pointer substrings arrays - c

Question

Im trying to copy part of a string to another string using pointers. My resulting string starts to copy at the correct place though it doesn't stop after exceeding the count. Also the string isn't copy from the source string rather than from the result parameter
#include <stdio.h>
char *getSub(const char *orig, int start, int count, char *res);
int main(void)
{
const char orig[] = "one two three";
char res[] = "123456789012345678";
printf("%s\n",getSub(orig, 4, 3, res));
return 0;
}
char *getSub(const char *orig, int start, int count, char *res)
{
const char *sCopy = orig;
while (*orig)
{
if (start >= (orig - sCopy)) && (res-sCopy < count))
{
*res++ = *orig++;
}
else
*orig++;
}
return res;
}

Answer 1

The big mistake is that you're calculating the difference of two unrelated pointers, res - sCopy (I suppose sourceCopy is also sCopy in the real code, or the other way round). Calculating the difference of pointers is only meaningful if both pointers point into (or one past the end of) the same array. As written, whether anything gets copied at all depends on the arbitrary locations of the two arrays.
if (start >= (orig - sourceCopy)) && (res-sCopy < c))
{
*res++ = *orig++;
}
else
*orig++;
anyway, that doesn't count how many characters are copied if any are copied at all.
Another mistake is that you don't 0-terminate the copy.
A correct implementation would be
char *getSub(const char *orig, int start, int count, char *res)
{
char *from = orig, *to = res;
// check whether the starting position is within orig
for( ; start > 0; --start, ++from)
{
if (*from == 0)
{
res[0] = 0;
return res;
}
}
// copy up to count characters from from to to
for( ; count > 0 && *from; --count)
{
*to++ = *from++;
}
// 0-terminate
*to = 0;
// return start of copy, change to return to if end should be returned
return res;
}

Answer 2

There are at least two problems with your code.
res - sCopy makes no sense because they are pointing at different objects.
You haven't null-terminated the destination string.

Answer 3

#include <string.h>
char *getSub(const char *orig, int start, int count, char *res){
int i,j,len = strlen(orig), limit = start + count;
if(res == NULL) return NULL;
if(start >= len || start < 0 || orig == NULL){
*res = '\0';
return res;
}
for(j=0,i=start;i<len && i < limit;++i){
res[j++]=orig[i];
}
res[j]='\0';
return res;
}