代码：

1 long __cdecl atol( 2   const char *nptr 3   ) 4 { 5   int c; /* current char */ 6   long total; /* current total */ 7   int sign; /* if '-', then negative, otherwise positive */ 8  9   /* skip whitespace */10   while ( isspace((int)(unsigned char)*nptr) )11   ++nptr;12 13   c = (int)(unsigned char)*nptr++; sign = c; /* save sign indication */14   if (c == '-' || c == '+')15   c = (int)(unsigned char)*nptr++; /* skip sign */16 17   total = 0;18 19   while (isdigit(c)) {20   total = 10 * total + (c - '0'); /* accumulate digit */21   c = (int)(unsigned char)*nptr++; /* get next char */22   }23 24   if (sign == '-')25   return -total;26   else27   return total; /* return result, negated if necessary */28 }

为什么要用 (int)(unsigned char)？

因为在char 类型强制转换成int类型时，若char类型数是负数（最高位为1）有些编译器会将最高字节填充0xFF

1 for(int i = 128; i < 131; i++) 2     { 3         int c = (char)i; 4         int d = (unsigned char)i; 5         printf("%d %d\n", c, d); 6     } 7  8 输出： 9 -128 12810 -127 12911 -126 13012 请按任意键继续. . .

因为isspace(），isdigit()这类函数接受一个int 参数，参数的值必须是一个可以用unsigned char 表示的值或者是EOF，以下是 man 手册的原文：

int isspace(int c);

int isdigit(int c);

...

These functions check whether c, which must have the value of anunsigned char or EOF,falls into a cetern charcter class according to the current locale.

所以需要用c = (int)(unsigned char)*nptr++，来确保从 *nptr++ 到 c 是进行零扩展而不是符号扩展，保证 c 中存放的是一个unsigned char 所能表示的值。