源码：微软strcpy，strcat和strcmp的实现源代码 itoa atoi

/***

*char *strcpy(dst, src) - copy one string over another

*

*Purpose:

* Copies the string src into the spot specified by

* dest; assumes enough room.

*

*Entry:

* char * dst - string over which "src" is to be copied

* const char * src - string to be copied over "dst"

*

*Exit:

* The address of "dst"

*

*Exceptions:

*******************************************************************************/

char* strcpy(char * dst, const char * src)

{

char * cp = dst;

while( *cp++ = *src++ )

; /* Copy src over dst */

return( dst );

}

/***

*char *strcat(dst, src) - concatenate (append) one string to another

*

*Purpose:

* Concatenates src onto the end of dest. Assumes enough

* space in dest.

*

*Entry:

* char *dst - string to which "src" is to be appended

* const char *src - string to be appended to the end of "dst"

*

*Exit:

* The address of "dst"

*

*Exceptions:

*

*******************************************************************************/

Char* strcat ( char * dst , const char * src )

{

char * cp = dst;

while( *cp )

cp++; /* find end of dst */

while( *cp++ = *src++ ) ; /* Copy src to end of dst */

return( dst ); /* return dst */

}

/***

*strcmp - compare two strings, returning less than, equal to, or greater than

*

*Purpose:

* STRCMP compares two strings and returns an integer

* to indicate whether the first is less than the second, the two are

* equal, or whether the first is greater than the second.

*

* Comparison is done byte by byte on an UNSIGNED basis, which is to

* say that Null (0) is less than any other character (1-255).

*

*Entry:

* const char * src - string for left-hand side of comparison

* const char * dst - string for right-hand side of comparison

*

*Exit:

* returns -1 if src < dst

* returns 0 if src == dst

* returns +1 if src > dst

*

*Exceptions:

*

*******************************************************************************/

int strcmp ( const char* src, const char* dst )

{

int ret = 0 ;

while( ! (ret = *(unsigned char *)src - *(unsigned char *)dst) && *dst)

++src, ++dst;

if ( ret < 0 )

ret = -1 ;

else if ( ret > 0 )

ret = 1 ;

return( ret );

}

itoa atoi

void itoa(int num,char str[] )
{
int sign = num,i = 0,j = 0;
char temp[11];

if(sign<0)//判断是否是一个负数
{
num = -num;
}

do
{
temp[i] = num%10 + '0';
num/=10;
i++;
}while(num>0);

if(sign<0)
{
temp[i++] = '-';//对于负数,要加以负号
}

temp[i] = '\0';

i--;

while(i>=0)//反向操作
{
str[j] = temp[i];
j++;
i--;
}

str[j] = '\0';
}

int atoi(char s[])
{
int i = 0,sum = 0,sign;    //输入的数前面可能还有空格或制表符应加判断

while(' '==s[i]||'\t'==s[i])
{
i++;
}

sign = ('-'==s[i])?-1:1;

if('-'==s[i]||'+'==s[i])
{
i++;
}

while(s[i]!='\0')
{
sum = s[i]-'0'+sum*10;
i++;
}

return sign*sum;
}

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