Hello,
I was trying to understand more on strtok_r() function with following code:
The problem is with the loop at line: string1 = tmp1 where I want move the pointer to the next token
Explanation and correction is greatly appreciated.
Make sure to follow proper usage of this function, where on the first call the 1st argument should point to the input string, then on the following calls it should be NULL.
Thanks Corona!
My last thread related to strtok() function in this forum triggered this one , that I had hard time to understand the pointer moving along the line:
Quote:
tok = strtok(NULL, " \r\n\t"); // NULL means 'continue from last token'.
It doesn't work "because" of the extra pointer. You found a way around the syntax error, it's valid to assign a pointer to a pointer, but I can't imagine it's actually giving the intended results.
Every time you give strtok_r a string as the first parameter, you are telling it to start over. Keep giving it the same string, and you will keep getting the same results. Give it NULL instead, and it will advance.
That is really not how you're supposed to be using it. I repeat, this is how it's supposed to work:
Last edited by Corona688; 01-16-2014 at 04:13 PM..
Yes, your code worked perfectly and much simpler. The only thing bugs me is the NULL for the second strtok_r():
Quote:
word=strtok_r(string1, sep, &tmp1);
word=strtok_r(NULL, sep, &tmp1); // NULL means 'continue from last token'.
Swallowing it without digestion bothers me a lot. I was thinking this way according to the explanation:
Quote:
char *strtok_r(char *restrict s, const char *restrict sep, char **restrict lasts);
In the first call to strtok_r(), s points to a null-terminated string, sep to a null-terminated string of separator characters, and the value pointed to by lasts is ignored. The strtok_r() function shall return a pointer to the first character of the first token, write a null character into s immediately following the returned token, and update the pointer to which lasts points. In subsequent calls, s is a NULL pointer and lasts shall be unchanged from the previous call so that subsequent calls shall move through the string s, returning successive tokens until no tokens remain. The separator string sep may be different from call to call. When no token remains in s, a NULL pointer shall be returned.
:
But not sure, especially without the new pointer ptr. It seems I have to just remember and use it in your way. Trying more to get the prototype of strtok_r(), and to understand reentrancy, a related concept new to me. I hate C but I love it more.
Ok, MUST follow the usage of the function as migurus said:
Yes, your code worked perfectly and much simpler. The only thing bugs me is the NULL for the second strtok_r()
From man strtok:
strtok() wants a NULL because somewhere inside, there is an if(string==NULL) { // Use the string we had last time and literally no other reason. It's just a weird old library call that insists you use it in a very particular way.
There is no point worrying what the contents of tmp1 are either. It might not even be the same in a different libc.
I think your confusion is related to the concept of re-entrancy. A re-entrant function, if you call it twice with the exact same parameters, would do the exact same thing. strtok() violates this, because it remembers what string you gave it last time you called it.
Imagine you're breaking up the string "a:b:c|d:e:f|g:h:i" with strtok upon "|". It gives you "a:b:c", and you call strtok() again with ":" to break it into "a", "b", "c". In doing so, strtok() will forget the original string!
This is perfectly okay with strtok_r since you can give them different tmp variables. Those variables, not the function itself, will remember where it was last, so there is no conflict.
Last edited by Corona688; 01-16-2014 at 05:23 PM..
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