strchr causing segmentation fault in C

Question

I am having problem with a C program. I know that strchr() is causing the problem, and it is returning

Segmentation Fault

My code is as follows:

char *pointer;
pointer = strchr(string, character);

I don't know why I'm getting the error message. One thing I can guess is that my 'string' sometimes doesn't contain the character. Could that be the reason why?

How can I stop it, as I don't have control over the input string?

Full code

int index(const char *string, char character)
{
    const char *pointer;
    pointer = strchr(string, character);

    if(pointer)
        return (pointer - string);
    else
        return -1;
}

can you show the rest of your code ? you can misused strchr .... — Mzf, Mar 18 '14 at 15:55
This function looks OK, the problem could be in the caller. Maybe it didn't properly allocate the string. — Barmar, Mar 18 '14 at 16:00
@Barmar forgot to mention that string could also be null sometimes, is that why then? — Jordan, Mar 18 '14 at 16:02
The only thing that's completely obvious about what you are doing wrong is not using a debugger. — Hans Passant, Mar 18 '14 at 16:03
If `string` is null, then `strchr` may not like it. You probably want to check if `string` is NULL explicitly in this function first and return `-1` or something if it is. If `string` is a valid pointer to a NULL, then that's an empty string and your code shown for `index` would be fine for that case. — lurker, Mar 18 '14 at 16:03
@Jordan Yes, that's why. `strchr()` doesn't work when the string is null. — Barmar, Mar 18 '14 at 16:06
Also the result of substracting two pointers is `ptrdiff_t`. — alk, Mar 18 '14 at 17:30
@alk, it differs from what documentation? From what I know, `null` is never _defined_ in standard C, only `NULL`. — FaranAiki, May 06 '22 at 00:47

score 0 · Answer 1 · answered Mar 18 '14 at 16:03

First , try to print the input for your method.

From here

The strchr function searches string for the first occurrence of c. The null character terminating string is included in the search.

Are you sure that your string is legal? (meaning ending with null)

I think this what may cause your problem

Alberto Coletta · Answer 2 · 2014-03-18T16:19:51.390

The strchr(char *string, int c) function returns a pointer to the first occurrence of character c in string or a NULL pointer if no matching character is found.

Since the null character terminating string is included in the search, you have to be sure that your string is null-terminated, otherwise the search goes out of bounds.

If you'd like to print the string or performing other operations with the result, check first the pointer; in your specific case:

if(pointer != NULL)
   printf(...);
   [...]

score 0 · Answer 3 · answered Mar 18 '14 at 18:21

To write save code always perform input validation:

Either the hard way (if NULL is not supposed to be passed in):

ssize_t index(const char * string, const char character)
{
  assert(NULL != string);

  const char * pointer = strchr(string, character);

  if (pointer)
  {
    return (pointer - string);
  }
  else
  {
    return -1;
  }
}

Or the smooth way (silently treat NULL the same way as ""):

ssize_t index(const char * string, const char character)
{
  const char * pointer = NULL;

  if (string)
  {
    pointer = strchr(string, character);
  }

  if (pointer)
  {
    return (pointer - string);
  }
  else
  {
    return -1;
  }
}

strchr causing segmentation fault in C

3 Answers3