strtok()

[CeeCee]

I was reading The C Programming Language by Kernighan and Ritchie. I noticed a reference to a function in the standard library header string.h that I might be able to find a lot of uses for in my programs, strtok().
Could someone show me a bit of code describing how this function works, as my book doesn't say how to use it, only what the function declaration is and a description of what it does.

-CeeCee

[quzah]

It's amazing what a board search for the word strtok can produce...

Quzah.

[CeeCee]

Okay, point made. But I still have a few questions

#include <stdio.h>
#include <string.h>

int main ( ) {
  char test_msg[]="This is a test message";
  char *p;
  char *delim = " ";
  char *words[20];
  int i = 0, j;

  p = strtok(test_msg,delim);
  while ( p ) {
    words[i] = p;
    p = strtok(NULL,delim);
    i++;
  } 

  printf ( "%d\n", i );
  for ( j = 0 ; j < i ; j++ ){
    printf("'%s'\n", words[j]);
  }
}

p = strtok(NULL,delim);
What does this do and why? I know that it assigns a token to the char pointer p, and that spaces don't count as tokens, but what is NULL there for?

[Fordy]

I think the first call to strtok returns a pointer to the first word ("This" in this case). The subsequent calls to NULL force the pointer up the string to the next token without reassigning it to anywhere else.

I think anyway.....

[quzah]

> I think the first call to strtok returns a pointer to the first word
> ("This" in this case). The subsequent calls to NULL force the
> pointer up the string to the next token without reassigning it to
> anywhere else.

Yup. Good explaination. For another description, try here.

I like the comment:

"Never use this function."

Quzah.

[CeeCee]

Never use this function? I find that a bit funny because if it weren't meant to be used, why have it? Besides, if I wanted to extract tokens from a string then the last thing I would want is to leave the string unchanged. Even if for some strange reason I did I could copy the string and change the copy.

Is that just the opinion of the person who wrote that?

[Stoned_Coder]

All you have to remember about strtok is that if you need to keep the original data then tokenise a copy. The modification of the original data is the only downside to strtok.