glibc的几个有用的处理二进制位的内置函数

— Built-in Function: int __builtin_ffs (unsigned int x)

Returns one plus the index of the least significant 1-bit of x, or if x is zero, returns zero.

返回右起第一个‘1’的位置。

— Built-in Function: int __builtin_clz (unsigned int x)

Returns the number of leading 0-bits in x, starting at the most significant bit position. If x is 0, the result is undefined.

返回左起第一个‘1’之前0的个数。

— Built-in Function: int __builtin_ctz (unsigned int x)

Returns the number of trailing 0-bits in x, starting at the least significant bit position. If x is 0, the result is undefined.

返回右起第一个‘1’之后的0的个数。

— Built-in Function: int __builtin_popcount (unsigned int x)

Returns the number of 1-bits in x.

返回‘1’的个数。

— Built-in Function: int __builtin_parity (unsigned int x)

Returns the parity of x, i.e. the number of 1-bits in x modulo 2.

返回‘1’的个数的奇偶性。

— Built-in Function: int __builtin_ffsl (unsigned long)

Similar to __builtin_ffs, except the argument type is unsigned long.

— Built-in Function: int __builtin_clzl (unsigned long)

Similar to __builtin_clz, except the argument type is unsigned long.

— Built-in Function: int __builtin_ctzl (unsigned long)

Similar to __builtin_ctz, except the argument type is unsigned long.

— Built-in Function: int __builtin_popcountl (unsigned long)

Similar to __builtin_popcount, except the argument type is unsigned long.

— Built-in Function: int __builtin_parityl (unsigned long)

Similar to __builtin_parity, except the argument type is unsigned long.

— Built-in Function: int __builtin_ffsll (unsigned long long)

Similar to __builtin_ffs, except the argument type is unsigned long long.

— Built-in Function: int __builtin_clzll (unsigned long long)

Similar to __builtin_clz, except the argument type is unsigned long long.

— Built-in Function: int __builtin_ctzll (unsigned long long)

Similar to __builtin_ctz, except the argument type is unsigned long long.

— Built-in Function: int __builtin_popcountll (unsigned long long)

Similar to __builtin_popcount, except the argument type is unsigned long long.

— Built-in Function: int __builtin_parityll (unsigned long long)

Similar to __builtin_parity, except the argument type is unsigned long long.

【实验程序】

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这里存在一个为题，希望知道的朋友能够解释，就是为什么不能用函数指针指向这些内置函数。

【测试结果】

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