C segmentation fault xor char array

Question

I want to xor a certain value on all of my chars in a char array. I have some code that works all fine except for the xor line:

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

void obfuscate(FILE *file, char *input, char *variableName)
{
    int i;
    int size = (int)strlen(input);
    char a = 65;
    fprintf(file, "%s: %s -> ", variableName, input);

    for(i = 0; i < size; i++)
    {
        input[i] = (char)(input[i] ^ a);//I am causing a segmentation fault
        fprintf(file, "0x%x, ", input[i]);
    }
    return;
}

int main()
{
    char *a = "MyString";
    FILE *file = fopen("out.txt", "w");
    obfuscate(file, a, "a");
    fclose(file);
}

I use the normal gcc compiler with gcc in.c -o out.o on a linux machine.

If I comment the line with the xor operation it works just fine. Any help is highly appreciated, my own research did not result in a solution.

Answer 1

Try below solution with dynamic memory.
In char *a = "MyString"; char *in = malloc(sizeof(a));
a is pointing to code section we can't alter it. & in is pointing to heap section, so we can modify it any no.of times.

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

void obfuscate(FILE *file, char *input, char *variableName)
{
    int i;
    int size = (int)strlen(input);
    char a = 65;
    fprintf(file, "%s: %s -> ", variableName, input);

    for(i = 0; i < size; i++)
    {
        input[i] = (char)(input[i] ^ a);//I am causing a segmentation fault
        fprintf(file, "0x%x, ", input[i]);
    }
    return;
}

int main()
{
    char *a = "MyString";
    char *in = malloc(sizeof(a));
    strcpy(in, a);
    FILE *file = fopen("out.txt", "w");
    obfuscate(file, in, "a");
    fclose(file);
    free(in);
}