linux gcc 裏面的變量命名的陷阱

linux的gcc編譯器自帶了很多宏定義，倘若我們的變量跟系統的宏定義相同，編譯就會錯誤。

如下面這段代碼：

#include<stdio.h>
int main(int argc,char **argv){
    int linux = 10;
    int unix = 2;
    return 0;
}

運行命令編譯

# gcc -o main mian.c

出現如下錯誤：

 linux.c: In function ‘main’:
linux.c:10:9: error: expected identifier or ‘(’ before numeric constant
     int linux = 10;
         ^
linux.c:11:9: error: expected identifier or ‘(’ before numeric constant
     int unix = 2;
         ^

運行命令預編譯
# gcc -E main.c

出現如下：

# 9 "linux.c" 2
int main(int argc,char **argv){
    int 1 = 10;
    int 1 = 2;
    return 0;
}

發現 linux 和unix都被預編譯成了 1.

解決方法：使用如下三種方法都可以編譯通過

gcc -std=c99  main.c -o main
gcc -std=c90  main.c -o main
gcc -std=c11  main.c -o main

使用如下三種方法都不可以編譯通過，原因是gcc 默認是gnu標準的。

gcc -std=gnu89  main.c -o main
gcc -std=gnu90  main.c -o main
gcc -std=gnu11  main.c -o main

運行命令

#gcc -dM -E - < /dev/null

輸出gcc編譯處理器的所有宏定義，其中就包含了linux unix的宏定義，這也是爲什麼我們用gnu的標準都編譯不通過。

#define __DBL_MIN_EXP__ (-1021)
#define __pentiumpro__ 1
#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 ## LL
#define __CHAR_BIT__ 8
#define __UINT8_MAX__ 255
#define __WINT_MAX__ 4294967295U
#define __ORDER_LITTLE_ENDIAN__ 1234
#define __SIZE_MAX__ 4294967295U
#define __WCHAR_MAX__ 2147483647L
#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__ 2
#define __unix__ 1
#define __GCC_ATOMIC_CHAR32_T_LOCK_FREE 2
#define __UINT_FAST64_MAX__ 18446744073709551615ULL
#define __SIG_ATOMIC_TYPE__ int
#define __DBL_MIN_10_EXP__ (-307)
#define __FINITE_MATH_ONLY__ 0
#define __GNUC_PATCHLEVEL__ 4
#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 __UINTMAX_TYPE__ long long unsigned int
#define __linux 1
#define __DEC32_EPSILON__ 1E-6DF
#define __unix 1
#define __UINT32_MAX__ 4294967295U
#define __LDBL_MAX_EXP__ 16384
#define __WINT_MIN__ 0U
#define __linux__ 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__ 4
#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 __INT32_MAX__ 2147483647
#define __SIZEOF_LONG__ 4
#define __STDC_IEC_559__ 1
#define __STDC_ISO_10646__ 201103L
#define __UINT16_C(c) c
#define __DECIMAL_DIG__ 21
#define __gnu_linux__ 1
#define __LDBL_HAS_QUIET_NAN__ 1
#define __GNUC__ 4
#define __FLT_HAS_DENORM__ 1
#define __SIZEOF_LONG_DOUBLE__ 12
#define __BIGGEST_ALIGNMENT__ 16
#define __i686 1
#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__ int
#define __LDBL_HAS_DENORM__ 1
#define __DEC128_MAX__ 9.999999999999999999999999999999999E6144DL
#define __INT_LEAST32_MAX__ 2147483647
#define __DEC32_MIN__ 1E-95DF
#define __DBL_MAX_EXP__ 1024
#define __DEC128_EPSILON__ 1E-33DL
#define __ATOMIC_HLE_RELEASE 131072
#define __PTRDIFF_MAX__ 2147483647
#define __STDC_NO_THREADS__ 1
#define __ATOMIC_HLE_ACQUIRE 65536
#define __LONG_LONG_MAX__ 9223372036854775807LL
#define __SIZEOF_SIZE_T__ 4
#define __SIZEOF_WINT_T__ 4
#define __GCC_HAVE_DWARF2_CFI_ASM 1
#define __GXX_ABI_VERSION 1002
#define __FLT_MIN_EXP__ (-125)
#define __i686__ 1
#define __INT_FAST64_TYPE__ long long int
#define __DBL_MIN__ ((double)2.22507385850720138309e-308L)
#define __DECIMAL_BID_FORMAT__ 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 __i386 1
#define __FLT_MANT_DIG__ 24
#define __VERSION__ "4.8.4"
#define __UINT64_C(c) c ## ULL
#define _STDC_PREDEF_H 1
#define __GCC_ATOMIC_INT_LOCK_FREE 2
#define __FLOAT_WORD_ORDER__ __ORDER_LITTLE_ENDIAN__
#define __STDC_IEC_559_COMPLEX__ 1
#define __INT32_C(c) c
#define __DEC64_EPSILON__ 1E-15DD
#define __ORDER_PDP_ENDIAN__ 3412
#define __DEC128_MIN_EXP__ (-6142)
#define __code_model_32__ 1
#define __INT_FAST32_TYPE__ int
#define __UINT_LEAST16_TYPE__ short unsigned int
#define unix 1
#define __INT16_MAX__ 32767
#define __i386__ 1
#define __SIZE_TYPE__ unsigned int
#define __UINT64_MAX__ 18446744073709551615ULL
#define __INT8_TYPE__ signed char
#define __ELF__ 1
#define __FLT_RADIX__ 2
#define __INT_LEAST16_TYPE__ short int
#define __LDBL_EPSILON__ 1.08420217248550443401e-19L
#define __UINTMAX_C(c) c ## ULL
#define __SIG_ATOMIC_MAX__ 2147483647
#define __GCC_ATOMIC_WCHAR_T_LOCK_FREE 2
#define __SIZEOF_PTRDIFF_T__ 4
#define __DEC32_SUBNORMAL_MIN__ 0.000001E-95DF
#define __pentiumpro 1
#define __INT_FAST16_MAX__ 2147483647
#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__ 2147483647L
#define __DEC128_SUBNORMAL_MIN__ 0.000000000000000000000000000000001E-6143DL
#define __FLT_HAS_INFINITY__ 1
#define __UINT_FAST16_TYPE__ 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__ 2147483647
#define linux 1
#define __LDBL_MANT_DIG__ 64
#define __DBL_HAS_QUIET_NAN__ 1
#define __SIG_ATOMIC_MIN__ (-__SIG_ATOMIC_MAX__ - 1)
#define __INTPTR_TYPE__ int
#define __UINT16_TYPE__ short unsigned int
#define __WCHAR_TYPE__ long int
#define __SIZEOF_FLOAT__ 4
#define __UINTPTR_MAX__ 4294967295U
#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 __INT64_TYPE__ long long int
#define __FLT_MAX_EXP__ 128
#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__ unsigned int
#define __UINT_LEAST32_TYPE__ unsigned int
#define __SIZEOF_SHORT__ 2
#define __LDBL_MIN_EXP__ (-16381)
#define __INT_LEAST8_MAX__ 127
#define __SSP__ 1
#define __LDBL_MAX_10_EXP__ 4932
#define __ATOMIC_RELAXED 0
#define __DBL_EPSILON__ ((double)2.22044604925031308085e-16L)
#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 __DEC_EVAL_METHOD__ 2
#define __ORDER_BIG_ENDIAN__ 4321
#define __UINT32_C(c) c ## U
#define __INTMAX_MAX__ 9223372036854775807LL
#define __BYTE_ORDER__ __ORDER_LITTLE_ENDIAN__
#define __FLT_DENORM_MIN__ 1.40129846432481707092e-45F
#define __INT8_MAX__ 127
#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 long int
#define i386 1
#define __DEC128_MAX_EXP__ 6145
#define __ATOMIC_CONSUME 1
#define __GNUC_MINOR__ 8
#define __UINTMAX_MAX__ 18446744073709551615ULL
#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__ int
#define __ATOMIC_SEQ_CST 5
#define __UINT32_TYPE__ unsigned int
#define __UINTPTR_TYPE__ 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__ 4294967295U
#define __GNUC_GNU_INLINE__ 1
#define __GCC_ATOMIC_SHORT_LOCK_FREE 2
#define __UINT_FAST8_TYPE__ unsigned char
#define __ATOMIC_ACQ_REL 4
#define __ATOMIC_RELEASE 3

我們繼續深究。分別運行如下命令，得出：

$ cpp --std=gnu99 -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=gnu89 -dM < /dev/null | grep linux
#define __linux 1
#define __linux__ 1
#define __gnu_linux__ 1
#define linux 1
$ cpp --std=c89 -dM < /dev/null | grep linux
#define __linux 1
#define __linux__ 1
#define __gnu_linux__ 1
$ cpp --std=c90 -dM < /dev/null | grep linux
#define __linux 1
#define __linux__ 1
#define __gnu_linux__ 1
$ cpp --std=gnu90 -dM < /dev/null | grep linux
#define __linux 1
#define __linux__ 1
#define __gnu_linux__ 1
#define linux 1
$ cpp --std=c11 -dM < /dev/null | grep linux
#define __linux 1
#define __linux__ 1
#define __gnu_linux__ 1
$ cpp --std=gnu11 -dM < /dev/null | grep linux
#define __linux 1
#define __linux__ 1
#define __gnu_linux__ 1
#define linux 1

不難發現gnu的標準裏都有linux的宏定義，但在c的標準裏就去掉了。

運行命令# man gcc

裏面有段話

 -std=
           Determine the language standard.   This option is currently only supported when compiling C or C++.


           The compiler can accept several base standards, such as c90 or c++98, and GNU dialects of those standards, such as gnu90 or gnu++98.  When a base standard is specified, the compiler accepts all
           programs following that standard plus those using GNU extensions that do not contradict it.  For example, -std=c90 turns off certain features of GCC that are incompatible with ISO C90, such as the
           "asm" and "typeof" keywords, but not other GNU extensions that do not have a meaning in ISO C90, such as omitting the middle term of a "?:" expression. On the other hand, when a GNU dialect of a
           standard is specified, all features supported by the compiler are enabled, even when those features change the meaning of the base standard.  As a result, some strict-conforming programs may be
           rejected.  The particular standard is used by -Wpedantic to identify which features are GNU extensions given that version of the standard. For example -std=gnu90 -Wpedantic warns about C++ style
           // comments, while -std=gnu99 -Wpedantic does not.

c89，c90或者c11都爲了兼容官方 ISO的標準去掉了一些特性。

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