Why does writing to a string literal in this C program segfault?

Question

#include<stdio.h>

void main()
{
    char *p="nyks";

    p[2]='n';

    printf("%s",p);
}

This crashes with a SEGMENTATION FAULT. Can someone explain why?

Answer 1

It is undefined behavior to try to overwrite a string literal. C99 §6.4.5/6:

If the program attempts to modify such an array, the behavior is undefined.

This is restated in Appendix J.2 (undefined behavior).

If you instead do:

char p[] = "nyks";

you can allocate and initialize an automatic (stack) character array. In that case, it is perfectly fine to modify elements.

Answer 2

The standard dictates that literal strings are defined const. You cannot change it.

The compiler places the literal in a readonly memory section. You can output the assembly and observe this. If you are using GCC it is done via the -s flag. It will place the string in a .rodata section.