Segfault when trying to index pointer to pointers in function

Question

I\'m trying to do something with an array (malloc-ed), namely arr of a custom struct. The array is passed by reference to a function. I get a segfault whenever I tr

Answer 1

Your problem is the line

(*arr[1])->i = 3;

Because the subscripting operator's evaluation precedes the dereferencing's evaluation it is equivalent to the following:

(*(arr[1]))->i = 3;

This is obviously wrong. You need

(*arr)[1]->i = 3;

therefore.

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Notes:

• do not cast the result of malloc
• add #include <stdlib.h> to resolve the warning
• adding an extra level of indirection (foo*** pointing to foo**) is unnecessary; just copy by value

• (in addition to the upper note) a good old 1D array should actually be sufficient in your case

• call free after malloc

Answer 2

The warning you get is because you forgot to #include <stdlib.h>, so malloc is not declared, so the compiler assumes it should return int. This can lead to all kinds of fun problems. (And you should remove those casts.)

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The other problem is in this line: (*arr[1])->i = 3;

Postfix operators (like []) bind tighter than prefix operators (like *), so *arr[1] parses as *(arr[1]).

You can write (*arr)[1]->i instead to fix this, but as it turns out, your function never actually modifies *arr, so there's no reason to pass arr (the other arr, the one in main)'s address to it. Just do this:

void doSomething(foo **arr)
{
  arr[1]->i = 3;
}

and call it as doSomething(arr).