Question

I’m trying to write a response into a variable, and I can’t figure out how to do it.

This doesn’t work – screws up memory, but no protection errors:

for (int i = 0; i < 20; i++) {
    list[i] = 'a';
}

Same with this – memory screwed up:

for (int i = 0; i < 20; i++) {
    *(((int*)(list))+i) = 'a';
}
//I don't think this is a string issues as this doesn't help:
//*(((int*)(list))+20) = '\0';

This causes a bus error:

for (int i = 0; i < 20; i++) {
    *list[i] = 'a';
}

This works as desired:

*list = "aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa";

What am I doing wrong?

P.S. list is char**.

Answer 1

In C, a pointer can be used to represent an array, and a single string is an array of char, or in other words, a char *. This means that a char ** is an array of strings. So if you want to put characters into the first string (assuming that memory has already been allocated to it), you should use list[0][i] = 'a'; in the first loop – i.e., put 'a' into position i of the 0th string.

Another way of interpreting a char ** (which is the one I suspect is the one you’re supposed to use) is that it is a pointer that points to a pointer that points to an array of char. In that case, you can use the “outer” pointer to modify what the inner pointer points to; this can be used to first allocate the string and then write to it:

*list = malloc(21); // Allocate 21 bytes and make the pointer that 'list' points to refer to that memory
for (int i = 0; i < 20; i++) {
    (*list)[i] = 'a';
}
(*list)[20] = '\0'; // Also, you need the null terminator at the end of the string

In memory, this looks like this:

list ---> (another pointer) ---> |a|a|a|a|a|a|...|a|a|\0|