常用CRT字符串函数源码

////////////////////memcpy/////////////////////////
void * __cdecl memcpy (
        void * dst,
        const void * src,
        size_t count
        )
{
        void * ret = dst;

        while (count--) {
                *(char *)dst = *(char *)src;
                dst = (char *)dst + 1;
                src = (char *)src + 1;
        }

        return(ret);
}
/////////////////////memset/////////////////
void * __cdecl memset (
        void *dst,
        int val,
        size_t count
        )
{
        void *start = dst;

        while (count--) {
                *(char *)dst = (char)val;
                dst = (char *)dst + 1;
        }

        return(start);
}
///////////////////////strcat//////////////////
char * __cdecl 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 */

}
///////////////////////strcpy//////////////////////
char * __cdecl strcpy(char * dst, const char * src)
{
        char * cp = dst;

        while( *cp++ = *src++ )
                ;               /* Copy src over dst */

        return( dst );
}
/////////////////////////strchr////////////////////////////
char * __cdecl strchr (
        const char * string,
        int ch
        )
{
        while (*string && *string != (char)ch)
                string++;

        if (*string == (char)ch)
                return((char *)string);
        return(NULL);
}
///////////////////////strcmp//////////////////////////
int __cdecl 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 );
}
///////////////////////strlen/////////////////////////
size_t __cdecl strlen (
        const char * str
        )
{
        const char *eos = str;

        while( *eos++ ) ;

        return( (int)(eos - str - 1) );
}
//////////////////////strncat//////////////////////////
char * __cdecl strncat (
        char * front,
        const char * back,
        size_t count
        )
{
        char *start = front;

        while (*front++)
                ;
        front--;

        while (count--)
                if (!(*front++ = *back++))
                        return(start);

        *front = '\0';
        return(start);
}
/////////////////////////strncmp//////////////////////////
int __cdecl strncmp (
        const char * first,
        const char * last,
        size_t count
        )
{
        if (!count)
                return(0);

        while (--count && *first && *first == *last)
        {
                first++;
                last++;
        }

        return( *(unsigned char *)first - *(unsigned char *)last );
}
//////////////////////////strncpy/////////////////////////////
char * __cdecl strncpy (
        char * dest,
        const char * source,
        size_t count
        )
{
        char *start = dest;

        while (count && (*dest++ = *source++))    /* copy string */
                count--;

        if (count)                              /* pad out with zeroes */
                while (--count)
                        *dest++ = '\0';

        return(start);
}
////////////////////////////strnset//////////////////////////////
char * __cdecl _strnset (
        char * string,
        int val,
        size_t count
        )
{
        char *start = string;

        while (count-- && *string)
                *string++ = (char)val;

        return(start);
}
///////////////////////////strset//////////////////////
char * __cdecl _strset (
        char * string,
        int val
        )
{
        char *start = string;

        while (*string)
                *string++ = (char)val;

        return(start);
}
////////////////////////strrchr///////////////////////
char * __cdecl strrchr (
        const char * string,
        int ch
        )
{
        char *start = (char *)string;

        while (*string++)                       /* find end of string */
                ;
                                                /* search towards front */
        while (--string != start && *string != (char)ch)
                ;

        if (*string == (char)ch)                /* char found ? */
                return( (char *)string );

        return(NULL);
}
//////////////////////////strrev////////////////////////
char * __cdecl _strrev (
        char * string
        )
{
        char *start = string;
        char *left = string;
        char ch;

        while (*string++)                 /* find end of string */
                ;
        string -= 2;

        while (left < string)
        {
                ch = *left;
                *left++ = *string;
                *string-- = ch;
        }

        return(start);
}
////////////////////////////strstr////////////////////////////
char * __cdecl strstr (
        const char * str1,
        const char * str2
        )
{
        char *cp = (char *) str1;
        char *s1, *s2;

        if ( !*str2 )
            return((char *)str1);

        while (*cp)
        {
                s1 = cp;
                s2 = (char *) str2;

                while ( *s1 && *s2 && !(*s1-*s2) )
                        s1++, s2++;

                if (!*s2)
                        return(cp);

                cp++;
        }

        return(NULL);

}