What happens when you assign a character string to a char* instead of writing element by element?

Question

I have a question about strings that hopefully someone can help with. I realise that doing something like:

char* word = malloc(4);
word = "THE";

is not really correct (as word is really a pointer to the first element as I understand it) and so we are trying to assign a character string to a pointer. However, in code in partially works and I would like to know what is going on. Below is some code to illustrate what I mean (with the standard way of writing one character at a time to the array as a comparison).

#include <stdio.h>
#include <stdlib.h>


int main(void)
{  
    char* word = malloc(4);
    word[0] = 'T';
    word[1] = 'H';
    word[2] = 'E';
    word[3] = '\0';

    // printing out where string is stored and the string
    printf("POINTER: %p\n", word);
    printf("STRING: %s\n", word);

    /* 
     * altering an element and reprinting. The string prints out
     * altered and memory location prints out unaltered as expected 
     * in my output 
     */ 
    word[0] = '!';
    printf("POINTER: %p\n", word);
    printf("STRING: %s\n", word);

    /*
     * trying the dodgy way. In my output the pointer is altered and
     * the string prints out fine. Why?
     */
    word = "CAT";
    printf("POINTER: %p\n", word);
    printf("STRING: %s\n", word);

    /*
     * it only partially works though as altering a single element
     * leads to a segmentation fault
     */
    word[0] = '!';
    printf("POINTER: %p\n", word);
    printf("STRING: %s\n", word);

    free(word);


}

So clearly a string is being stored somewhere but in a different location from what malloc() gave us and then after we cannot alter elements (we get a segmentation fault) only read them. If anyone can explain to me what exactly is happening I would be very appreciative!

Answer 1

I'll let you visualize this for yourself. Go here http://www.pythontutor.com/c.html#mode=edit and paste your code, then hit "visualize execution".

You'll see graphically what's happening in memory.

EDIT: I'm sorry, the server seems to be down, so I'll try to use Python to show you what the site would have shown:

And finally I'll direct you to a question regarding strings stored in read-only section in memory, like char* a = "AAAAAA":
https://stackoverflow.com/questions/164194/why-do-i-get-a-segmentation-fault-when-writing-to-a-string-initialized-with-cha