Problem with strtok at the end of a string.

Hi everyone,
I am trying to write a little program that takes a line of comma-separated data from stdin, and insert a new entry inside it. Unfortunately it always segfaults when reading the last entry and I can't figure out why. Here's my code; can anyone help? I'm pretty sure the line I've marked with ***** is the culprit.

Code:

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

int main(int argc, char *argv[]) {
   char line[1024], tokn[1024], outline[1024];
   int i, j=1;

   i = atoi( argv[2] );
   strcpy( outline, "" );
   fgets( line, 1024, stdin );
   printf( "%s,%d,%d\n", line, strlen(line), line[strlen(line)]=='\0' );             //make sure "line" is a proper, \0 terminated string, which it is
   if( i<=1 ) { 
      printf( "%s,%s", argv[1], line );    //Case when we have to put the desired string at the front
      return 0;
   }

   strcpy( tokn, strtok( line, ",\0" ) );
   while( 1 ) {                             
      if( i==j ) {strcat( outline, argv[1] );                     //have to put the string here
         strcat( outline, "," );                                  //append comma
      }
      strcat( outline, tokn );                                    //output token from original string
      strcat( outline, ","  );                                    //append comma
      strcpy( tokn, strtok( NULL, ",\0" ) );                      //get next token *****
      if(!tokn) break;                                            //break from while loop if end of string is reached
      printf( "%s\n", outline );
      fflush(stdout);                                             
      j++;
   }
   
   if( i>j ) {
      strcat( outline, argv[1] );   //Case when we have to put the desired string at the end
      strcat( outline, "," );
   } 
   outline[strlen(outline)-1] = '\0';         //remove trailing comma
   printf( "%s\n", outline );
   
   return 0;
}

strtok can, and will, return NULL when it is out of data; and you can't use NULL inside a strcpy. Do it in steps. Especially since you can't ever leave the loop: tokn is always a non-NULL address, so !tokn is never true.

Ah, that makes sense. Fixed it. Thanks.