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
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