Pointer mystery/noobish issue

Question

I am originally a Java programmer who is now struggling with C and specifically C's pointers.

The idea on my mind is to receive a string, from the user, on a command line, into a character pointer. I then want to access its individual elements. The idea is later to devise a function that will reverse the elements' order. (I want to work with anagrams in texts.)

My code is

#include <stdio.h>

char *string; 


int main(void)
{
printf("Enter a string: ");
scanf("%s\n",string);
putchar(*string);

int i;
for (i=0; i<3;i++)
{
  string--;
}
putchar(*string);

}

(Sorry, Code marking doesn't work). What I am trying to do is to have a first shot at accessing individual elements. If the string is "Santillana" and the pointer is set at the very beginning (after scanf()), the content *string ought to be an S. If unbeknownst to me the pointer should happen to be set at the '\0' after scanf(), backing up a few steps (string-- repeated) ought to produce something in the way of a character with *string. Both these putchar()'s, though, produce a Segmentation fault.

I am doing something fundamentally wrong and something fundamental has escaped me. I would be eternally grateful for any advice about my shortcomings, most of all of any tips of books/resources where these particular problems are illuminated. Two thick C books and the reference manual have proved useless as far as this.

Answer 1

On a side note, using scanf(3) is bad practice. You may want to look into fgets(3) or similar functions that avoid common pitfalls that are associated with scanf(3).

Answer 2

You haven't allocated space for the string. You'll need something like:

char string[1024];

You also should not be decrementing the variable string. If it is an array, you can't do that.

You could simply do:

putchar(string[i]);

Or you can use a pointer (to the proposed array):

char *str = string;

for (i = 0; i < 3; i++)
    str++;

putchar(*str);

But you could shorten that loop to:

str += 3;

or simply write:

putchar(*(str+3));

Etc.

---

You should check that scanf() is successful. You should limit the size of the input string to avoid buffer (stack) overflows:

if (scanf("%1023s", string) != 1)
    ...something went wrong — probably EOF without any data...

Note that %s skips leading white space, and then reads characters up to the next white space (a simple definition of 'word'). Adding the newline to the format string makes little difference. You could consider "%1023[^\n]\n" instead; that looks for up to 1023 non-newlines followed by a newline.

You should start off avoiding global variables. Sometimes, they're necessary, but not in this example.