strtok()

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

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

Quzah.

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

Code:

#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?

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.....

> 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.

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?

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.