What's the difference between a string and a user entered string in C

Question

I'm using with a smaller piece of code to test functionality for a larger (beginner) program, but I don't understand the difference between two strings.

I found and used:

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

int main()
{

char *string, *found;

string = strdup ("1/2/3");
printf("Orig: '%s'\n",string);

while ((found = strsep(&string,"/")) != NULL )
  printf ("%s\n",found);

return (0);
}

and this print the tokens one at a time.

Then when I try and move to a user entered string:

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

int main()
{
  char string[13],
  char *found, *cp = string;

  fprintf(stderr, "\nEnter string: ");
  scanf("%12s",string);
  printf("Original string: '%s'\n",string);

  while((found =  strsep(&cp,"/,-")) != NULL )
    printf("%s\n",found);

  return(0);
}

I get a seg fault. I understand the basics of pointers, arrays and strings, but clearly I'm missing something, and would love for someone to tell me what it is!

Also - if I change printf("%s\n",found); to printf("%i\n",found); I get some junk integers returned, but always the correct amount, e.g. If I enter 1/2/3 I get three lines of integers, 1111/2222 I get two lines.

Thanks!

-Edit- There was an adittional problem with strsep, detailed here. Thanks all.

Answer 1

In the first piece of code, string is assigned the return value of strdup, which allocates space for the string to duplicate and returns a pointer to that allocated space.

In the second piece of code, string uninitialized when it is passed to scanf, so scanf is reading the invalid value in that pointer and attempting to dereference it. This invokes undefined behavior which in this case manifests as a crash.

You need to set aside space for the user's string. A simple way to do this is to create an array of a given size:

char string[80];

Then tell scanf how many characters it can read in:

 scanf("%79s",string);