atoi和itoa的实现

#include <cruntime.h>
#include <stdlib.h>
#include <ctype.h>



long __cdecl atol(
const char *nptr
)
{
int c;
long total;
int sign;


while ( isspace((int)(unsigned char)*nptr) )
++nptr;

c = (int)(unsigned char)*nptr++;
sign = c;
if (c == ‘‘-‘‘ || c == ‘‘+‘‘)
c = (int)(unsigned char)*nptr++;

total = 0;

while (isdigit(c)) {
total = 10 * total + (c - ‘‘0‘‘);
c = (int)(unsigned char)*nptr++;
}

if (sign == ‘‘-‘‘)
return -total;
else
return total;
}




int __cdecl atoi(
const char *nptr
)
{
return (int)atol(nptr);
}

#ifndef _NO_INT64

__int64 __cdecl _atoi64(
const char *nptr
)
{
int c;
__int64 total;
int sign;


while ( isspace((int)(unsigned char)*nptr) )
++nptr;

c = (int)(unsigned char)*nptr++;
sign = c;
if (c == ‘‘-‘‘ || c == ‘‘+‘‘)
c = (int)(unsigned char)*nptr++;

total = 0;

while (isdigit(c)) {
total = 10 * total + (c - ‘‘0‘‘);
c = (int)(unsigned char)*nptr++;
}

if (sign == ‘‘-‘‘)
return -total;
else
return total;
}

#endif


#include <msvcrt/errno.h>
#include <msvcrt/stdlib.h>
#include <msvcrt/internal/file.h>
char* _itoa(int value, char* string, int radix)
{
char tmp[33];
char* tp = tmp;
int i;
unsigned v;
int sign;
char* sp;

if (radix > 36 || radix <= 1)
{
__set_errno(EDOM);
return 0;
}

sign = (radix == 10 && value < 0);
if (sign)
v = -value;
else
v = (unsigned)value;
while (v || tp == tmp)
{
i = v % radix;
v = v / radix;
if (i < 10)
*tp++ = i+‘‘0‘‘;
else
*tp++ = i + ‘‘a‘‘ - 10;
}

if (string == 0)
string = (char*)malloc((tp-tmp)+sign+1);
sp = string;

if (sign)
*sp++ = ‘‘-‘‘;
while (tp > tmp)
*sp++ = *--tp;
*sp = 0;
return string;
} 

atoi和itoa的实现

原文：http://blog.csdn.net/pishum/article/details/19327141