strtok is causing segmentation fault

The following program is supposed to convert the string into "pig latin." "This is a test string" is supposed to become "hisTay siay aay esttay tringsay." Basically move the first letter to the end and then add "ay." For some reason my program is producing a segmentation fault. Specifically the strtok right after the while condition is what's causing it. I'm not sure why. Is it because I'm passing NULL to the strtok?

Code:

---

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

int main()
{
  int i;
  char *tokenPtr;
  char string[] = "This is a test string";

  tokenPtr = strtok( string, " " );

  while ( *tokenPtr != NULL ) {
    tokenPtr = strtok( NULL, " " );
    for ( i = 1; (*(tokenPtr + i) != '\0') && (*(tokenPtr + i) != NULL); ++i ) {
      printf( "%c", *(tokenPtr + i) );

      //    printf( "%cay ", *tokenPtr );
    }
    printf( "%cay ", *tokenPtr );
  }

  printf( "\n" );

  return 0;
}

---

You definately don't enter a for loop if tokenPtr is returned NULL...
Fix:

Code:

---

while( (tokenPtr = strtok( NULL, " ")) != NULL)
{
    ....
}

---

Also in the for loop the second condition: (*(tokenPtr + i) != NULL) is not needed and not making sense anyway. A character should not be compared to a pointer, as NULL is...

And 'i' should start at 0 I suppose.

To clarify the strtok statement right after the while condition is to move the pointer on to the next word. Without it, the program prints the first word infinitely.

while ( *tokenPtr != NULL )
Should be
while ( tokenPtr != NULL )

Putting
tokenPtr = strtok( NULL, " " );
in the beginning of the loop - you skip the first word

(*(tokenPtr + i) != NULL) - has no sence left is a char - right is pointer - don't you pay attention to warnings?

Not if you make the loop control statement assign tokenPtr as well as testing whether to enter the loop. I didn't tell you to remove the call to strtok, but place it in a better position.

Quote:

---

Originally Posted by vart

Putting
tokenPtr = strtok( NULL, " " );
in the beginning of the loop - you skip the first word

---

Yes you are right silly me...
It should go better like:

Code:

---

tokenPtr = strtok(string, " ");
if( tokenPtr )
{
    do
    {
        ....
    }
    while( (tokenPtr = strtok(NULL, " ") != NULL);
}

---

To handle any case and act on all tokens.

The do while statement worked well. I don't see why though. Nothing was fundamentally changed as far as I can tell.

Quote:

---

Originally Posted by yougene

The do while statement worked well. I don't see why though.

---

And what exactly?

I guess the better way to put it is I don't see why my original program didn't work. Not including the variable / pointer comparison.

You have *tokenPtr in a loop condition, when tokenPtr is a NULL pointer - dereferencing it causes the crash

Quote:

---

Originally Posted by yougene

I guess the better way to put it is I don't see why my original program didn't work. Not including the variable / pointer comparison.

---

When strtok was done with tokens and returned NULL, your original program happily entered a for loop, and dereferenced the NULL pointer, which caused a segmentation fault.

Thanks, I'm going to go digest this.