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atoi() & itoa()函数的内部的实现

/***
*atox.c - atoi and atol conversion
*
* Copyright (c) 1989-1997, Microsoft Corporation. All rights reserved.
*
*Purpose:
* Converts a character string into an int or long.
*
*******************************************************************************/


#include
#include
#include

/***
*long atol(char *nptr) - Convert string to long
*
*Purpose:
* Converts ASCII string pointed to by nptr to binary.
* Overflow is not detected.
*
*Entry:
* nptr = ptr to string to convert
*
*Exit:
* return long int value of the string
*
*Exceptions:
* None - overflow is not detected.
*
*******************************************************************************/

long __cdecl atol(
const char *nptr
)
{
int c; /* current char */
long total; /* current total */
int sign; /* if ''-'', then negative, otherwise positive */

/* skip whitespace */
while ( isspace((int)(unsigned char)*nptr) )
++nptr;

c = (int)(unsigned char)*nptr++;
sign = c; /* save sign indication */
if (c == ''-'' || c == ''+'')
c = (int)(unsigned char)*nptr++; /* skip sign */

total = 0;

while (isdigit(c)) {
total = 10 * total + (c - ''0''); /* accumulate digit */
c = (int)(unsigned char)*nptr++; /* get next char */
}

if (sign == ''-'')
return -total;
else
return total; /* return result, negated if necessary */
}


/***
*int atoi(char *nptr) - Convert string to long
*
*Purpose:
* Converts ASCII string pointed to by nptr to binary.
* Overflow is not detected. Because of this, we can just use
* atol().
*
*Entry:
* nptr = ptr to string to convert
*
*Exit:
* return int value of the string
*
*Exceptions:
* None - overflow is not detected.
*
*******************************************************************************/

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