Question

I have this custom string copy function and I’ve noticed that the *dest argument changes upon dereferencing it and modifying its contents:

char *copyArray(char *dest, char *src)
{
    char *a = dest;

    while (*dest++ = *src++)
        ;   
    char *b = dest;

    if ( (a-b) != 0)  
        printf("Dest pointer has changed\n");

    return dest;                                                                        
}

And if you consider the following code:

int main()
{
    char name [] = "Hello, there!";

    char *new = copyArray(name, "bye");

    printf("New is '%s', name is '%s'\n", new, name); 

    return 0;
}

I get the following output:

Dest pointer has changed
New is 'o, there!', name is 'bye'

Why does this happen?

My intent was for *new to point to the same location name[] points to, but obviously as the address changed, it points to a different place.

I feel that what’s happened is that once the copying ended, C moved the original contents of the destination string minus the number of characters changed (4 for “bye”) to a new address and assigned that to *dest.

Is this what’s really happening? And could somebody please explain to me why?

This is not really work related, I’m just trying to understand better how pointers behave.

Thank you very much for your replies!

Answer 1

The problem occurs here

while (*dest++ = *src++)
    ;   

which in plain english means copy the thing pointed to by src into the thing pointed to by dest, and after that increment both src and dest (that is, advance both pointers to point to the next thing).

Use a copy of the pointer if you want to avoid changing it – eg the a variable you already created, and return that one.