In C, strtok causing my program to crash even when compiling example code from different sources

Question

using the code demonstrated below, the program crashes. It is a code I copy-pasted from this source and haven't modified it at all, yet strtok seems to cause the program to crash.

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

int main()
{
   char str[80] = "This is - www.tutorialspoint.com - website";
   const char s[2] = "-";
   char *token;

   /* get the first token */
   token = strtok(str, s);

   /* walk through other tokens */
   while( token != NULL ) 
   {
      printf( " %s\n", token );

      token = strtok(NULL, s);
   }

   return(0);
}

I can't seem to find the cause for this. I've tried looking for a source to the strtok function, and came across this:

char * __cdecl strtok(char *s1, const char *delimit)
{
    static char *lastToken = NULL; /* UNSAFE SHARED STATE! */
    char *tmp;

    /* Skip leading delimiters if new string. */
    if ( s1 == NULL ) {
        s1 = lastToken;
        if (s1 == NULL)         /* End of story? */
            return NULL;
    } else {
        s1 += strspn(s1, delimit);
    }

    /* Find end of segment */
    tmp = strpbrk(s1, delimit);
    if (tmp) {
        /* Found another delimiter, split string and save state. */
        *tmp = '\0'; //->This seems to be the line at fault<-
        lastToken = tmp + 1;
    } else {
        /* Last segment, remember that. */
        lastToken = NULL;
    }

    return s1;
}

Making a local copy and using it instead in my code resulted with the same issue, and after using a few prints it seems that after the first call to the function, it crashes, specifically, strpbrk returns the correct value, which can be accessed (By using a printf to it's return value), but when assigned the value \0 it causes the program to crash (apparent by being unable to print any message entered after it).

That's as far as I can get, does anybody have any idea what's going on?

Thanks in advance for your replies.

Your code looks correct to me. There must be a bug in your compiler or its installation of associated libraries etc. — M.M, May 08 '16 at 14:31
(except for leading delimiters, which is questionable) The second version doesn't treat multiple consecutive delimiters as one (as strtok() does). BTW: strtok() is a terrible function. Better avoid it. and don't try to emultate it. — wildplasser, May 08 '16 at 14:37
Could the issue be that you are modifying a string literal? Does str[0]=0 crash as well? — mvds, May 08 '16 at 15:24

score 2 · Accepted Answer · answered May 08 '16 at 14:34

Please have a look that the code is working. The problem might be a local error in your local environment and configuration. You can try with another computer or follow the link and see that you get the expected output.

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

int main()
{
   char str[80] = "This is - www.tutorialspoint.com - website";
   const char s[2] = "-";
   char *token;

   /* get the first token */
   token = strtok(str, s);

   /* walk through other tokens */
   while( token != NULL ) 
   {
      printf( " %s\n", token );

      token = strtok(NULL, s);
   }

   return(0);
}

If you run the compiled program with an analysis program such as Valgrind you will notice that there are either no errors reported from the code or error where you can debug the crash and get a nice error message where in the code the program crashes.

In C, strtok causing my program to crash even when compiling example code from different sources

1 Answers1