C standard header file for non-printable/control character constants - c

So, whenever I need to reference certain ASCII control characters I find myself doing junk like this:
#define STX 2
#define ETX 3
Is there some C header file found in most development environments (LINUX probably) that define these constants for me so that I don't have to use magic numbers or define them all the time? Something like <stdint.h>.
I've been writing C code for 35 years and I've never come across such a header file.


The best I can do for you off the top of my head:
1. Using the Linux Documentation Project's manpage source
This file is likely to be present somewhere on your machine; the path I used is where you'll find it on a Debian/Ubuntu install. The list only includes C0 codes, but maybe that's good enough:
$ gunzip -c /usr/share/man/man7/ascii.7.gz |
> awk '$1~/0[0-3][0-7]/{printf "#define C0_%-3s %2d\n", $4, $2}'
#define C0_NUL 0
#define C0_SOH 1
#define C0_STX 2
#define C0_ETX 3
#define C0_EOT 4
#define C0_ENQ 5
#define C0_ACK 6
#define C0_BEL 7
#define C0_BS 8
#define C0_HT 9
#define C0_LF 10
#define C0_VT 11
#define C0_FF 12
#define C0_CR 13
#define C0_SO 14
#define C0_SI 15
#define C0_DLE 16
#define C0_DC1 17
#define C0_DC2 18
#define C0_DC3 19
#define C0_DC4 20
#define C0_NAK 21
#define C0_SYN 22
#define C0_ETB 23
#define C0_CAN 24
#define C0_EM 25
#define C0_SUB 26
#define C0_ESC 27
#define C0_FS 28
#define C0_GS 29
#define C0_RS 30
#define C0_US 31
2. Using the Unicode Character Database.
This includes a lot more, with lots of Unicode control characters, some of them in the astral plane, although obviously you can filter it. It also includes some extra abbreviation names. The file NamedAliases.txt can be downloaded from https://www.unicode.org/Public/14.0.0/ucd/NameAliases.txt or you can grab the entire UCD as a zipped archive at https://www.unicode.org/Public/zipped/14.0.0/
I truncated the output at 60 lines; the entire list is 353 lines.
awk -F\; '$3=="abbreviation"
{printf "#define CTL_%-8s 0x%s\n", $2, $1}' \
UCD/NameAliases.txt
#define CTL_NUL 0x0000
#define CTL_SOH 0x0001
#define CTL_STX 0x0002
#define CTL_ETX 0x0003
#define CTL_EOT 0x0004
#define CTL_ENQ 0x0005
#define CTL_ACK 0x0006
#define CTL_BEL 0x0007
#define CTL_BS 0x0008
#define CTL_HT 0x0009
#define CTL_TAB 0x0009
#define CTL_LF 0x000A
#define CTL_NL 0x000A
#define CTL_EOL 0x000A
#define CTL_VT 0x000B
#define CTL_FF 0x000C
#define CTL_CR 0x000D
#define CTL_SO 0x000E
#define CTL_SI 0x000F
#define CTL_DLE 0x0010
#define CTL_DC1 0x0011
#define CTL_DC2 0x0012
#define CTL_DC3 0x0013
#define CTL_DC4 0x0014
#define CTL_NAK 0x0015
#define CTL_SYN 0x0016
#define CTL_ETB 0x0017
#define CTL_CAN 0x0018
#define CTL_EOM 0x0019
#define CTL_SUB 0x001A
#define CTL_ESC 0x001B
#define CTL_FS 0x001C
#define CTL_GS 0x001D
#define CTL_RS 0x001E
#define CTL_US 0x001F
#define CTL_SP 0x0020
#define CTL_DEL 0x007F
#define CTL_PAD 0x0080
#define CTL_HOP 0x0081
#define CTL_BPH 0x0082
#define CTL_NBH 0x0083
#define CTL_IND 0x0084
#define CTL_NEL 0x0085
#define CTL_SSA 0x0086
#define CTL_ESA 0x0087
#define CTL_HTS 0x0088
#define CTL_HTJ 0x0089
#define CTL_VTS 0x008A
#define CTL_PLD 0x008B
#define CTL_PLU 0x008C
#define CTL_RI 0x008D
#define CTL_SS2 0x008E
#define CTL_SS3 0x008F
#define CTL_DCS 0x0090
#define CTL_PU1 0x0091
#define CTL_PU2 0x0092
#define CTL_STS 0x0093
#define CTL_CCH 0x0094
#define CTL_MW 0x0095
#define CTL_SPA 0x0096
...

Related

Strange token stringification in C with linux-* variable or string [duplicate]

Why does the C preprocessor in GCC interpret the word linux (small letters) as the constant 1?
test.c:
#include <stdio.h>
int main(void)
{
int linux = 5;
return 0;
}
Result of $ gcc -E test.c (stop after the preprocessing stage):
....
int main(void)
{
int 1 = 5;
return 0;
}
Which of course yields an error.
(BTW: There is no #define linux in the stdio.h file.)

In the Old Days (pre-ANSI), predefining symbols such as unix and vax was a way to allow code to detect at compile time what system it was being compiled for. There was no official language standard back then (beyond the reference material at the back of the first edition of K&R), and C code of any complexity was typically a complex maze of #ifdefs to allow for differences between systems. These macro definitions were generally set by the compiler itself, not defined in a library header file. Since there were no real rules about which identifiers could be used by the implementation and which were reserved for programmers, compiler writers felt free to use simple names like unix and assumed that programmers would simply avoid using those names for their own purposes.
The 1989 ANSI C standard introduced rules restricting what symbols an implementation could legally predefine. A macro predefined by the compiler could only have a name starting with two underscores, or with an underscore followed by an uppercase letter, leaving programmers free to use identifiers not matching that pattern and not used in the standard library.
As a result, any compiler that predefines unix or linux is non-conforming, since it will fail to compile perfectly legal code that uses something like int linux = 5;.
As it happens, gcc is non-conforming by default -- but it can be made to conform (reasonably well) with the right command-line options:
gcc -std=c90 -pedantic ... # or -std=c89 or -ansi
gcc -std=c99 -pedantic
gcc -std=c11 -pedantic
See the gcc manual for more details.
gcc will be phasing out these definitions in future releases, so you shouldn't write code that depends on them. If your program needs to know whether it's being compiled for a Linux target or not it can check whether __linux__ is defined (assuming you're using gcc or a compiler that's compatible with it). See the GNU C preprocessor manual for more information.
A largely irrelevant aside: the "Best One Liner" winner of the 1987 International Obfuscated C Code Contest, by David Korn (yes, the author of the Korn Shell) took advantage of the predefined unix macro:
main() { printf(&unix["\021%six\012\0"],(unix)["have"]+"fun"-0x60);}
It prints "unix", but for reasons that have absolutely nothing to do with the spelling of the macro name.
I don't want to post any spoilers here, and I encourage anyone reading this to try to understand that code on their own first. But if you really want to give up, I've posted an explanation here:
https://gist.github.com/Keith-S-Thompson/6920347

This appears to be an (undocumented) "GNU extension": [correction: I finally found a mention in the docs. See below.]
The following command uses the -dM option to print all preprocessor defines; since the input "file" is empty, it shows exactly the predefined macros. It was run with gcc-4.7.3 on a standard ubuntu install. You can see that the preprocessor is standard-aware. In total, there 243 macros with -std=gnu99 and 240 with -std=c99; I filtered the output for relevance.
$ cpp --std=c89 -dM < /dev/null | grep linux
#define __linux 1
#define __linux__ 1
#define __gnu_linux__ 1
$ cpp --std=gnu89 -dM < /dev/null | grep linux
#define __linux 1
#define __linux__ 1
#define __gnu_linux__ 1
#define linux 1
$ cpp --std=c99 -dM < /dev/null | grep linux
#define __linux 1
#define __linux__ 1
#define __gnu_linux__ 1
$ cpp --std=gnu99 -dM < /dev/null | grep linux
#define __linux 1
#define __linux__ 1
#define __gnu_linux__ 1
#define linux 1
The "gnu standard" versions also #define unix. (Using c11 and gnu11 produces the same results.)
I suppose they had their reasons, but it seems to me to make the default installation of gcc (which compiles C code with -std=gnu89 unless otherwise specified) non-conformant, and -- as in this question -- surprising. Polluting the global namespace with macros whose names don't begin with an underscore is not permitted in a conformant implementation. (6.8.10p2: "Any other predefined macro names shall begin with a leading underscore followed by an uppercase letter or a second
underscore," but, as mentioned in Appendix J.5 (portability issues), such names are often predefined.)
When I originally wrote this answer, I wasn't able to find any documentation in gcc about this issue, but I did finally discover it, not in C implementation-defined behaviour nor in C extensions but in the cpp manual section 3.7.3, where it notes that:
We are slowly phasing out all predefined macros which are outside the reserved namespace. You should never use them in new programs…

Because linux is a built-in macro defined when the compiler is running on, or compiling for (if it is a cross-compiler), Linux.
There are a lot of such predefined macros. With GCC, you can use:
cp /dev/null emptyfile.c
gcc -E -dM emptyfile.c
to get a list of macros. (I've not managed to persuade GCC to accept /dev/null directly, but
the empty file seems to work OK.) With GCC 4.8.1 running on Mac OS X 10.8.5, I got the output:
#define __DBL_MIN_EXP__ (-1021)
#define __UINT_LEAST16_MAX__ 65535
#define __ATOMIC_ACQUIRE 2
#define __FLT_MIN__ 1.17549435082228750797e-38F
#define __UINT_LEAST8_TYPE__ unsigned char
#define __INTMAX_C(c) c ## L
#define __CHAR_BIT__ 8
#define __UINT8_MAX__ 255
#define __WINT_MAX__ 2147483647
#define __ORDER_LITTLE_ENDIAN__ 1234
#define __SIZE_MAX__ 18446744073709551615UL
#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__ ((double)4.94065645841246544177e-324L)
#define __GCC_HAVE_SYNC_COMPARE_AND_SWAP_8 1
#define __GCC_ATOMIC_CHAR_LOCK_FREE 2
#define __FLT_EVAL_METHOD__ 0
#define __GCC_ATOMIC_CHAR32_T_LOCK_FREE 2
#define __x86_64 1
#define __UINT_FAST64_MAX__ 18446744073709551615ULL
#define __SIG_ATOMIC_TYPE__ int
#define __DBL_MIN_10_EXP__ (-307)
#define __FINITE_MATH_ONLY__ 0
#define __GNUC_PATCHLEVEL__ 1
#define __UINT_FAST8_MAX__ 255
#define __DEC64_MAX_EXP__ 385
#define __INT8_C(c) c
#define __UINT_LEAST64_MAX__ 18446744073709551615ULL
#define __SHRT_MAX__ 32767
#define __LDBL_MAX__ 1.18973149535723176502e+4932L
#define __UINT_LEAST8_MAX__ 255
#define __GCC_ATOMIC_BOOL_LOCK_FREE 2
#define __APPLE_CC__ 1
#define __UINTMAX_TYPE__ long unsigned int
#define __DEC32_EPSILON__ 1E-6DF
#define __UINT32_MAX__ 4294967295U
#define __LDBL_MAX_EXP__ 16384
#define __WINT_MIN__ (-__WINT_MAX__ - 1)
#define __SCHAR_MAX__ 127
#define __WCHAR_MIN__ (-__WCHAR_MAX__ - 1)
#define __INT64_C(c) c ## LL
#define __DBL_DIG__ 15
#define __GCC_ATOMIC_POINTER_LOCK_FREE 2
#define __SIZEOF_INT__ 4
#define __SIZEOF_POINTER__ 8
#define __USER_LABEL_PREFIX__ _
#define __STDC_HOSTED__ 1
#define __LDBL_HAS_INFINITY__ 1
#define __FLT_EPSILON__ 1.19209289550781250000e-7F
#define __LDBL_MIN__ 3.36210314311209350626e-4932L
#define __DEC32_MAX__ 9.999999E96DF
#define __strong
#define __INT32_MAX__ 2147483647
#define __SIZEOF_LONG__ 8
#define __APPLE__ 1
#define __UINT16_C(c) c
#define __DECIMAL_DIG__ 21
#define __LDBL_HAS_QUIET_NAN__ 1
#define __DYNAMIC__ 1
#define __GNUC__ 4
#define __MMX__ 1
#define __FLT_HAS_DENORM__ 1
#define __SIZEOF_LONG_DOUBLE__ 16
#define __BIGGEST_ALIGNMENT__ 16
#define __DBL_MAX__ ((double)1.79769313486231570815e+308L)
#define __INT_FAST32_MAX__ 2147483647
#define __DBL_HAS_INFINITY__ 1
#define __DEC32_MIN_EXP__ (-94)
#define __INT_FAST16_TYPE__ short int
#define __LDBL_HAS_DENORM__ 1
#define __DEC128_MAX__ 9.999999999999999999999999999999999E6144DL
#define __INT_LEAST32_MAX__ 2147483647
#define __DEC32_MIN__ 1E-95DF
#define __weak
#define __DBL_MAX_EXP__ 1024
#define __DEC128_EPSILON__ 1E-33DL
#define __SSE2_MATH__ 1
#define __ATOMIC_HLE_RELEASE 131072
#define __PTRDIFF_MAX__ 9223372036854775807L
#define __amd64 1
#define __tune_core2__ 1
#define __ATOMIC_HLE_ACQUIRE 65536
#define __LONG_LONG_MAX__ 9223372036854775807LL
#define __SIZEOF_SIZE_T__ 8
#define __SIZEOF_WINT_T__ 4
#define __GXX_ABI_VERSION 1002
#define __FLT_MIN_EXP__ (-125)
#define __INT_FAST64_TYPE__ long long int
#define __DBL_MIN__ ((double)2.22507385850720138309e-308L)
#define __LP64__ 1
#define __DEC128_MIN__ 1E-6143DL
#define __REGISTER_PREFIX__
#define __UINT16_MAX__ 65535
#define __DBL_HAS_DENORM__ 1
#define __UINT8_TYPE__ unsigned char
#define __NO_INLINE__ 1
#define __FLT_MANT_DIG__ 24
#define __VERSION__ "4.8.1"
#define __UINT64_C(c) c ## ULL
#define __GCC_ATOMIC_INT_LOCK_FREE 2
#define __FLOAT_WORD_ORDER__ __ORDER_LITTLE_ENDIAN__
#define __INT32_C(c) c
#define __DEC64_EPSILON__ 1E-15DD
#define __ORDER_PDP_ENDIAN__ 3412
#define __DEC128_MIN_EXP__ (-6142)
#define __INT_FAST32_TYPE__ int
#define __UINT_LEAST16_TYPE__ short unsigned int
#define __INT16_MAX__ 32767
#define __ENVIRONMENT_MAC_OS_X_VERSION_MIN_REQUIRED__ 1080
#define __SIZE_TYPE__ long unsigned int
#define __UINT64_MAX__ 18446744073709551615ULL
#define __INT8_TYPE__ signed char
#define __FLT_RADIX__ 2
#define __INT_LEAST16_TYPE__ short int
#define __LDBL_EPSILON__ 1.08420217248550443401e-19L
#define __UINTMAX_C(c) c ## UL
#define __SSE_MATH__ 1
#define __k8 1
#define __SIG_ATOMIC_MAX__ 2147483647
#define __GCC_ATOMIC_WCHAR_T_LOCK_FREE 2
#define __SIZEOF_PTRDIFF_T__ 8
#define __x86_64__ 1
#define __DEC32_SUBNORMAL_MIN__ 0.000001E-95DF
#define __INT_FAST16_MAX__ 32767
#define __UINT_FAST32_MAX__ 4294967295U
#define __UINT_LEAST64_TYPE__ long long unsigned int
#define __FLT_HAS_QUIET_NAN__ 1
#define __FLT_MAX_10_EXP__ 38
#define __LONG_MAX__ 9223372036854775807L
#define __DEC128_SUBNORMAL_MIN__ 0.000000000000000000000000000000001E-6143DL
#define __FLT_HAS_INFINITY__ 1
#define __UINT_FAST16_TYPE__ short unsigned int
#define __DEC64_MAX__ 9.999999999999999E384DD
#define __CHAR16_TYPE__ short unsigned int
#define __PRAGMA_REDEFINE_EXTNAME 1
#define __INT_LEAST16_MAX__ 32767
#define __DEC64_MANT_DIG__ 16
#define __INT64_MAX__ 9223372036854775807LL
#define __UINT_LEAST32_MAX__ 4294967295U
#define __GCC_ATOMIC_LONG_LOCK_FREE 2
#define __INT_LEAST64_TYPE__ long long int
#define __INT16_TYPE__ short int
#define __INT_LEAST8_TYPE__ signed char
#define __DEC32_MAX_EXP__ 97
#define __INT_FAST8_MAX__ 127
#define __INTPTR_MAX__ 9223372036854775807L
#define __LITTLE_ENDIAN__ 1
#define __SSE2__ 1
#define __LDBL_MANT_DIG__ 64
#define __CONSTANT_CFSTRINGS__ 1
#define __DBL_HAS_QUIET_NAN__ 1
#define __SIG_ATOMIC_MIN__ (-__SIG_ATOMIC_MAX__ - 1)
#define __code_model_small__ 1
#define __k8__ 1
#define __INTPTR_TYPE__ long int
#define __UINT16_TYPE__ short unsigned int
#define __WCHAR_TYPE__ int
#define __SIZEOF_FLOAT__ 4
#define __pic__ 2
#define __UINTPTR_MAX__ 18446744073709551615UL
#define __DEC64_MIN_EXP__ (-382)
#define __INT_FAST64_MAX__ 9223372036854775807LL
#define __GCC_ATOMIC_TEST_AND_SET_TRUEVAL 1
#define __FLT_DIG__ 6
#define __UINT_FAST64_TYPE__ long long unsigned int
#define __INT_MAX__ 2147483647
#define __MACH__ 1
#define __amd64__ 1
#define __INT64_TYPE__ long long int
#define __FLT_MAX_EXP__ 128
#define __ORDER_BIG_ENDIAN__ 4321
#define __DBL_MANT_DIG__ 53
#define __INT_LEAST64_MAX__ 9223372036854775807LL
#define __GCC_ATOMIC_CHAR16_T_LOCK_FREE 2
#define __DEC64_MIN__ 1E-383DD
#define __WINT_TYPE__ int
#define __UINT_LEAST32_TYPE__ unsigned int
#define __SIZEOF_SHORT__ 2
#define __SSE__ 1
#define __LDBL_MIN_EXP__ (-16381)
#define __INT_LEAST8_MAX__ 127
#define __SIZEOF_INT128__ 16
#define __LDBL_MAX_10_EXP__ 4932
#define __ATOMIC_RELAXED 0
#define __DBL_EPSILON__ ((double)2.22044604925031308085e-16L)
#define _LP64 1
#define __UINT8_C(c) c
#define __INT_LEAST32_TYPE__ int
#define __SIZEOF_WCHAR_T__ 4
#define __UINT64_TYPE__ long long unsigned int
#define __INT_FAST8_TYPE__ signed char
#define __DBL_DECIMAL_DIG__ 17
#define __FXSR__ 1
#define __DEC_EVAL_METHOD__ 2
#define __UINT32_C(c) c ## U
#define __INTMAX_MAX__ 9223372036854775807L
#define __BYTE_ORDER__ __ORDER_LITTLE_ENDIAN__
#define __FLT_DENORM_MIN__ 1.40129846432481707092e-45F
#define __INT8_MAX__ 127
#define __PIC__ 2
#define __UINT_FAST32_TYPE__ unsigned int
#define __CHAR32_TYPE__ unsigned int
#define __FLT_MAX__ 3.40282346638528859812e+38F
#define __INT32_TYPE__ int
#define __SIZEOF_DOUBLE__ 8
#define __FLT_MIN_10_EXP__ (-37)
#define __INTMAX_TYPE__ long int
#define __DEC128_MAX_EXP__ 6145
#define __ATOMIC_CONSUME 1
#define __GNUC_MINOR__ 8
#define __UINTMAX_MAX__ 18446744073709551615UL
#define __DEC32_MANT_DIG__ 7
#define __DBL_MAX_10_EXP__ 308
#define __LDBL_DENORM_MIN__ 3.64519953188247460253e-4951L
#define __INT16_C(c) c
#define __STDC__ 1
#define __PTRDIFF_TYPE__ long int
#define __ATOMIC_SEQ_CST 5
#define __UINT32_TYPE__ unsigned int
#define __UINTPTR_TYPE__ long unsigned 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 __GCC_ATOMIC_LLONG_LOCK_FREE 2
#define __LDBL_DIG__ 18
#define __FLT_DECIMAL_DIG__ 9
#define __UINT_FAST16_MAX__ 65535
#define __GNUC_GNU_INLINE__ 1
#define __GCC_ATOMIC_SHORT_LOCK_FREE 2
#define __SSE3__ 1
#define __UINT_FAST8_TYPE__ unsigned char
#define __ATOMIC_ACQ_REL 4
#define __ATOMIC_RELEASE 3
That's 236 macros from an empty file. When I added #include <stdio.h> to the file, the number of macros defined went up to 505. These includes all sorts of platform-identifying macros.

From info gcc (emphasis mine):
-ansi
In C mode, this is equivalent to -std=c90. In C++ mode, it is
equivalent to -std=c++98.
This turns off certain features of GCC that are incompatible with
ISO C90 (when compiling C code), or of standard C++ (when compiling
C++ code), such as the asm and typeof keywords, and predefined
macros such as 'unix' and 'vax' that identify the type of system
you are using. It also enables the undesirable and rarely used ISO
trigraph feature. For the C compiler, it disables recognition of
C++ style // comments as well as the inline keyword.
(It uses vax in the example instead of linux because when it was written maybe it was more popular ;-).
The basic idea is that GCC only tries to fully comply with the ISO standards when it is invoked with the -ansi option.

Use this command
gcc -dM -E - < /dev/null
to get this
#define _LP64 1
#define _STDC_PREDEF_H 1
#define __ATOMIC_ACQUIRE 2
#define __ATOMIC_ACQ_REL 4
#define __ATOMIC_CONSUME 1
#define __ATOMIC_HLE_ACQUIRE 65536
#define __ATOMIC_HLE_RELEASE 131072
#define __ATOMIC_RELAXED 0
#define __ATOMIC_RELEASE 3
#define __ATOMIC_SEQ_CST 5
#define __BIGGEST_ALIGNMENT__ 16
#define __BYTE_ORDER__ __ORDER_LITTLE_ENDIAN__
#define __CHAR16_TYPE__ short unsigned int
#define __CHAR32_TYPE__ unsigned int
#define __CHAR_BIT__ 8
#define __DBL_DECIMAL_DIG__ 17
#define __DBL_DENORM_MIN__ ((double)4.94065645841246544177e-324L)
#define __DBL_DIG__ 15
#define __DBL_EPSILON__ ((double)2.22044604925031308085e-16L)
#define __DBL_HAS_DENORM__ 1
#define __DBL_HAS_INFINITY__ 1
#define __DBL_HAS_QUIET_NAN__ 1
#define __DBL_MANT_DIG__ 53
#define __DBL_MAX_10_EXP__ 308
#define __DBL_MAX_EXP__ 1024
#define __DBL_MAX__ ((double)1.79769313486231570815e+308L)
#define __DBL_MIN_10_EXP__ (-307)
#define __DBL_MIN_EXP__ (-1021)
#define __DBL_MIN__ ((double)2.22507385850720138309e-308L)
#define __DEC128_EPSILON__ 1E-33DL
#define __DEC128_MANT_DIG__ 34
#define __DEC128_MAX_EXP__ 6145
#define __DEC128_MAX__ 9.999999999999999999999999999999999E6144DL
#define __DEC128_MIN_EXP__ (-6142)
#define __DEC128_MIN__ 1E-6143DL
#define __DEC128_SUBNORMAL_MIN__ 0.000000000000000000000000000000001E-6143DL
#define __DEC32_EPSILON__ 1E-6DF
#define __DEC32_MANT_DIG__ 7
#define __DEC32_MAX_EXP__ 97
#define __DEC32_MAX__ 9.999999E96DF
#define __DEC32_MIN_EXP__ (-94)
#define __DEC32_MIN__ 1E-95DF
#define __DEC32_SUBNORMAL_MIN__ 0.000001E-95DF
#define __DEC64_EPSILON__ 1E-15DD
#define __DEC64_MANT_DIG__ 16
#define __DEC64_MAX_EXP__ 385
#define __DEC64_MAX__ 9.999999999999999E384DD
#define __DEC64_MIN_EXP__ (-382)
#define __DEC64_MIN__ 1E-383DD
#define __DEC64_SUBNORMAL_MIN__ 0.000000000000001E-383DD
#define __DECIMAL_BID_FORMAT__ 1
#define __DECIMAL_DIG__ 21
#define __DEC_EVAL_METHOD__ 2
#define __ELF__ 1
#define __FINITE_MATH_ONLY__ 0
#define __FLOAT_WORD_ORDER__ __ORDER_LITTLE_ENDIAN__
#define __FLT_DECIMAL_DIG__ 9
#define __FLT_DENORM_MIN__ 1.40129846432481707092e-45F
#define __FLT_DIG__ 6
#define __FLT_EPSILON__ 1.19209289550781250000e-7F
#define __FLT_EVAL_METHOD__ 0
#define __FLT_HAS_DENORM__ 1
#define __FLT_HAS_INFINITY__ 1
#define __FLT_HAS_QUIET_NAN__ 1
#define __FLT_MANT_DIG__ 24
#define __FLT_MAX_10_EXP__ 38
#define __FLT_MAX_EXP__ 128
#define __FLT_MAX__ 3.40282346638528859812e+38F
#define __FLT_MIN_10_EXP__ (-37)
#define __FLT_MIN_EXP__ (-125)
#define __FLT_MIN__ 1.17549435082228750797e-38F
#define __FLT_RADIX__ 2
#define __FXSR__ 1
#define __GCC_ASM_FLAG_OUTPUTS__ 1
#define __GCC_ATOMIC_BOOL_LOCK_FREE 2
#define __GCC_ATOMIC_CHAR16_T_LOCK_FREE 2
#define __GCC_ATOMIC_CHAR32_T_LOCK_FREE 2
#define __GCC_ATOMIC_CHAR_LOCK_FREE 2
#define __GCC_ATOMIC_INT_LOCK_FREE 2
#define __GCC_ATOMIC_LLONG_LOCK_FREE 2
#define __GCC_ATOMIC_LONG_LOCK_FREE 2
#define __GCC_ATOMIC_POINTER_LOCK_FREE 2
#define __GCC_ATOMIC_SHORT_LOCK_FREE 2
#define __GCC_ATOMIC_TEST_AND_SET_TRUEVAL 1
#define __GCC_ATOMIC_WCHAR_T_LOCK_FREE 2
#define __GCC_HAVE_DWARF2_CFI_ASM 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_IEC_559 2
#define __GCC_IEC_559_COMPLEX 2
#define __GNUC_MINOR__ 3
#define __GNUC_PATCHLEVEL__ 0
#define __GNUC_STDC_INLINE__ 1
#define __GNUC__ 6
#define __GXX_ABI_VERSION 1010
#define __INT16_C(c) c
#define __INT16_MAX__ 0x7fff
#define __INT16_TYPE__ short int
#define __INT32_C(c) c
#define __INT32_MAX__ 0x7fffffff
#define __INT32_TYPE__ int
#define __INT64_C(c) c ## L
#define __INT64_MAX__ 0x7fffffffffffffffL
#define __INT64_TYPE__ long int
#define __INT8_C(c) c
#define __INT8_MAX__ 0x7f
#define __INT8_TYPE__ signed char
#define __INTMAX_C(c) c ## L
#define __INTMAX_MAX__ 0x7fffffffffffffffL
#define __INTMAX_TYPE__ long int
#define __INTPTR_MAX__ 0x7fffffffffffffffL
#define __INTPTR_TYPE__ long int
#define __INT_FAST16_MAX__ 0x7fffffffffffffffL
#define __INT_FAST16_TYPE__ long int
#define __INT_FAST32_MAX__ 0x7fffffffffffffffL
#define __INT_FAST32_TYPE__ long int
#define __INT_FAST64_MAX__ 0x7fffffffffffffffL
#define __INT_FAST64_TYPE__ long int
#define __INT_FAST8_MAX__ 0x7f
#define __INT_FAST8_TYPE__ signed char
#define __INT_LEAST16_MAX__ 0x7fff
#define __INT_LEAST16_TYPE__ short int
#define __INT_LEAST32_MAX__ 0x7fffffff
#define __INT_LEAST32_TYPE__ int
#define __INT_LEAST64_MAX__ 0x7fffffffffffffffL
#define __INT_LEAST64_TYPE__ long int
#define __INT_LEAST8_MAX__ 0x7f
#define __INT_LEAST8_TYPE__ signed char
#define __INT_MAX__ 0x7fffffff
#define __LDBL_DENORM_MIN__ 3.64519953188247460253e-4951L
#define __LDBL_DIG__ 18
#define __LDBL_EPSILON__ 1.08420217248550443401e-19L
#define __LDBL_HAS_DENORM__ 1
#define __LDBL_HAS_INFINITY__ 1
#define __LDBL_HAS_QUIET_NAN__ 1
#define __LDBL_MANT_DIG__ 64
#define __LDBL_MAX_10_EXP__ 4932
#define __LDBL_MAX_EXP__ 16384
#define __LDBL_MAX__ 1.18973149535723176502e+4932L
#define __LDBL_MIN_10_EXP__ (-4931)
#define __LDBL_MIN_EXP__ (-16381)
#define __LDBL_MIN__ 3.36210314311209350626e-4932L
#define __LONG_LONG_MAX__ 0x7fffffffffffffffLL
#define __LONG_MAX__ 0x7fffffffffffffffL
#define __LP64__ 1
#define __MMX__ 1
#define __NO_INLINE__ 1
#define __ORDER_BIG_ENDIAN__ 4321
#define __ORDER_LITTLE_ENDIAN__ 1234
#define __ORDER_PDP_ENDIAN__ 3412
#define __PIC__ 2
#define __PIE__ 2
#define __PRAGMA_REDEFINE_EXTNAME 1
#define __PTRDIFF_MAX__ 0x7fffffffffffffffL
#define __PTRDIFF_TYPE__ long int
#define __REGISTER_PREFIX__
#define __SCHAR_MAX__ 0x7f
#define __SEG_FS 1
#define __SEG_GS 1
#define __SHRT_MAX__ 0x7fff
#define __SIG_ATOMIC_MAX__ 0x7fffffff
#define __SIG_ATOMIC_MIN__ (-__SIG_ATOMIC_MAX__ - 1)
#define __SIG_ATOMIC_TYPE__ int
#define __SIZEOF_DOUBLE__ 8
#define __SIZEOF_FLOAT128__ 16
#define __SIZEOF_FLOAT80__ 16
#define __SIZEOF_FLOAT__ 4
#define __SIZEOF_INT128__ 16
#define __SIZEOF_INT__ 4
#define __SIZEOF_LONG_DOUBLE__ 16
#define __SIZEOF_LONG_LONG__ 8
#define __SIZEOF_LONG__ 8
#define __SIZEOF_POINTER__ 8
#define __SIZEOF_PTRDIFF_T__ 8
#define __SIZEOF_SHORT__ 2
#define __SIZEOF_SIZE_T__ 8
#define __SIZEOF_WCHAR_T__ 4
#define __SIZEOF_WINT_T__ 4
#define __SIZE_MAX__ 0xffffffffffffffffUL
#define __SIZE_TYPE__ long unsigned int
#define __SSE2_MATH__ 1
#define __SSE2__ 1
#define __SSE_MATH__ 1
#define __SSE__ 1
#define __SSP_STRONG__ 3
#define __STDC_HOSTED__ 1
#define __STDC_IEC_559_COMPLEX__ 1
#define __STDC_IEC_559__ 1
#define __STDC_ISO_10646__ 201605L
#define __STDC_NO_THREADS__ 1
#define __STDC_UTF_16__ 1
#define __STDC_UTF_32__ 1
#define __STDC_VERSION__ 201112L
#define __STDC__ 1
#define __UINT16_C(c) c
#define __UINT16_MAX__ 0xffff
#define __UINT16_TYPE__ short unsigned int
#define __UINT32_C(c) c ## U
#define __UINT32_MAX__ 0xffffffffU
#define __UINT32_TYPE__ unsigned int
#define __UINT64_C(c) c ## UL
#define __UINT64_MAX__ 0xffffffffffffffffUL
#define __UINT64_TYPE__ long unsigned int
#define __UINT8_C(c) c
#define __UINT8_MAX__ 0xff
#define __UINT8_TYPE__ unsigned char
#define __UINTMAX_C(c) c ## UL
#define __UINTMAX_MAX__ 0xffffffffffffffffUL
#define __UINTMAX_TYPE__ long unsigned int
#define __UINTPTR_MAX__ 0xffffffffffffffffUL
#define __UINTPTR_TYPE__ long unsigned int
#define __UINT_FAST16_MAX__ 0xffffffffffffffffUL
#define __UINT_FAST16_TYPE__ long unsigned int
#define __UINT_FAST32_MAX__ 0xffffffffffffffffUL
#define __UINT_FAST32_TYPE__ long unsigned int
#define __UINT_FAST64_MAX__ 0xffffffffffffffffUL
#define __UINT_FAST64_TYPE__ long unsigned int
#define __UINT_FAST8_MAX__ 0xff
#define __UINT_FAST8_TYPE__ unsigned char
#define __UINT_LEAST16_MAX__ 0xffff
#define __UINT_LEAST16_TYPE__ short unsigned int
#define __UINT_LEAST32_MAX__ 0xffffffffU
#define __UINT_LEAST32_TYPE__ unsigned int
#define __UINT_LEAST64_MAX__ 0xffffffffffffffffUL
#define __UINT_LEAST64_TYPE__ long unsigned int
#define __UINT_LEAST8_MAX__ 0xff
#define __UINT_LEAST8_TYPE__ unsigned char
#define __USER_LABEL_PREFIX__
#define __VERSION__ "6.3.0 20170406"
#define __WCHAR_MAX__ 0x7fffffff
#define __WCHAR_MIN__ (-__WCHAR_MAX__ - 1)
#define __WCHAR_TYPE__ int
#define __WINT_MAX__ 0xffffffffU
#define __WINT_MIN__ 0U
#define __WINT_TYPE__ unsigned int
#define __amd64 1
#define __amd64__ 1
#define __code_model_small__ 1
#define __gnu_linux__ 1
#define __has_include(STR) __has_include__(STR)
#define __has_include_next(STR) __has_include_next__(STR)
#define __k8 1
#define __k8__ 1
#define __linux 1
#define __linux__ 1
#define __pic__ 2
#define __pie__ 2
#define __unix 1
#define __unix__ 1
#define __x86_64 1
#define __x86_64__ 1
#define linux 1
#define unix 1

Compiler error using AVR Atmel Studio library config files

// Created by lenovo on 8/27/2017.
//
#ifndef UART_UART_CFG_H
#define UART_UART_CFG_H
#define UART_BaudRate 1200 //9600UL
#define CLK 16
#define UART_Parity NONE
#define UART_TX EN_TX
#define UART_RX EN_RX
#define UART_STARTBITS 1
#define UART_STOPBITS 1
#define UART_DATABITS EightBits
#endif //UART_UART_CFG_H
This is the error "called object is not a function or pointer"
and this my private file that have all word's addresses inside the config file. The first file:
//
// Created by lenovo on 8/27/2017.
//
#ifndef UART_UART_PRIV_H
#define UART_UART_PRIV_H
/* Main PINS */
#define UCSRA *((volatile u8 *)0x2B)
#define UCSRB *((volatile u8 *)0x2A)
#define UCSRC *((volatile u8 *)0x40)
#define UBRRL *((volatile u8 *)0x29)
#define UBRRH *((volatile u8 *)0x40)
#define UDR *((volatile u8 *)0x2C)
/* END Main PINS */
#define NONE 0x00
#define twoBit 0x08
#define oneBit 0x00
/* Bits */
#define fiveBits 0x00
#define SixBits 0x02
#define SevenBits 0x04
#define EightBits 0x06
/* End Bits */
#define DIS 0 // Disable
#define EN 1 // Enable
#define UART_9600 9600UL
#endif //UART_UART_PRIV_H
Private file that have some addresses for my Microcontroller ATmega16. Moreover, my config file is reference to private file; which have keys that defined in private. For example, in UART_Partit I wrote NONE and NONE address defined in private but it shows error

You need to include your file with declarations.
#ifndef UART_UART_CFG_H
#define UART_UART_CFG_H
#include "uart_priv.h"
#define UART_BaudRate 1200 //9600UL
#define CLK 16
#define UART_Parity NONE
#define UART_TX EN_TX
#define UART_RX EN_RX
#define UART_STARTBITS 1
#define UART_STOPBITS 1
#define UART_DATABITS EightBits
#endif //UART_UART_CFG_H
and there are no EN_TX or EN_RX defines for these constants in your second file.

scanf equivalent with arrow key support in c

Is there some functions in c which behaves like scanf (taking inputs from the keyboard) while being able to move the cursor in the input by pressing the arrow keys, like in a terminal ?

There are no function in C standard to do that.
You can look at ncurses

In addition to previous suggestions, if you want to do a little work you can "roll your own" using the vt100 escape sequences, which most (probably all) terminal programs continue to support. Here's a small sample program,
#include <stdio.h>
#include <string.h>
#include "vt100.h"
int main ( int argc, char *argv[] )
{
char cupstr[99];
char reply[99];
strcpy(cupstr,_cup(12,25));
printf("%sEnter Data ---> ",cupstr);
fgets(reply,88,stdin);
strcpy(cupstr,_cup(13,25));
printf("%sEchoed Data --> %s\n",cupstr,reply);
}
And the big part is that vt100.h containing all the escape sequences,
/* ------------------------------------------------------------------------- *
VT100.H (c)1989-1999, John Forkosh Associates VT-100 Graphics
* ------------------------------------------------------------------------- */
#define bel printf("\007")
#define esc printf("\033")
#define csi printf("\033[")
#define lscreen printf("\033[?5h")
#define dscreen printf("\033[?5l")
#define rev_vid printf("\033[7m")
#define blink printf("\033[5m")
#define under printf("\033[4m")
#define bold printf("\033[1m")
#define norm_vid printf("\033[0m")
#define wide_vid printf("\033#6")
#define high_vid2 printf("\033#4")
#define high_vid1 printf("\033#3")
#define graphic printf("\033(0")
#define no_graph printf("\033(B")
#define no_window printf("\033[1;24r")
#define no_att printf("\033[0;22;24;25;27m")
#define cup(row,col) printf("\033[%d;%dH",(row),(col))
#define stbm(top,bot) printf("\033[%d;%dr",(top),(bot))
#define ri printf("\033M")
#define el printf("\033[K")
#define ed0 printf("\033[0J")
#define bar graphic;printf("x");no_graph
#define wipe(row,col) cup(row,col);printf("\033[J")
#define clr wipe(0,0);norm_vid
/* ------------------------------------------------------------------------- *
String equivalents ... Note: _cup() and _stbm() only accept literal args.
* ------------------------------------------------------------------------- */
#define _bel "\007"
#define _esc "\033"
#define _csi "\033["
#define _lscreen "\033[?5h"
#define _dscreen "\033[?5l"
#define _rev_vid "\033[7m"
#define _blink "\033[5m"
#define _under "\033[4m"
#define _bold "\033[1m"
#define _norm_vid "\033[0m"
#define _wide_vid "\033#6"
#define _high_vid2 "\033#4"
#define _high_vid1 "\033#3"
#define _graphic "\033(0"
#define _no_graph "\033(B"
#define _no_window "\033[1;24r"
#define _no_att "\033[0;22;24;25;27m"
#define _cup(row,col) "\033[" #row ";" #col "\110"
#define _stbm(top,bot) "\033[" #top ";" #bot "\162"
#define _ri "\033M"
#define _el "\033[K"
#define _ed0 "\033[0J"
#define _bar "\033(0x\033(B"
#define _wipe_clr "\033[0;0H\033[2J"
#define _clr "\033[0;0H\033[2J\033(B\033[0m\033[1;24r"
/* ------------------------------------------------------------------------- */
These escape sequences let you program all the stuff that used to look so cool on vt100's, vt220's, etc, a long, long time ago (but right here in this galaxy).

If clause for bit cheking replaced with bit manipulation using C

I have following chunk of code:
// **** CONTROL REGISTER 4 SETUP ****
ctrl|=(uint8_t)(LIS3DSH_InitStruct->CR4_Odr);
if(LIS3DSH_InitStruct->CR4_Bdu)
ctrl|=(1<<LIS3DSH_CR4_BDU_POSITION);
if(LIS3DSH_InitStruct->CR4_Zen)
ctrl|=(1<<LIS3DSH_CR4_Z_AXIS_POSITION);
if(LIS3DSH_InitStruct->CR4_Yen)
ctrl|=(1<<LIS3DSH_CR4_Y_AXIS_POSITION);
if(LIS3DSH_InitStruct->CR4_Xen)
ctrl|=(1<<LIS3DSH_CR4_X_AXIS_POSITION);
LIS3DSH_Write(&ctrl,
LIS3DSH_CTRL_REG4_ADDR,
sizeof(ctrl));
delay(1000000);
// **** END OF CONTROL REGISTER 4 SETUP ****
Now, I am pretty sure these if clauses (since this is embedded world) can be replaced with bit manipulation techniques, can someone show me how? I know this is pretty stupid question, but I've simply forgot these things ...
Here is header file with structure:
#ifndef __STM32F4_DISCOVERY_LIS3DS_H
#define __STM32F4_DISCOVERY_LIS3DS_H
#include "stm32f4xx.h"
#define LIS3DSH_CTRL_REG1_ADDR 0x21
#define LIS3DSH_CTRL_REG2_ADDR 0x22
#define LIS3DSH_CTRL_REG3_ADDR 0x23
#define LIS3DSH_CTRL_REG4_ADDR 0x20
#define LIS3DSH_CTRL_REG5_ADDR 0x24
#define LIS3DSH_CTRL_REG6_ADDR 0x25
#define LIS3DSH_INFO1_REG_ADDR 0x0d
#define LIS3DSH_INFO2_REG_ADDR 0x0e
#define LIS3DSH_WHOAMI_REG_ADDR 0x0f
#define LIS3DSH_STATUS_REG_ADDR 0x27
#define LIS3DSH_TEMPERATURE_REG_ADDR 0x0c
#define LIS3DSH_OUT_X_L_REG_ADDR 0x28
#define LIS3DSH_OUT_X_H_REG_ADDR 0x29
#define LIS3DSH_OUT_Y_L_REG_ADDR 0x2a
#define LIS3DSH_OUT_Y_H_REG_ADDR 0x2b
#define LIS3DSH_OUT_Z_L_REG_ADDR 0x2c
#define LIS3DSH_OUT_Z_H_REG_ADDR 0x2d
#define LIS3DSH_FLAG_ZXYOR ((uint8_t)0x00)
#define LIS3DSH_FLAG_ZOR ((uint8_t)0x01)
#define LIS3DSH_FLAG_YOR ((uint8_t)0x02)
#define LIS3DSH_FLAG_XOR ((uint8_t)0x03)
#define LIS3DSH_FLAG_ZXYDA ((uint8_t)0x04)
#define LIS3DSH_FLAG_ZDA ((uint8_t)0x05)
#define LIS3DSH_FLAG_YDA ((uint8_t)0x06)
#define LIS3DSH_FLAG_XDA ((uint8_t)0x07)
#define DEVICE_ID ((uint8_t)0x3f)
#define LIS3DSH_SM1_INT_TO_PIN_INT1 ((uint8_t)0x00)
#define LIS3DSH_SM1_INT_TO_PIN_INT2 ((uint8_t)0x01)
#define LIS3DSH_SM1_DISABLE ((uint8_t)0x00)
#define LIS3DSH_SM1_ENABLE ((uint8_t)0x01)
#define LIS3DSH_SM2_INT_TO_PIN_INT1 ((uint8_t)0x00)
#define LIS3DSH_SM2_INT_TO_PIN_INT2 ((uint8_t)0x01)
#define LIS3DSH_SM2_DISABLE ((uint8_t)0x00)
#define LIS3DSH_SM2_ENABLE ((uint8_t)0x01)
#define LIS3DSH_CR3_DREN_TO_INT1_DISABLE ((uint8_t)0x00)
#define LIS3DSH_CR3_DREN_TO_INT1_ENABLE ((uint8_t)0x01)
#define LIS3DSH_CR3_IEA_ACTIVE_LOW ((uint8_t)0x00)
#define LIS3DSH_CR3_IEA_ACTIVE_HIGH ((uint8_t)0x01)
#define LIS3DSH_CR3_IEL_LATCHED ((uint8_t)0x00)
#define LIS3DSH_CR3_IEL_PULSED ((uint8_t)0x01)
#define LIS3DSH_CR3_INT2_DISABLED ((uint8_t)0x00)
#define LIS3DSH_CR3_INT2_ENABLED ((uint8_t)0x01)
#define LIS3DSH_CR3_INT1_DISABLED ((uint8_t)0x00)
#define LIS3DSH_CR3_INT1_ENABLED ((uint8_t)0x01)
#define LIS3DSH_CR3_VFILT_DISABLED ((uint8_t)0x00)
#define LIS3DSH_CR3_VFILT_ENABLED ((uint8_t)0x01)
#define LIS3DSH_CR3_NO_SOFT_RESET ((uint8_t)0x00)
#define LIS3DSH_CR3_SOFT_RESET ((uint8_t)0x01)
#define LIS3DSH_CR4_ODR_POWER_DOWN ((uint8_t)0x00)
#define LIS3DSH_CR4_ODR_3f125HZ ((uint8_t)0x01)
#define LIS3DSH_CR4_ODR_6f25HZ ((uint8_t)0x02)
#define LIS3DSH_CR4_ODR_12f5HZ ((uint8_t)0x03)
#define LIS3DSH_CR4_ODR_25HZ ((uint8_t)0x04)
#define LIS3DSH_CR4_ODR_50HZ ((uint8_t)0x05)
#define LIS3DSH_CR4_ODR_100HZ ((uint8_t)0x06)
#define LIS3DSH_CR4_ODR_400HZ ((uint8_t)0x07)
#define LIS3DSH_CR4_ODR_800HZ ((uint8_t)0x08)
#define LIS3DSH_CR4_ODR_1600HZ ((uint8_t)0x09)
#define LIS3DSH_CR4_BDU_DISABLED ((uint8_t)0x00)
#define LIS3DSH_CR4_BDU_ENABLED ((uint8_t)0x01)
#define LIS3DSH_CR4_BDU_POSITION ((uint8_t)0x04)
#define LIS3DSH_CR4_Z_AXIS_DISABLED ((uint8_t)0x00)
#define LIS3DSH_CR4_Z_AXIS_ENABLED ((uint8_t)0x01)
#define LIS3DSH_CR4_Z_AXIS_POSITION ((uint8_t)0x05)
#define LIS3DSH_CR4_X_AXIS_DISABLED ((uint8_t)0x00)
#define LIS3DSH_CR4_X_AXIS_ENABLED ((uint8_t)0x01)
#define LIS3DSH_CR4_X_AXIS_POSITION ((uint8_t)0x07)
#define LIS3DSH_CR4_Y_AXIS_DISABLED ((uint8_t)0x00)
#define LIS3DSH_CR4_Y_AXIS_ENABLED ((uint8_t)0x01)
#define LIS3DSH_CR4_Y_AXIS_POSITION ((uint8_t)0x06)
#define LIS3DSH_CR5_BW_800HZ ((uint8_t)0x00)
#define LIS3DSH_CR5_BW_400HZ ((uint8_t)0x01)
#define LIS3DSH_CR5_BW_200HZ ((uint8_t)0x02)
#define LIS3DSH_CR5_BW_50HZ ((uint8_t)0x03)
#define LIS3DSH_CR5_FSCALE_2G ((uint8_t)0x00)
#define LIS3DSH_CR5_FSCALE_4G ((uint8_t)0x01)
#define LIS3DSH_CR5_FSCALE_6G ((uint8_t)0x02)
#define LIS3DSH_CR5_FSCALE_8G ((uint8_t)0x03)
#define LIS3DSH_CR5_FSCALE_16G ((uint8_t)0x04)
#define LIS3DSH_CR5_ST_DISABLE ((uint8_t)0x00)
#define LIS3DSH_CR5_ST_POSITIVE ((uint8_t)0x01)
#define LIS3DSH_CR5_ST_NEGATIVE ((uint8_t)0x02)
#define LIS3DSH_CR5_ST_NOT_ALLOWED ((uint8_t)0x03)
#define LIS3DSH_CR5_MODE_4_WIRE_INTERFACE ((uint8_t)0x00)
#define LIS3DSH_CR5_MODE_3_WIRE_INTERFACE ((uint8_t)0x01)
#define LIS3DSH_CR6_FORCE_REBOOT_DISABLE ((uint8_t)0x00)
#define LIS3DSH_CR6_FORCE_REBOOT_ENABLE ((uint8_t)0x01)
#define LIS3DSH_CR6_FIFO_DISABLED ((uint8_t)0x00)
#define LIS3DSH_CR6_FIFO_ENABLED ((uint8_t)0x01)
#define LIS3DSH_CR6_WTM_DISABLED ((uint8_t)0x00)
#define LIS3DSH_CR6_WTM_ENABLED ((uint8_t)0x01)
#define LIS3DSH_CR6_ADDINC_DISABLED ((uint8_t)0x00)
#define LIS3DSH_CR6_ADDINC_ENABLED ((uint8_t)0x01)
#define LIS3DSH_CR6_FIFO_EMPTY_TO_INT1_DISABLED ((uint8_t)0x00)
#define LIS3DSH_CR6_FIFO_EMPTY_TO_INT1_ENABLED ((uint8_t)0x01)
#define LIS3DSH_CR6_FIFO_WTM_TO_INT1_DISABLED ((uint8_t)0x00)
#define LIS3DSH_CR6_FIFO_WTM_TO_INT1_ENABLED ((uint8_t)0x01)
#define LIS3DSH_CR6_FIFO_OVERRUN_TO_INT1_DISABLED ((uint8_t)0x00)
#define LIS3DSH_CR6_FIFO_OVERRUN_TO_INT1_ENABLED ((uint8_t)0x01)
#define LIS3DSH_CR6_BOOT_TO_INT2_DISABLED ((uint8_t)0x00)
#define LIS3DSH_CR6_BOOT_TO_INT2_ENABLED ((uint8_t)0x01)
#define LIS3DSH_SPI SPI1
#define LIS3DSH_SPI_CLK RCC_APB2Periph_SPI1
#define LIS3DSH_SPI_SCK_PIN GPIO_Pin_5
#define LIS3DSH_SPI_SCK_GPIO_PORT GPIOA
#define LIS3DSH_SPI_SCK_GPIO_CLK RCC_AHB1Periph_GPIOA
#define LIS3DSH_SPI_SCK_SOURCE GPIO_PinSource5
#define LIS3DSH_SPI_SCK_AF GPIO_AF_SPI1
#define LIS3DSH_SPI_MISO_PIN GPIO_Pin_6
#define LIS3DSH_SPI_MISO_GPIO_PORT GPIOA
#define LIS3DSH_SPI_MISO_GPIO_CLK RCC_AHB1Periph_GPIOA
#define LIS3DSH_SPI_MISO_SOURCE GPIO_PinSource6
#define LIS3DSH_SPI_MISO_AF GPIO_AF_SPI1
#define LIS3DSH_SPI_MOSI_PIN GPIO_Pin_7
#define LIS3DSH_SPI_MOSI_GPIO_PORT GPIOA
#define LIS3DSH_SPI_MOSI_GPIO_CLK RCC_AHB1Periph_GPIOA
#define LIS3DSH_SPI_MOSI_SOURCE GPIO_PinSource7
#define LIS3DSH_SPI_MOSI_AF GPIO_AF_SPI1
#define LIS3DSH_SPI_CS_PIN GPIO_Pin_3
#define LIS3DSH_SPI_CS_GPIO_PORT GPIOE
#define LIS3DSH_SPI_CS_GPIO_CLK RCC_AHB1Periph_GPIOE
#define LIS3DSH_SPI_INT1_PIN GPIO_Pin_0
#define LIS3DSH_SPI_INT1_GPIO_PORT GPIOE
#define LIS3DSH_SPI_INT1_GPIO_CLK RCC_AHB1Periph_GPIOE
#define LIS3DSH_SPI_INT1_EXTI_LINE EXTI_Line0
#define LIS3DSH_SPI_INT1_EXTI_PORT_SOURCE EXTI_PortSourceGPIOE
#define LIS3DSH_SPI_INT1_EXTI_PIN_SOURCE EXTI_PinSource0
#define LIS3DSH_SPI_INT1_EXTI_IRQn EXTI0_IRQn
#define LIS3DSH_SPI_INT2_PIN GPIO_Pin_1
#define LIS3DSH_SPI_INT2_GPIO_PORT GPIOE
#define LIS3DSH_SPI_INT2_GPIO_CLK RCC_AHB1Periph_GPIOE
#define LIS3DSH_SPI_INT2_EXTI_LINE EXTI_Line1
#define LIS3DSH_SPI_INT2_EXTI_PORT_SOURCE EXTI_PortSourceGPIOE
#define LIS3DSH_SPI_INT2_EXTI_PIN_SOURCE EXTI_PinSource1
#define LIS3DSH_SPI_INT2_EXTI_IRQn EXTI1_IRQn
#define LIS3DSH_CS_LOW() GPIO_ResetBits(LIS3DSH_SPI_CS_GPIO_PORT, LIS3DSH_SPI_CS_PIN)
#define LIS3DSH_CS_HIGH() GPIO_SetBits(LIS3DSH_SPI_CS_GPIO_PORT, LIS3DSH_SPI_CS_PIN)
#define LIS3DSH_FLAG_TIMEOUT ((uint32_t)0x1000)
typedef struct
{
// **** Control Register 1 ****
uint8_t SM1_Hysteresis;
uint8_t SM1_Pin;
uint8_t SM1_Enable;
// **** END OF Control Register 1 ****
// **** Control Register 2 ****
uint8_t SM2_Hysteresis;
uint8_t SM2_Pin;
uint8_t SM2_Enable;
// **** END OF Control Register 2 ****
// **** Control Register 3 ****
uint8_t CR3_Dren;
uint8_t CR3_Iea;
uint8_t CR3_Iel;
uint8_t CR3_Int2En;
uint8_t CR3_Int1En;
uint8_t CR3_Vfilt;
uint8_t CR3_Strt;
// **** END OF Control Register 3
// **** Control Register 4 ****
uint8_t CR4_Odr;
uint8_t CR4_Bdu;
uint8_t CR4_Zen;
uint8_t CR4_Yen;
uint8_t CR4_Xen;
// **** END OF Control Register 4
// **** Control Register 5 ****
uint8_t CR5_Bw;
uint8_t CR5_Fscale;
uint8_t CR5_St;
uint8_t CR5_Sim;
// **** END OF Control Register 5
// **** Control Register 6 ****
uint8_t CR6_Boot;
uint8_t CR6_FifoEn;
uint8_t CR6_WtmEn;
uint8_t CR6_AddInc;
uint8_t CR6_P1Empty;
uint8_t CR6_P1Wtm;
uint8_t CR6_P1OverRun;
uint8_t CR6_P2Boot;
// **** END OF Control Register 6
} LIS3DSH_InitTypeDef;
typedef struct
{
uint16_t x;
uint16_t y;
uint16_t z;
} accel_vector;
extern LIS3DSH_InitTypeDef deviceLIS3DSH;
extern GPIO_InitTypeDef portLIS3DSH;
extern SPI_InitTypeDef busLIS3DSH;
extern accel_vector accelData;
void LIS3DSH_Init(LIS3DSH_InitTypeDef* LIS3DSH_InitStruct);
uint32_t LIS3DSH_TIMEOUT_UserCallback(void);
void LIS3DSH_AccelInit(void);
void LIS3DSH_Read(uint8_t* pBuffer,
uint8_t ReadAddr,
uint16_t NumByteToRead);
void LIS3DSH_Write(uint8_t* pBuffer,
uint8_t WriteAddr,
uint16_t NumByteToWrite);
void LIS3DSH_FailureHandler(void);
accel_vector LIS3DSH_ReadData(void);
void LIS3DSH_SoftReset(void);
#endif
ctrl is type uin8_t and I am using IAR Embedded Workbench with C and STM32F4Discovery board. Here is a code that inits part of LIS3DSH_IniStruct:
deviceLIS3DSH.CR4_Odr=LIS3DSH_CR4_ODR_1600HZ;
deviceLIS3DSH.CR4_Bdu=LIS3DSH_CR4_BDU_ENABLED;
deviceLIS3DSH.CR4_Zen=LIS3DSH_CR4_Z_AXIS_ENABLED;
deviceLIS3DSH.CR4_Yen=LIS3DSH_CR4_Y_AXIS_ENABLED;
deviceLIS3DSH.CR4_Xen=LIS3DSH_CR4_X_AXIS_ENABLED;
and deviceLIS3DSH is declared as:
LIS3DSH_InitTypeDef deviceLIS3DSH;

The same code but with the if-statements replaced with pure bit manipulation:
// **** CONTROL REGISTER 4 SETUP ****
ctrl |= (LIS3DSH_InitStruct->CR4_Odr);
ctrl |= (LIS3DSH_InitStruct->CR4_Bdu << LIS3DSH_CR4_BDU_POSITION);
ctrl |= (LIS3DSH_InitStruct->CR4_Zen << LIS3DSH_CR4_Z_AXIS_POSITION);
ctrl |= (LIS3DSH_InitStruct->CR4_Yen << LIS3DSH_CR4_Y_AXIS_POSITION);
ctrl |= (LIS3DSH_InitStruct->CR4_Xen << LIS3DSH_CR4_X_AXIS_POSITION);
LIS3DSH_Write(&ctrl,
LIS3DSH_CTRL_REG4_ADDR,
sizeof(ctrl));
delay(1000000);
// **** END OF CONTROL REGISTER 4 SETUP ****

These are already bit manipulation.
if(LIS3DSH_InitStruct->CR4_Bdu)
This tests if the integer inside is 0 or not. The test is a logical operator, so it checks all the bits. Internally, it might be a tst assembly instruction.
ctrl|=(1<<LIS3DSH_CR4_BDU_POSITION);
Here is some actual bit manipulation. ctrl has 8 bits, and we want to make one of these a 1, even it is already is 1.
So we take 1 which is 00000001 and shift the bits left to get the 1 into the position we are concerned with. Which position is this? It is given by the macro LIS3DSH_CR4_BDU_POSITION which is defined to be a number from 0 to 7 in the header file.
So the right hand side becomes 1<<4 or 00010000 or 0x10 or 16.
The we or this number with ctrl via ctrl = ctrl | 0x10; Which has the effect of making sure the 5th bit of the ctrl integer is a 1. It is shorter to write ctrl |= (1<<4);

Why does the C preprocessor interpret the word "linux" as the constant "1"?

Why does the C preprocessor in GCC interpret the word linux (small letters) as the constant 1?
test.c:
#include <stdio.h>
int main(void)
{
int linux = 5;
return 0;
}
Result of $ gcc -E test.c (stop after the preprocessing stage):
....
int main(void)
{
int 1 = 5;
return 0;
}
Which of course yields an error.
(BTW: There is no #define linux in the stdio.h file.)

In the Old Days (pre-ANSI), predefining symbols such as unix and vax was a way to allow code to detect at compile time what system it was being compiled for. There was no official language standard back then (beyond the reference material at the back of the first edition of K&R), and C code of any complexity was typically a complex maze of #ifdefs to allow for differences between systems. These macro definitions were generally set by the compiler itself, not defined in a library header file. Since there were no real rules about which identifiers could be used by the implementation and which were reserved for programmers, compiler writers felt free to use simple names like unix and assumed that programmers would simply avoid using those names for their own purposes.
The 1989 ANSI C standard introduced rules restricting what symbols an implementation could legally predefine. A macro predefined by the compiler could only have a name starting with two underscores, or with an underscore followed by an uppercase letter, leaving programmers free to use identifiers not matching that pattern and not used in the standard library.
As a result, any compiler that predefines unix or linux is non-conforming, since it will fail to compile perfectly legal code that uses something like int linux = 5;.
As it happens, gcc is non-conforming by default -- but it can be made to conform (reasonably well) with the right command-line options:
gcc -std=c90 -pedantic ... # or -std=c89 or -ansi
gcc -std=c99 -pedantic
gcc -std=c11 -pedantic
See the gcc manual for more details.
gcc will be phasing out these definitions in future releases, so you shouldn't write code that depends on them. If your program needs to know whether it's being compiled for a Linux target or not it can check whether __linux__ is defined (assuming you're using gcc or a compiler that's compatible with it). See the GNU C preprocessor manual for more information.
A largely irrelevant aside: the "Best One Liner" winner of the 1987 International Obfuscated C Code Contest, by David Korn (yes, the author of the Korn Shell) took advantage of the predefined unix macro:
main() { printf(&unix["\021%six\012\0"],(unix)["have"]+"fun"-0x60);}
It prints "unix", but for reasons that have absolutely nothing to do with the spelling of the macro name.
I don't want to post any spoilers here, and I encourage anyone reading this to try to understand that code on their own first. But if you really want to give up, I've posted an explanation here:
https://gist.github.com/Keith-S-Thompson/6920347

This appears to be an (undocumented) "GNU extension": [correction: I finally found a mention in the docs. See below.]
The following command uses the -dM option to print all preprocessor defines; since the input "file" is empty, it shows exactly the predefined macros. It was run with gcc-4.7.3 on a standard ubuntu install. You can see that the preprocessor is standard-aware. In total, there 243 macros with -std=gnu99 and 240 with -std=c99; I filtered the output for relevance.
$ cpp --std=c89 -dM < /dev/null | grep linux
#define __linux 1
#define __linux__ 1
#define __gnu_linux__ 1
$ cpp --std=gnu89 -dM < /dev/null | grep linux
#define __linux 1
#define __linux__ 1
#define __gnu_linux__ 1
#define linux 1
$ cpp --std=c99 -dM < /dev/null | grep linux
#define __linux 1
#define __linux__ 1
#define __gnu_linux__ 1
$ cpp --std=gnu99 -dM < /dev/null | grep linux
#define __linux 1
#define __linux__ 1
#define __gnu_linux__ 1
#define linux 1
The "gnu standard" versions also #define unix. (Using c11 and gnu11 produces the same results.)
I suppose they had their reasons, but it seems to me to make the default installation of gcc (which compiles C code with -std=gnu89 unless otherwise specified) non-conformant, and -- as in this question -- surprising. Polluting the global namespace with macros whose names don't begin with an underscore is not permitted in a conformant implementation. (6.8.10p2: "Any other predefined macro names shall begin with a leading underscore followed by an uppercase letter or a second
underscore," but, as mentioned in Appendix J.5 (portability issues), such names are often predefined.)
When I originally wrote this answer, I wasn't able to find any documentation in gcc about this issue, but I did finally discover it, not in C implementation-defined behaviour nor in C extensions but in the cpp manual section 3.7.3, where it notes that:
We are slowly phasing out all predefined macros which are outside the reserved namespace. You should never use them in new programs…

Because linux is a built-in macro defined when the compiler is running on, or compiling for (if it is a cross-compiler), Linux.
There are a lot of such predefined macros. With GCC, you can use:
cp /dev/null emptyfile.c
gcc -E -dM emptyfile.c
to get a list of macros. (I've not managed to persuade GCC to accept /dev/null directly, but
the empty file seems to work OK.) With GCC 4.8.1 running on Mac OS X 10.8.5, I got the output:
#define __DBL_MIN_EXP__ (-1021)
#define __UINT_LEAST16_MAX__ 65535
#define __ATOMIC_ACQUIRE 2
#define __FLT_MIN__ 1.17549435082228750797e-38F
#define __UINT_LEAST8_TYPE__ unsigned char
#define __INTMAX_C(c) c ## L
#define __CHAR_BIT__ 8
#define __UINT8_MAX__ 255
#define __WINT_MAX__ 2147483647
#define __ORDER_LITTLE_ENDIAN__ 1234
#define __SIZE_MAX__ 18446744073709551615UL
#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__ ((double)4.94065645841246544177e-324L)
#define __GCC_HAVE_SYNC_COMPARE_AND_SWAP_8 1
#define __GCC_ATOMIC_CHAR_LOCK_FREE 2
#define __FLT_EVAL_METHOD__ 0
#define __GCC_ATOMIC_CHAR32_T_LOCK_FREE 2
#define __x86_64 1
#define __UINT_FAST64_MAX__ 18446744073709551615ULL
#define __SIG_ATOMIC_TYPE__ int
#define __DBL_MIN_10_EXP__ (-307)
#define __FINITE_MATH_ONLY__ 0
#define __GNUC_PATCHLEVEL__ 1
#define __UINT_FAST8_MAX__ 255
#define __DEC64_MAX_EXP__ 385
#define __INT8_C(c) c
#define __UINT_LEAST64_MAX__ 18446744073709551615ULL
#define __SHRT_MAX__ 32767
#define __LDBL_MAX__ 1.18973149535723176502e+4932L
#define __UINT_LEAST8_MAX__ 255
#define __GCC_ATOMIC_BOOL_LOCK_FREE 2
#define __APPLE_CC__ 1
#define __UINTMAX_TYPE__ long unsigned int
#define __DEC32_EPSILON__ 1E-6DF
#define __UINT32_MAX__ 4294967295U
#define __LDBL_MAX_EXP__ 16384
#define __WINT_MIN__ (-__WINT_MAX__ - 1)
#define __SCHAR_MAX__ 127
#define __WCHAR_MIN__ (-__WCHAR_MAX__ - 1)
#define __INT64_C(c) c ## LL
#define __DBL_DIG__ 15
#define __GCC_ATOMIC_POINTER_LOCK_FREE 2
#define __SIZEOF_INT__ 4
#define __SIZEOF_POINTER__ 8
#define __USER_LABEL_PREFIX__ _
#define __STDC_HOSTED__ 1
#define __LDBL_HAS_INFINITY__ 1
#define __FLT_EPSILON__ 1.19209289550781250000e-7F
#define __LDBL_MIN__ 3.36210314311209350626e-4932L
#define __DEC32_MAX__ 9.999999E96DF
#define __strong
#define __INT32_MAX__ 2147483647
#define __SIZEOF_LONG__ 8
#define __APPLE__ 1
#define __UINT16_C(c) c
#define __DECIMAL_DIG__ 21
#define __LDBL_HAS_QUIET_NAN__ 1
#define __DYNAMIC__ 1
#define __GNUC__ 4
#define __MMX__ 1
#define __FLT_HAS_DENORM__ 1
#define __SIZEOF_LONG_DOUBLE__ 16
#define __BIGGEST_ALIGNMENT__ 16
#define __DBL_MAX__ ((double)1.79769313486231570815e+308L)
#define __INT_FAST32_MAX__ 2147483647
#define __DBL_HAS_INFINITY__ 1
#define __DEC32_MIN_EXP__ (-94)
#define __INT_FAST16_TYPE__ short int
#define __LDBL_HAS_DENORM__ 1
#define __DEC128_MAX__ 9.999999999999999999999999999999999E6144DL
#define __INT_LEAST32_MAX__ 2147483647
#define __DEC32_MIN__ 1E-95DF
#define __weak
#define __DBL_MAX_EXP__ 1024
#define __DEC128_EPSILON__ 1E-33DL
#define __SSE2_MATH__ 1
#define __ATOMIC_HLE_RELEASE 131072
#define __PTRDIFF_MAX__ 9223372036854775807L
#define __amd64 1
#define __tune_core2__ 1
#define __ATOMIC_HLE_ACQUIRE 65536
#define __LONG_LONG_MAX__ 9223372036854775807LL
#define __SIZEOF_SIZE_T__ 8
#define __SIZEOF_WINT_T__ 4
#define __GXX_ABI_VERSION 1002
#define __FLT_MIN_EXP__ (-125)
#define __INT_FAST64_TYPE__ long long int
#define __DBL_MIN__ ((double)2.22507385850720138309e-308L)
#define __LP64__ 1
#define __DEC128_MIN__ 1E-6143DL
#define __REGISTER_PREFIX__
#define __UINT16_MAX__ 65535
#define __DBL_HAS_DENORM__ 1
#define __UINT8_TYPE__ unsigned char
#define __NO_INLINE__ 1
#define __FLT_MANT_DIG__ 24
#define __VERSION__ "4.8.1"
#define __UINT64_C(c) c ## ULL
#define __GCC_ATOMIC_INT_LOCK_FREE 2
#define __FLOAT_WORD_ORDER__ __ORDER_LITTLE_ENDIAN__
#define __INT32_C(c) c
#define __DEC64_EPSILON__ 1E-15DD
#define __ORDER_PDP_ENDIAN__ 3412
#define __DEC128_MIN_EXP__ (-6142)
#define __INT_FAST32_TYPE__ int
#define __UINT_LEAST16_TYPE__ short unsigned int
#define __INT16_MAX__ 32767
#define __ENVIRONMENT_MAC_OS_X_VERSION_MIN_REQUIRED__ 1080
#define __SIZE_TYPE__ long unsigned int
#define __UINT64_MAX__ 18446744073709551615ULL
#define __INT8_TYPE__ signed char
#define __FLT_RADIX__ 2
#define __INT_LEAST16_TYPE__ short int
#define __LDBL_EPSILON__ 1.08420217248550443401e-19L
#define __UINTMAX_C(c) c ## UL
#define __SSE_MATH__ 1
#define __k8 1
#define __SIG_ATOMIC_MAX__ 2147483647
#define __GCC_ATOMIC_WCHAR_T_LOCK_FREE 2
#define __SIZEOF_PTRDIFF_T__ 8
#define __x86_64__ 1
#define __DEC32_SUBNORMAL_MIN__ 0.000001E-95DF
#define __INT_FAST16_MAX__ 32767
#define __UINT_FAST32_MAX__ 4294967295U
#define __UINT_LEAST64_TYPE__ long long unsigned int
#define __FLT_HAS_QUIET_NAN__ 1
#define __FLT_MAX_10_EXP__ 38
#define __LONG_MAX__ 9223372036854775807L
#define __DEC128_SUBNORMAL_MIN__ 0.000000000000000000000000000000001E-6143DL
#define __FLT_HAS_INFINITY__ 1
#define __UINT_FAST16_TYPE__ short unsigned int
#define __DEC64_MAX__ 9.999999999999999E384DD
#define __CHAR16_TYPE__ short unsigned int
#define __PRAGMA_REDEFINE_EXTNAME 1
#define __INT_LEAST16_MAX__ 32767
#define __DEC64_MANT_DIG__ 16
#define __INT64_MAX__ 9223372036854775807LL
#define __UINT_LEAST32_MAX__ 4294967295U
#define __GCC_ATOMIC_LONG_LOCK_FREE 2
#define __INT_LEAST64_TYPE__ long long int
#define __INT16_TYPE__ short int
#define __INT_LEAST8_TYPE__ signed char
#define __DEC32_MAX_EXP__ 97
#define __INT_FAST8_MAX__ 127
#define __INTPTR_MAX__ 9223372036854775807L
#define __LITTLE_ENDIAN__ 1
#define __SSE2__ 1
#define __LDBL_MANT_DIG__ 64
#define __CONSTANT_CFSTRINGS__ 1
#define __DBL_HAS_QUIET_NAN__ 1
#define __SIG_ATOMIC_MIN__ (-__SIG_ATOMIC_MAX__ - 1)
#define __code_model_small__ 1
#define __k8__ 1
#define __INTPTR_TYPE__ long int
#define __UINT16_TYPE__ short unsigned int
#define __WCHAR_TYPE__ int
#define __SIZEOF_FLOAT__ 4
#define __pic__ 2
#define __UINTPTR_MAX__ 18446744073709551615UL
#define __DEC64_MIN_EXP__ (-382)
#define __INT_FAST64_MAX__ 9223372036854775807LL
#define __GCC_ATOMIC_TEST_AND_SET_TRUEVAL 1
#define __FLT_DIG__ 6
#define __UINT_FAST64_TYPE__ long long unsigned int
#define __INT_MAX__ 2147483647
#define __MACH__ 1
#define __amd64__ 1
#define __INT64_TYPE__ long long int
#define __FLT_MAX_EXP__ 128
#define __ORDER_BIG_ENDIAN__ 4321
#define __DBL_MANT_DIG__ 53
#define __INT_LEAST64_MAX__ 9223372036854775807LL
#define __GCC_ATOMIC_CHAR16_T_LOCK_FREE 2
#define __DEC64_MIN__ 1E-383DD
#define __WINT_TYPE__ int
#define __UINT_LEAST32_TYPE__ unsigned int
#define __SIZEOF_SHORT__ 2
#define __SSE__ 1
#define __LDBL_MIN_EXP__ (-16381)
#define __INT_LEAST8_MAX__ 127
#define __SIZEOF_INT128__ 16
#define __LDBL_MAX_10_EXP__ 4932
#define __ATOMIC_RELAXED 0
#define __DBL_EPSILON__ ((double)2.22044604925031308085e-16L)
#define _LP64 1
#define __UINT8_C(c) c
#define __INT_LEAST32_TYPE__ int
#define __SIZEOF_WCHAR_T__ 4
#define __UINT64_TYPE__ long long unsigned int
#define __INT_FAST8_TYPE__ signed char
#define __DBL_DECIMAL_DIG__ 17
#define __FXSR__ 1
#define __DEC_EVAL_METHOD__ 2
#define __UINT32_C(c) c ## U
#define __INTMAX_MAX__ 9223372036854775807L
#define __BYTE_ORDER__ __ORDER_LITTLE_ENDIAN__
#define __FLT_DENORM_MIN__ 1.40129846432481707092e-45F
#define __INT8_MAX__ 127
#define __PIC__ 2
#define __UINT_FAST32_TYPE__ unsigned int
#define __CHAR32_TYPE__ unsigned int
#define __FLT_MAX__ 3.40282346638528859812e+38F
#define __INT32_TYPE__ int
#define __SIZEOF_DOUBLE__ 8
#define __FLT_MIN_10_EXP__ (-37)
#define __INTMAX_TYPE__ long int
#define __DEC128_MAX_EXP__ 6145
#define __ATOMIC_CONSUME 1
#define __GNUC_MINOR__ 8
#define __UINTMAX_MAX__ 18446744073709551615UL
#define __DEC32_MANT_DIG__ 7
#define __DBL_MAX_10_EXP__ 308
#define __LDBL_DENORM_MIN__ 3.64519953188247460253e-4951L
#define __INT16_C(c) c
#define __STDC__ 1
#define __PTRDIFF_TYPE__ long int
#define __ATOMIC_SEQ_CST 5
#define __UINT32_TYPE__ unsigned int
#define __UINTPTR_TYPE__ long unsigned 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 __GCC_ATOMIC_LLONG_LOCK_FREE 2
#define __LDBL_DIG__ 18
#define __FLT_DECIMAL_DIG__ 9
#define __UINT_FAST16_MAX__ 65535
#define __GNUC_GNU_INLINE__ 1
#define __GCC_ATOMIC_SHORT_LOCK_FREE 2
#define __SSE3__ 1
#define __UINT_FAST8_TYPE__ unsigned char
#define __ATOMIC_ACQ_REL 4
#define __ATOMIC_RELEASE 3
That's 236 macros from an empty file. When I added #include <stdio.h> to the file, the number of macros defined went up to 505. These includes all sorts of platform-identifying macros.

From info gcc (emphasis mine):
-ansi
In C mode, this is equivalent to -std=c90. In C++ mode, it is
equivalent to -std=c++98.
This turns off certain features of GCC that are incompatible with
ISO C90 (when compiling C code), or of standard C++ (when compiling
C++ code), such as the asm and typeof keywords, and predefined
macros such as 'unix' and 'vax' that identify the type of system
you are using. It also enables the undesirable and rarely used ISO
trigraph feature. For the C compiler, it disables recognition of
C++ style // comments as well as the inline keyword.
(It uses vax in the example instead of linux because when it was written maybe it was more popular ;-).
The basic idea is that GCC only tries to fully comply with the ISO standards when it is invoked with the -ansi option.

Use this command
gcc -dM -E - < /dev/null
to get this
#define _LP64 1
#define _STDC_PREDEF_H 1
#define __ATOMIC_ACQUIRE 2
#define __ATOMIC_ACQ_REL 4
#define __ATOMIC_CONSUME 1
#define __ATOMIC_HLE_ACQUIRE 65536
#define __ATOMIC_HLE_RELEASE 131072
#define __ATOMIC_RELAXED 0
#define __ATOMIC_RELEASE 3
#define __ATOMIC_SEQ_CST 5
#define __BIGGEST_ALIGNMENT__ 16
#define __BYTE_ORDER__ __ORDER_LITTLE_ENDIAN__
#define __CHAR16_TYPE__ short unsigned int
#define __CHAR32_TYPE__ unsigned int
#define __CHAR_BIT__ 8
#define __DBL_DECIMAL_DIG__ 17
#define __DBL_DENORM_MIN__ ((double)4.94065645841246544177e-324L)
#define __DBL_DIG__ 15
#define __DBL_EPSILON__ ((double)2.22044604925031308085e-16L)
#define __DBL_HAS_DENORM__ 1
#define __DBL_HAS_INFINITY__ 1
#define __DBL_HAS_QUIET_NAN__ 1
#define __DBL_MANT_DIG__ 53
#define __DBL_MAX_10_EXP__ 308
#define __DBL_MAX_EXP__ 1024
#define __DBL_MAX__ ((double)1.79769313486231570815e+308L)
#define __DBL_MIN_10_EXP__ (-307)
#define __DBL_MIN_EXP__ (-1021)
#define __DBL_MIN__ ((double)2.22507385850720138309e-308L)
#define __DEC128_EPSILON__ 1E-33DL
#define __DEC128_MANT_DIG__ 34
#define __DEC128_MAX_EXP__ 6145
#define __DEC128_MAX__ 9.999999999999999999999999999999999E6144DL
#define __DEC128_MIN_EXP__ (-6142)
#define __DEC128_MIN__ 1E-6143DL
#define __DEC128_SUBNORMAL_MIN__ 0.000000000000000000000000000000001E-6143DL
#define __DEC32_EPSILON__ 1E-6DF
#define __DEC32_MANT_DIG__ 7
#define __DEC32_MAX_EXP__ 97
#define __DEC32_MAX__ 9.999999E96DF
#define __DEC32_MIN_EXP__ (-94)
#define __DEC32_MIN__ 1E-95DF
#define __DEC32_SUBNORMAL_MIN__ 0.000001E-95DF
#define __DEC64_EPSILON__ 1E-15DD
#define __DEC64_MANT_DIG__ 16
#define __DEC64_MAX_EXP__ 385
#define __DEC64_MAX__ 9.999999999999999E384DD
#define __DEC64_MIN_EXP__ (-382)
#define __DEC64_MIN__ 1E-383DD
#define __DEC64_SUBNORMAL_MIN__ 0.000000000000001E-383DD
#define __DECIMAL_BID_FORMAT__ 1
#define __DECIMAL_DIG__ 21
#define __DEC_EVAL_METHOD__ 2
#define __ELF__ 1
#define __FINITE_MATH_ONLY__ 0
#define __FLOAT_WORD_ORDER__ __ORDER_LITTLE_ENDIAN__
#define __FLT_DECIMAL_DIG__ 9
#define __FLT_DENORM_MIN__ 1.40129846432481707092e-45F
#define __FLT_DIG__ 6
#define __FLT_EPSILON__ 1.19209289550781250000e-7F
#define __FLT_EVAL_METHOD__ 0
#define __FLT_HAS_DENORM__ 1
#define __FLT_HAS_INFINITY__ 1
#define __FLT_HAS_QUIET_NAN__ 1
#define __FLT_MANT_DIG__ 24
#define __FLT_MAX_10_EXP__ 38
#define __FLT_MAX_EXP__ 128
#define __FLT_MAX__ 3.40282346638528859812e+38F
#define __FLT_MIN_10_EXP__ (-37)
#define __FLT_MIN_EXP__ (-125)
#define __FLT_MIN__ 1.17549435082228750797e-38F
#define __FLT_RADIX__ 2
#define __FXSR__ 1
#define __GCC_ASM_FLAG_OUTPUTS__ 1
#define __GCC_ATOMIC_BOOL_LOCK_FREE 2
#define __GCC_ATOMIC_CHAR16_T_LOCK_FREE 2
#define __GCC_ATOMIC_CHAR32_T_LOCK_FREE 2
#define __GCC_ATOMIC_CHAR_LOCK_FREE 2
#define __GCC_ATOMIC_INT_LOCK_FREE 2
#define __GCC_ATOMIC_LLONG_LOCK_FREE 2
#define __GCC_ATOMIC_LONG_LOCK_FREE 2
#define __GCC_ATOMIC_POINTER_LOCK_FREE 2
#define __GCC_ATOMIC_SHORT_LOCK_FREE 2
#define __GCC_ATOMIC_TEST_AND_SET_TRUEVAL 1
#define __GCC_ATOMIC_WCHAR_T_LOCK_FREE 2
#define __GCC_HAVE_DWARF2_CFI_ASM 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_IEC_559 2
#define __GCC_IEC_559_COMPLEX 2
#define __GNUC_MINOR__ 3
#define __GNUC_PATCHLEVEL__ 0
#define __GNUC_STDC_INLINE__ 1
#define __GNUC__ 6
#define __GXX_ABI_VERSION 1010
#define __INT16_C(c) c
#define __INT16_MAX__ 0x7fff
#define __INT16_TYPE__ short int
#define __INT32_C(c) c
#define __INT32_MAX__ 0x7fffffff
#define __INT32_TYPE__ int
#define __INT64_C(c) c ## L
#define __INT64_MAX__ 0x7fffffffffffffffL
#define __INT64_TYPE__ long int
#define __INT8_C(c) c
#define __INT8_MAX__ 0x7f
#define __INT8_TYPE__ signed char
#define __INTMAX_C(c) c ## L
#define __INTMAX_MAX__ 0x7fffffffffffffffL
#define __INTMAX_TYPE__ long int
#define __INTPTR_MAX__ 0x7fffffffffffffffL
#define __INTPTR_TYPE__ long int
#define __INT_FAST16_MAX__ 0x7fffffffffffffffL
#define __INT_FAST16_TYPE__ long int
#define __INT_FAST32_MAX__ 0x7fffffffffffffffL
#define __INT_FAST32_TYPE__ long int
#define __INT_FAST64_MAX__ 0x7fffffffffffffffL
#define __INT_FAST64_TYPE__ long int
#define __INT_FAST8_MAX__ 0x7f
#define __INT_FAST8_TYPE__ signed char
#define __INT_LEAST16_MAX__ 0x7fff
#define __INT_LEAST16_TYPE__ short int
#define __INT_LEAST32_MAX__ 0x7fffffff
#define __INT_LEAST32_TYPE__ int
#define __INT_LEAST64_MAX__ 0x7fffffffffffffffL
#define __INT_LEAST64_TYPE__ long int
#define __INT_LEAST8_MAX__ 0x7f
#define __INT_LEAST8_TYPE__ signed char
#define __INT_MAX__ 0x7fffffff
#define __LDBL_DENORM_MIN__ 3.64519953188247460253e-4951L
#define __LDBL_DIG__ 18
#define __LDBL_EPSILON__ 1.08420217248550443401e-19L
#define __LDBL_HAS_DENORM__ 1
#define __LDBL_HAS_INFINITY__ 1
#define __LDBL_HAS_QUIET_NAN__ 1
#define __LDBL_MANT_DIG__ 64
#define __LDBL_MAX_10_EXP__ 4932
#define __LDBL_MAX_EXP__ 16384
#define __LDBL_MAX__ 1.18973149535723176502e+4932L
#define __LDBL_MIN_10_EXP__ (-4931)
#define __LDBL_MIN_EXP__ (-16381)
#define __LDBL_MIN__ 3.36210314311209350626e-4932L
#define __LONG_LONG_MAX__ 0x7fffffffffffffffLL
#define __LONG_MAX__ 0x7fffffffffffffffL
#define __LP64__ 1
#define __MMX__ 1
#define __NO_INLINE__ 1
#define __ORDER_BIG_ENDIAN__ 4321
#define __ORDER_LITTLE_ENDIAN__ 1234
#define __ORDER_PDP_ENDIAN__ 3412
#define __PIC__ 2
#define __PIE__ 2
#define __PRAGMA_REDEFINE_EXTNAME 1
#define __PTRDIFF_MAX__ 0x7fffffffffffffffL
#define __PTRDIFF_TYPE__ long int
#define __REGISTER_PREFIX__
#define __SCHAR_MAX__ 0x7f
#define __SEG_FS 1
#define __SEG_GS 1
#define __SHRT_MAX__ 0x7fff
#define __SIG_ATOMIC_MAX__ 0x7fffffff
#define __SIG_ATOMIC_MIN__ (-__SIG_ATOMIC_MAX__ - 1)
#define __SIG_ATOMIC_TYPE__ int
#define __SIZEOF_DOUBLE__ 8
#define __SIZEOF_FLOAT128__ 16
#define __SIZEOF_FLOAT80__ 16
#define __SIZEOF_FLOAT__ 4
#define __SIZEOF_INT128__ 16
#define __SIZEOF_INT__ 4
#define __SIZEOF_LONG_DOUBLE__ 16
#define __SIZEOF_LONG_LONG__ 8
#define __SIZEOF_LONG__ 8
#define __SIZEOF_POINTER__ 8
#define __SIZEOF_PTRDIFF_T__ 8
#define __SIZEOF_SHORT__ 2
#define __SIZEOF_SIZE_T__ 8
#define __SIZEOF_WCHAR_T__ 4
#define __SIZEOF_WINT_T__ 4
#define __SIZE_MAX__ 0xffffffffffffffffUL
#define __SIZE_TYPE__ long unsigned int
#define __SSE2_MATH__ 1
#define __SSE2__ 1
#define __SSE_MATH__ 1
#define __SSE__ 1
#define __SSP_STRONG__ 3
#define __STDC_HOSTED__ 1
#define __STDC_IEC_559_COMPLEX__ 1
#define __STDC_IEC_559__ 1
#define __STDC_ISO_10646__ 201605L
#define __STDC_NO_THREADS__ 1
#define __STDC_UTF_16__ 1
#define __STDC_UTF_32__ 1
#define __STDC_VERSION__ 201112L
#define __STDC__ 1
#define __UINT16_C(c) c
#define __UINT16_MAX__ 0xffff
#define __UINT16_TYPE__ short unsigned int
#define __UINT32_C(c) c ## U
#define __UINT32_MAX__ 0xffffffffU
#define __UINT32_TYPE__ unsigned int
#define __UINT64_C(c) c ## UL
#define __UINT64_MAX__ 0xffffffffffffffffUL
#define __UINT64_TYPE__ long unsigned int
#define __UINT8_C(c) c
#define __UINT8_MAX__ 0xff
#define __UINT8_TYPE__ unsigned char
#define __UINTMAX_C(c) c ## UL
#define __UINTMAX_MAX__ 0xffffffffffffffffUL
#define __UINTMAX_TYPE__ long unsigned int
#define __UINTPTR_MAX__ 0xffffffffffffffffUL
#define __UINTPTR_TYPE__ long unsigned int
#define __UINT_FAST16_MAX__ 0xffffffffffffffffUL
#define __UINT_FAST16_TYPE__ long unsigned int
#define __UINT_FAST32_MAX__ 0xffffffffffffffffUL
#define __UINT_FAST32_TYPE__ long unsigned int
#define __UINT_FAST64_MAX__ 0xffffffffffffffffUL
#define __UINT_FAST64_TYPE__ long unsigned int
#define __UINT_FAST8_MAX__ 0xff
#define __UINT_FAST8_TYPE__ unsigned char
#define __UINT_LEAST16_MAX__ 0xffff
#define __UINT_LEAST16_TYPE__ short unsigned int
#define __UINT_LEAST32_MAX__ 0xffffffffU
#define __UINT_LEAST32_TYPE__ unsigned int
#define __UINT_LEAST64_MAX__ 0xffffffffffffffffUL
#define __UINT_LEAST64_TYPE__ long unsigned int
#define __UINT_LEAST8_MAX__ 0xff
#define __UINT_LEAST8_TYPE__ unsigned char
#define __USER_LABEL_PREFIX__
#define __VERSION__ "6.3.0 20170406"
#define __WCHAR_MAX__ 0x7fffffff
#define __WCHAR_MIN__ (-__WCHAR_MAX__ - 1)
#define __WCHAR_TYPE__ int
#define __WINT_MAX__ 0xffffffffU
#define __WINT_MIN__ 0U
#define __WINT_TYPE__ unsigned int
#define __amd64 1
#define __amd64__ 1
#define __code_model_small__ 1
#define __gnu_linux__ 1
#define __has_include(STR) __has_include__(STR)
#define __has_include_next(STR) __has_include_next__(STR)
#define __k8 1
#define __k8__ 1
#define __linux 1
#define __linux__ 1
#define __pic__ 2
#define __pie__ 2
#define __unix 1
#define __unix__ 1
#define __x86_64 1
#define __x86_64__ 1
#define linux 1
#define unix 1

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