How to reassign void pointer pointing to a string to a new string in C

Question

Let's say I have this

char *value = "old";

struct ConfigOpt {
    void *value;
};

struct ConfigOpt options[] = {
    {value}
};

options[0].value = "new";

printf("%s", value);

However, it prints out "old" instead of "new." Is there a way to set the variable to a new string by accessing it through a void pointer? I know I can pre-allocate the string, but I'd prefer to do it this way if possible.

Thanks,

Answer 1

Nothing in your code assigns value after the initializations. So obviously it will keep having the initialized value, i.e. a pointer to the string literal "old".

If your goal really is to change value via the array options, you can do something like:

struct ConfigOpt {
    void *value;
};

int main(void) {
    char *value = "old";
    printf("%s", value);

    struct ConfigOpt options[] = {
        {&value}
    };
    
    *(char**)options[0].value = "new";

    printf("%s", value);
    return 0;
}

This will print

oldnew

Explanation of changes:

        {&value}
         ^
         Address of

The void-pointer in options[0] must be initialized to the address of value, i.e. a pointer to value. Otherwise, you can't change the actual value of value.

    *(char**)options[0].value = "new"; 
    | ^^^^^^
    | Cast back to the original type
    ^
    Dereference of the pointer to assign a new value

Since options[0].value is a void pointer, you need to do a cast back to the original pointer type before you can use it for accessing the original object. The original object, i.e. value has type char*. Consequently, address of the original object has type char** which you need to use for the cast. Then when you want to access the original object, you need to dereference the char** pointer by adding a * in front - like you always do when accessing pointed-to objects.