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

Question

Why does the C preprocessor in GCC interpret the word linux  small letters  as the constant 1   test c    include  lt stdio h gt  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

User · Answer

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

User · Answer

Use this command   gcc -dM -E -  lt   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

User · Answer

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  lt   dev null   grep linux  define   linux 1  define   linux   1  define   gnu linux   1    cpp --std gnu89 -dM  lt   dev null   grep linux  define   linux 1  define   linux   1  define   gnu linux   1  define linux 1    cpp --std c99 -dM  lt   dev null   grep linux  define   linux 1  define   linux   1  define   gnu linux   1    cpp --std gnu99 -dM  lt   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 hellip

User · Answer

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 amp 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  amp 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

User · Answer

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  lt stdio h gt  to the file  the number of macros defined went up to 505   These includes all sorts of platform-identifying macros

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

Examples related to c

Examples related to linux

Examples related to gcc

Examples related to c-preprocessor