what difference between while(*p){p++;} ,while (*++p){;} and while(*p++){;}?

Question

It\'s about strcat function.

while (*p)
    p++;

and

while (*++p)
    ;

both works, but

Answer 1

Both

while (*p) p++;

and

while (*++p) ;

will advance p to point to the 0 terminator in a string, whereas

while (*p++) ;

will advance p to point one character past the 0 terminator.

To see why, let's assume the following characters in memory:

Address         0x00 0x01 0x02 0x03
-------         ---- ---- ---- ----
0x8000           'a'  'b'  'c'   0
0x8004           ...

Assume that p starts at address 0x8000. Here's how the first loop plays out:

1.  *p = 'a'
2.  p = 0x8001
3.  *p = 'b'
4.  p = 0x8002
5.  *p = 'c'
6.  p = 0x8003
7.  *p = 0
8.  end loop

Here's how the second loop plays out:

1.  p = 0x8001
2.  *p = 'b'
3.  p = 0x8002
4.  *p = 'c'
5.  p = 0x8003
6.  *p = 0
7.  end loop

And here's the last one:

1.  *p = 'a'
2.  p = 0x8001
3.  *p = 'b'
4.  p = 0x8002
5.  *p = 'c'
6.  p = 0x8003
7.  *p = 0;
8.  p = 0x8004
9.  end loop

In the last version, evaluating *p++ advances the pointer even if the value of *p is 0.