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字符数字转换 atoi 与 strtol

2024-10-29 15:28:01   209人阅读

原文：http://www.cnblogs.com/JefferyZhou/archive/2010/07/01/1769555.html

 

在很多时候我们都很清楚 atoX 系列函数: atoi , atol , atof
新来的一系列函数:  strtol,  strtoul, strtod 
通常有如下的关系：
1. 对应关系其中： 

          atoi   (把字符串转到整形)    --对应--   strtol  (把字符串转到长整形)           

          atol   (把字符串转到长整形)    --对应--   strtol  (把字符串转到长整形)           

          atof   (把字符串转到浮点数)    --对应--   strtod (把字符串转到浮点数) 
2. atoX 系列是 三十年前的函数 strtoX 系列是后十年产品


3. atoX 系列接口，没有成功失败的区别(标准实现中)，    strtoX 系列接口，有成功失败的区别


    比如：int i_atoi_lfs =  atoi(""); 与 int i_atoi_rfs = atoi("0"); 两个得到的是一样的，没有任何区别         

     而： int i_atoi_lfs =  strtol  ("", NULL,10); 与 int i_atoi_rfs = strtol  ("0", NULL,10);  得到的结果都是0，但是左边会置失败标志位。


4. msvcr80.dll  的具体实现：

 

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int __cdecl atoi(__in_z const char *_Str){  return _tstoi(_Str);  }   
int __cdecl _tstoi(     const _TCHAR *nptr        ){    return (int)_tstol(nptr);}
int __cdecl _tstoi(     const _TCHAR *nptr        ){    return (int)_tstol(nptr);}
long __cdecl _tstol(const _TCHAR *nptr){return _tcstol(nptr, NULL, 10);}
#define _tcstol   strtol
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<pre class="brush:cpp">extern "C" long __cdecl strtol (
        const char *nptr,
        char **endptr,
        int ibase
        )
{
    if (__locale_changed == 0)
    {
        return (long) strtoxl(&__initiallocalestructinfo, nptr, (const char **)endptr, ibase, 0);
    }
    else
    {
        return (long) strtoxl(NULL, nptr, (const char **)endptr, ibase, 0);
    }
}
</pre>
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static unsigned long __cdecl strtoxl (
        _locale_t plocinfo,
        const char *nptr,
        const char **endptr,
        int ibase,
        int flags
        )
{
        const char *p;
        char c;
        unsigned long number;
        unsigned digval;
        unsigned long maxval;
        _LocaleUpdate _loc_update(plocinfo);
 
        /* validation section */
        if (endptr != NULL)
        {
            /* store beginning of string in endptr */
            *endptr = (char *)nptr;
        }
        _VALIDATE_RETURN(nptr != NULL, EINVAL, 0L);
        _VALIDATE_RETURN(ibase == 0 || (2 <= ibase && ibase <= 36), EINVAL, 0L);
 
        p = nptr;                       /* p is our scanning pointer */
        number = 0;                     /* start with zero */
//1. 这里关注到，函数没有检查传入的原字符指针是否为空, 如果传递了一个空的就崩了....
        c = *p++;                       /* read char */
        while ( _isspace_l((int)(unsigned char)c, _loc_update.GetLocaleT()) )
            c = *p++;               /* skip whitespace */
 
//2. 不要期望能够 转换负负得正的字符串, 注意 "--100" 得到 0 ，  "-100" 得到 -100
        if (c == ‘-‘) {
            flags |= FL_NEG;        /* remember minus sign */
            c = *p++;
        }
        else if (c == ‘+‘)
            c = *p++;               /* skip sign */
//3.  基数是 2 到 36 的闭区间 ， [2, 36]
        if (ibase < 0 || ibase == 1 || ibase > 36) {
            /* bad base! */
            if (endptr)
                /* store beginning of string in endptr */
                *endptr = nptr;
            return 0L;              /* return 0 */
        }
 
//4. 如果转换的时候基数输入是0， 则基数取决于原字符的前面两个字符,
// 以非0开头的是 10进制字符串，
// 以0x或者0X开头的是 16进制字符串,
// 而仅仅以 0开头的是 8进制
        else if (ibase == 0) {
            /* determine base free-lance, based on first two chars of
               string */
            if (c != ‘0‘)
                ibase = 10;
            else if (*p == ‘x‘ || *p == ‘X‘)
                ibase = 16;
            else
                ibase = 8;
        }
 
//  {{{   源码里面，这个地方 有这么一段 暂时不知道是干嘛的, 在我看来貌似是多余的
        if (ibase == 0) {
            /* determine base free-lance, based on first two chars of
               string */
            if (c != ‘0‘)
                ibase = 10;
            else if (*p == ‘x‘ || *p == ‘X‘)
                ibase = 16;
            else
                ibase = 8;
        }
//}}}
 
// 5. 如果是 16 进制，则跳过0x 或者 0X 的前缀
        if (ibase == 16) {
            /* we might have 0x in front of number; remove if there */
            if (c == ‘0‘ && (*p == ‘x‘ || *p == ‘X‘)) {
                ++p;
                c = *p++;       /* advance past prefix */
            }
        }
 
// 6. 下面就是读取字符串，然后按照 local 解析应用的数值, 如果在转换过程中出现各种情况都会对标志位flags 进行标记
        /* if our number exceeds this, we will overflow on multiply */
        maxval = ULONG_MAX / ibase;
 
 
        for (;;) {      /* exit in middle of loop */
            /* convert c to value */
            if ( __ascii_isdigit_l((int)(unsigned char)c, _loc_update.GetLocaleT()) )
                digval = c - ‘0‘;
            else if ( __ascii_isalpha_l((int)(unsigned char)c, _loc_update.GetLocaleT()) )
                digval = __ascii_toupper(c) - ‘A‘ + 10;
            else
                break;
            if (digval >= (unsigned)ibase)
                break;          /* exit loop if bad digit found */
 
            /* record the fact we have read one digit */
            flags |= FL_READDIGIT;
 
            /* we now need to compute number = number * base + digval,
               but we need to know if overflow occured.  This requires
               a tricky pre-check. */
 
            if (number < maxval || (number == maxval &&
                        (unsigned long)digval <= ULONG_MAX % ibase)) {
                /* we won‘t overflow, go ahead and multiply */
                number = number * ibase + digval;
            }
            else {
                /* we would have overflowed -- set the overflow flag */
                flags |= FL_OVERFLOW;
                if (endptr == NULL) {
                    /* no need to keep on parsing if we
                       don‘t have to return the endptr. */
                    break;
                }
            }
 
            c = *p++;               /* read next digit */
        }
 
        --p;                            /* point to place that stopped scan */
 
        if (!(flags & FL_READDIGIT)) {
            /* no number there; return 0 and point to beginning of
               string */
            if (endptr)
                /* store beginning of string in endptr later on */
                p = nptr;
            number = 0L;            /* return 0 */
        }
        else if ( (flags & FL_OVERFLOW) ||
                ( !(flags & FL_UNSIGNED) &&
                  ( ( (flags & FL_NEG) && (number > -LONG_MIN) ) ||
                    ( !(flags & FL_NEG) && (number > LONG_MAX) ) ) ) )
        {
            /* overflow or signed overflow occurred */
            errno = ERANGE;                  //(老的实现方式和新的实现方式区别主要在这里, 新版友记录转换过程)
            if ( flags & FL_UNSIGNED )
                number = ULONG_MAX;
            else if ( flags & FL_NEG )
                number = (unsigned long)(-LONG_MIN);
            else
                number = LONG_MAX;
        }
 
        if (endptr != NULL)
            /* store pointer to char that stopped the scan */
            *endptr = p;
 
        if (flags & FL_NEG)
            /* negate result if there was a neg sign */
            number = (unsigned long)(-(long)number);
 
        return number;                  /* done. */
}

 

 

所以 atoi 已经等同于strtol 

Sign,  Clown , 2010.07.01 . 23:32 . HDPY

[本文原创，转载请注明出处，在文章末尾提供原文链接http://www.cnblogs.com/JefferyZhou/，否则一旦发现，将按字节每人民币收费，绝不论价]

字符数字转换 atoi 与 strtol


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