How to work with pointer to pointer to structure in C?

maybe (*foo)->member = 1 (if it's dynamically allocated)

Due to operator precedence, you need to put parentheses around this:

(*foo)->member = 1;

You can use a temp variable to improve readability. For example:

Ttype *temp = *foo;
temp->member = 1;

If you have control of this and allowed to use C++, the better way is to use reference. For example:

void changeMember(Ttype *&foo) {
   foo->member = 1;
}

Try

(*foo)->member = 1;

You need to explicitly use the * first. Otherwise it's an attempt to dereference member.