807L
#define __LONG_LONG_MAX__ 9223372036854775807LL
#define __WCHAR_MAX__ 2147483647
#define __WCHAR_MIN__ (-__WCHAR_MAX__ - 1)
#define __WINT_MAX__ 4294967295U
#define __WINT_MIN__ 0U
#define __PTRDIFF_MAX__ 9223372036854775807L
#define __SIZE_MAX__ 18446744073709551615UL
#define __INTMAX_MAX__ 9223372036854775807L
#define __INTMAX_C(c) c ## L
#define __UINTMAX_MAX__ 18446744073709551615UL
#define __UINTMAX_C(c) c ## UL
#define __SIG_ATOMIC_MAX__ 2147483647
#define __SIG_ATOMIC_MIN__ (-__SIG_ATOMIC_MAX__ - 1)
#define __INT8_MAX__ 127
#define __INT16_MAX__ 32767
#define __INT32_MAX__ 2147483647
#define __INT64_MAX__ 9223372036854775807L
#define __UINT8_MAX__ 255
#define __UINT16_MAX__ 65535
#define __UINT32_MAX__ 4294967295U
#define __UINT64_MAX__ 18446744073709551615UL
#define __INT_LEAST8_MAX__ 127
#define __INT8_C(c) c
#define __INT_LEAST16_MAX__ 32767
#define __INT16_C(c) c
#define __INT_LEAST32_MAX__ 2147483647
#define __INT32_C(c) c
#define __INT_LEAST64_MAX__ 9223372036854775807L
#define __INT64_C(c) c ## L
#define __UINT_LEAST8_MAX__ 255
#define __UINT8_C(c) c
#define __UINT_LEAST16_MAX__ 65535
#define __UINT16_C(c) c
#define __UINT_LEAST32_MAX__ 4294967295U
#define __UINT32_C(c) c ## U
#define __UINT_LEAST64_MAX__ 18446744073709551615UL
#define __UINT64_C(c) c ## UL
#define __INT_FAST8_MAX__ 127
#define __INT_FAST16_MAX__ 9223372036854775807L
#define __INT_FAST32_MAX__ 9223372036854775807L
#define __INT_FAST64_MAX__ 9223372036854775807L
#define __UINT_FAST8_MAX__ 255
#define __UINT_FAST16_MAX__ 18446744073709551615UL
#define __UINT_FAST32_MAX__ 18446744073709551615UL
#define __UINT_FAST64_MAX__ 18446744073709551615UL
#define __INTPTR_MAX__ 9223372036854775807L
#define __UINTPTR_MAX__ 18446744073709551615UL
#define __FLT_EVAL_METHOD__ 0
#define __DEC_EVAL_METHOD__ 2
#define __FLT_RADIX__ 2
#define __FLT_MANT_DIG__ 24
#define __FLT_DIG__ 6
#define __FLT_MIN_EXP__ (-125)
#define __FLT_MIN_10_EXP__ (-37)
#define __FLT_MAX_EXP__ 128
#define __FLT_MAX_10_EXP__ 38
#define __FLT_DECIMAL_DIG__ 9
#define __FLT_MAX__ 3.40282346638528859812e+38F
#define __FLT_MIN__ 1.17549435082228750797e-38F
#define __FLT_EPSILON__ 1.19209289550781250000e-7F
#define __FLT_DENORM_MIN__ 1.40129846432481707092e-45F
#define __FLT_HAS_DENORM__ 1
#define __FLT_HAS_INFINITY__ 1
#define __FLT_HAS_QUIET_NAN__ 1
#define __DBL_MANT_DIG__ 53
#define __DBL_DIG__ 15
#define __DBL_MIN_EXP__ (-1021)
#define __DBL_MIN_10_EXP__ (-307)
#define __DBL_MAX_EXP__ 1024
#define __DBL_MAX_10_EXP__ 308
#define __DBL_DECIMAL_DIG__ 17
#define __DBL_MAX__ double(1.79769313486231570815e+308L)
#define __DBL_MIN__ double(2.22507385850720138309e-308L)
#define __DBL_EPSILON__ double(2.22044604925031308085e-16L)
#define __DBL_DENORM_MIN__ double(4.94065645841246544177e-324L)
#define __DBL_HAS_DENORM__ 1
#define __DBL_HAS_INFINITY__ 1
#define __DBL_HAS_QUIET_NAN__ 1
#define __LDBL_MANT_DIG__ 64
#define __LDBL_DIG__ 18
#define __LDBL_MIN_EXP__ (-16381)
#define __LDBL_MIN_10_EXP__ (-4931)
#define __LDBL_MAX_EXP__ 16384
#define __LDBL_MAX_10_EXP__ 4932
#define __DECIMAL_DIG__ 21
#define __LDBL_MAX__ 1.18973149535723176502e+4932L
#define __LDBL_MIN__ 3.36210314311209350626e-4932L
#define __LDBL_EPSILON__ 1.08420217248550443401e-19L
#define __LDBL_DENORM_MIN__ 3.64519953188247460253e-4951L
#define __LDBL_HAS_DENORM__ 1
#define __LDBL_HAS_INFINITY__ 1
#define __LDBL_HAS_QUIET_NAN__ 1
#define __DEC32_MANT_DIG__ 7
#define __DEC32_MIN_EXP__ (-94)
#define __DEC32_MAX_EXP__ 97
#define __DEC32_MIN__ 1E-95DF
#define __DEC32_MAX__ 9.999999E96DF
#define __DEC32_EPSILON__ 1E-6DF
#define __DEC32_SUBNORMAL_MIN__ 0.000001E-95DF
#define __DEC64_MANT_DIG__ 16
#define __DEC64_MIN_EXP__ (-382)
#define __DEC64_MAX_EXP__ 385
#define __DEC64_MIN__ 1E-383DD
#define __DEC64_MAX__ 9.999999999999999E384DD
#define __DEC64_EPSILON__ 1E-15DD
#define __DEC64_SUBNORMAL_MIN__ 0.000000000000001E-383DD
#define __DEC128_MANT_DIG__ 34
#define __DEC128_MIN_EXP__ (-6142)
#define __DEC128_MAX_EXP__ 6145
#define __DEC128_MIN__ 1E-6143DL
#define __DEC128_MAX__ 9.999999999999999999999999999999999E6144DL
#define __DEC128_EPSILON__ 1E-33DL
#define __DEC128_SUBNORMAL_MIN__ 0.000000000000000000000000000000001E-6143DL
#define __REGISTER_PREFIX__
#define __USER_LABEL_PREFIX__
#define _FORTIFY_SOURCE 2
#define __GNUC_GNU_INLINE__ 1
#define __NO_INLINE__ 1
#define __STRICT_ANSI__ 1
#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 __GCC_HAVE_SYNC_COMPARE_AND_SWAP_8 1
#define __GCC_HAVE_DWARF2_CFI_ASM 1
#define __PRAGMA_REDEFINE_EXTNAME 1
#define __SSP__ 1
#define __SIZEOF_INT128__ 16
#define __SIZEOF_WCHAR_T__ 4
#define __SIZEOF_WINT_T__ 4
#define __SIZEOF_PTRDIFF_T__ 8
#define __amd64 1
#define __amd64__ 1
#define __x86_64 1
#define __x86_64__ 1
#define __k8 1
#define __k8__ 1
#define __MMX__ 1
#define __SSE__ 1
#define __SSE2__ 1
#define __SSE_MATH__ 1
#define __SSE2_MATH__ 1
#define __gnu_linux__ 1
#define __linux 1
#define __linux__ 1
#define __unix 1
#define __unix__ 1
#define __ELF__ 1
#define __DECIMAL_BID_FORMAT__ 1
# 1 "<command-line>"
#define _GNU_SOURCE 1
# 1 "empty.cpp"

The result's telling me that _GNU_SOURCE is defined at command line.

How can I turn it off? I don't want to #undef _GNU_SOURCE in every source file.

Answer 1

Did some search on the internet, and found quite a lot of complaints about this.

It is in fact that libstdc++ needs this macro to be defined.

Simple answer, currently _GNU_SOURCE is required to be defined for libstdc++, and it won't be fixed in a short period of time.

There is even an faq on the libstd++ doc:

On Solaris, g++ (but not gcc) always defines the preprocessor macro _XOPEN_SOURCE. On GNU/Linux, the same happens with _GNU_SOURCE. (This is not an exhaustive list; other macros and other platforms are also affected.)

These macros are typically used in C library headers, guarding new versions of functions from their older versions. The C++ standard library includes the C standard library, but it requires the C90 version, which for backwards-compatibility reasons is often not the default for many vendors.

More to the point, the C++ standard requires behavior which is only available on certain platforms after certain symbols are defined. Usually the issue involves I/O-related typedefs. In order to ensure correctness, the compiler simply predefines those symbols.

Note that it's not enough to #define them only when the library is being built (during installation). Since we don't have an 'export' keyword, much of the library exists as headers, which means that the symbols must also be defined as your programs are parsed and compiled.

To see which symbols are defined, look for CPLUSPLUS_CPP_SPEC in the gcc config headers for your target (and try changing them to see what happens when building complicated code). You can also run "g++ -E -dM - < /dev/null" to display a list of predefined macros for any particular installation.

This has been discussed on the mailing lists quite a bit.

This method is something of a wart. We'd like to find a cleaner solution, but nobody yet has contributed the time.