C - Unexpected Segmentation Fault on strtok(...)

Question

I am using strtok(...) of the library and it appears to be working fine until the end condition, where it results in a segmentation fault and program crash. The API claims that strtok(...) will output a NULL when there are no more tokens to be found, which meant, I thought, that you had to catch this NULL in order to terminate any loops that you were running using strtok(...). What do I need to do to catch this NULL to prevent my program from crashing? I imagined the NULL was allowed for use as a terminating condition.

I have prepared a SSCCE for you to observe this behavior. I need strtok(...) to work for a much larger piece of software I am writing, and I am getting the exact same segmentation behavior. The output at the command line is shown below this code vignette (yes I know you use <...> to enclose libraries, but I was having difficulty getting this post to display the code libraries). I am using gcc version 4.5.3, on a Windows 8 OS, and below shows two different flavors of how I imagine one could try to catch the NULL in a loop.

#include <stdlib.h>
#include <stdio.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <string.h>

main(){
  char* from = "12.34.56.78";
  char * ch = ".";
  char * token = strtok(from, ch);
  printf("%s\n",token);
  while(token != NULL){
    token = strtok(NULL, ch);
    printf("%s\n", token);
  }
  printf("Broke out of loop!");
  while(strcmp(token, 0) != 0){
    printf("%s\n",token);
    token = strtok(NULL, ch);
  }
}

############ OUTPUT: ############

$ ./test
12
34
56
78
Segmentation fault (core dumped)

Answer 1

strtok modifies its first argument. You are passing it a string from read-only memory, and the segfault occurs when strtok tries to change it. Try changing from:

char* from = "12.34.56.78";

to

char from[] = "12.34.56.78";