strcpy, strcat, strcmp, strlen,memcpy

/***
*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)
{
assert((dst != NULL) && (src != NULL));
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;
}

size_t strlen(const char * str){ 
const char *cp =  str;
while  (*cp++ );
return (cp - str - 1 );
}

void* memcpy(void* sdt, const void* src, unsigned int len)
{
register char * d;
register char * s;
if (0 == len)
{
return dst;
}
if (dst > src)
{
d = (char *)dst + len - 1;
s = (char *)src + len - 1;
while (len >= 4)
{
*d-- = *s--;
*d-- = *s--;
*d-- = *s--;
*d-- = *s--;
len -= 4;
}
while (len--)
{
*d-- = *s--;
}
}
else if (dst < src)
{
d = (char *)dst;
s = (char *)src;
while (len >= 4)
{
*d++ = *s++;
*d++ = *s++;
*d++ = *s++;
*d++ = *s++;
len -= 4;
}
while (len--)
{
*d++ = *s++;
}
}
return dst;
}