Why am I getting segmantation fault (core dumped)

You have not allocate any memory for this string, the quick and dirty fix would be:

#include <stdio.h>
int main()
{
    char s[255],i;
    printf("Enter a string: ");
    scanf("%s",s);
    for(i=0; s[i]!='\0'; ++i);
    printf("Length of string: %d \n",i);
    return 0;
}

char *s;

doesn't allocate any memory for the scanf read - it just declares a pointer - so you need to use (for example)

s = malloc(100);

to get enough space for a 99 character string (plus the terminating nul) iirc

the issue then is, how do you ensure your user doesn't type more than 99 characters ? - and the answer is, using scanf like this, you cant - its 'dangerous'

[edit] I have seen talk of a specifier for scanf, that alludes to using "%ms" gets scanf to allocate the memory for you, but, I cannot attest to this form of usage [/edit]

[edit] I seem to remember that you could also a length specifier with scanf, but, you'd still have to use the malloc or declare s as an array of char, and, it gets messy when the user types over the number of characters you've allowed for [edit]

- you should use something safer, like fgets() instead

'g'