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

// move p forward as long as it points to a non-0 character,
// leave it pointing to a 0 character.
while (*p) p++;  

// move p forward until it points to a 0 character, skipping the first 
// character before you start
while (*++p);

// move p forward until it points one past a 0 character
while (*p++);

So given that (1) "works": (2) also works if p initially points to a non-empty string. (3) doesn't work at all because p ends up pointing to a different place.

*++p increments p and then evaluates to whatever p now points to. *p++ evaluates to whatever p initially points to, but also increments p. Hence (2) and (3) are different.

(1) and (3) are different because (3) executes the p++, then it decides whether to stop. (1) first looks at *p to decide whether to stop, and if not stopping it executes the p++.