How to duplicate strncat (original) - c++

Question

My stringNAdd function will duplicate strncat (original). I cannot accept arrays as parameters, but pointers. I wonder if my code right?
Here is the fixed code:
#include <string>
#include <iostream>
using namespace std;
char *stringNAdd(char str1[], char str2[],size_t num);
int main()
{
char dest[50] = "Using strncat function,";
char src[50] = " this part is added and this is ignored";
cout<< strncat(dest, src, 20) << endl;
cout << stringNAdd(dest, src, 20) << endl;
cin.get();
return 0;
}
char *stringNAdd(char str1[], char str2[],size_t num){
size_t str1_len = strlen(str1);
size_t i;
for (i=0; i < num && str2[i] != '\0'; i++)
i==num;
str1[str1_len+i] = str2[i];
str1[str1_len+i] = '\0';
return str1;
}
Output:
Using strncat function, this part is added
Using strncat function, this part is added

Answer 1

The problem is that you don't do the test of both functions in the same conditions: once you've executed strncat(), the dest already contains the longer concatenated version.
The second problem is that dest was already enlarged by 15 chars. It has therefore an initial length of 38 chars + the null terminator before calling stringNAdd(). Adding 15 more chars result in a string of 53 chars plus a null terminator, which is 4 chars longer than your array. So you'll get a buffer overflow, hence memory corruption and undefined behavior.
But all this is related to the testing conditions: your clone works fine.
Suggestions:
Run your functions in distinct blocks, and define your testing variables local to that block:
{
char dest[50] = "Using strncat function,";
char src[50] = " this part is added and this is ignored";
cout<< strncat(dest, src, 15) << endl;
cout << strlen(dest)<<endl;
}
{
char dest[50] = "Using strncat function,";
char src[50] = " this part is added and this is ignored";
cout << stringNAdd(dest, src, 15) << endl;
}
Think of a more secure version of your function, in which you would have an additional argument with the total length of the destination array to prevent these errors. This would increase the security of your code. By the way, this is what Microsoft does with strncat_s().
Finally, you could ask your teacher why he/she still lets you work with cstrings, when there are the so much more convenient and secure std::string, and that he certainly could find more modern exercises with the same pedagogical benefits.

Answer 2

Here is equivalent based on https://opensource.apple.com/source/Libc/Libc-167/gen.subproj/i386.subproj/strncat.c
#include <iostream>
char *strnadd(char *dst, const char *src, size_t n)
{
// abort if source is empty
if (n != 0)
{
// copy pointers
char *d = dst;
const char *s = src;
// find end of destination str
while (*d != 0)
d++;
// start copying chars from source str to the end of destination str
// until either source string ends or number of chars copied
// destination string has to be long enough to accommodate source
do
{
if ((*d = *s++) == 0)
break;
d++;
}
while (--n != 0);
// add null termination
*d = 0;
}
// return the resulting string
return dst;
}
int main()
{
char strCat[50];
char strAdd[50];
strcpy(strCat, "string1");
strcpy(strAdd, "string1");
char const *str2 = "string2";
std::cout << strncat(strCat, str2, 6) << std::endl;
std::cout << strnadd(strAdd, str2, 6) << std::endl;
return 0;
}
Prints:
string1string
string1string