Use strtok in a for loop

Question

To loop over a string str I used:

for (tok = strtok(str, ";"); tok && *tok; tok = strtok(NULL, ";"))  
{  
   //do stuff  
}

I would like to understand how this loop works. It seems to me:

(1) tok = strtok(str, ";"); //initialization of tok with the first token in str    
(2) tok = strtok(NULL, ";"); // go to the next token in str? how does this work? 
(3) tok && *tok;  //this stops the loop when tok =NULL or *tok=NULL 

I would appreciate your help!

Answer 1

Here's a sample strtok implementation: http://bxr.su/o/lib/libc/string/strtok.c#strtok

As you see in the code, it uses a static character pointer internally (pretty much every version I've seen store a pointer, either as a global variable or as a static variable as in the case above). This version calls the reentrant strtok_r (and the side effect of the line if (s == NULL && (s = *last) == NULL) is to use the last pointer if NULL is passed)

Answer 2

(2) tok = strtok(NULL, ";"); // go to the next token in str? how does this work?

That's exactly how strtok() works. By sending NULL as the first parameter, you signal that strtok() should continue with the string which was sent to it during the last call. If you want to know the exact implementation details, you will need to look at the source code for strtok(). Most likely it uses a static local variable.