Question

I am trying reverse a string.

This is the code I tried:

#include<stdio.h>
#include<string.h>

int main(){
    char *c="I am a good boy";
    printf("\n The input string is : %s\n",c);
    printf("\n The length of the string is : %d\n",strlen(c));
    int i,j;
    char temp;
    int len=strlen(c);
    for(i=0,j=len-1;i<=j;i++,j--)
    {
            temp=c[i];
            c[i]=c[j];
            c[j]=temp;
    //printf("%c\t%c\n",*(c+i),*(c+(len-i-1)));
    }
    printf("\n reversed string is : %s\n\n",c);
}

The code outputs a Bus error : 10.

But if I rewrite the same code as:

int main(void)
{
    char *str;
    str="I am a good boy";
    int i,j;
    char temp;
    int len=strlen(str);
    char *ptr=NULL;
    ptr=malloc(sizeof(char)*(len));
    ptr=strcpy(ptr,str);
    for (i=0, j=len-1; i<=j; i++, j--)
    {
        temp=ptr[i];
        ptr[i]=ptr[j];
        ptr[j]=temp;
    }
    printf("The reverse of the string is : %s\n",ptr);
}

It works perfectly fine.

Why is the first code throwing bus error or segmentation fault?

Answer 1

The bus error occurs because in many (if not most or all modern) C compilers, string literals are allocated in read-only memory.

You are reversing the string in place. In your first code snippet you are trying to write into a string literal. Not a good idea.

In the second case, you malloc’d a string which put it on the heap. It is now safe to reverse that string in place.

ADDENDUM

To the commenter who asked about segfaults versus bus errors, that is a great question. I have seen both. Here is a bus error on a mac:

$ cat bus.c 
char* s = "abc"; int main() {s[0]='d'; return 0;}

$ gcc --version bus.c && ./a.out
i686-apple-darwin10-gcc-4.2.1 (GCC) 4.2.1 (Apple Inc. build 5659)
Copyright (C) 2007 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

Bus error

On other operating systems/compilers you may indeed get a segfault.