How to access struct member via pointer to pointer

Question

typedef struct {
    int a;
} stu, *pstdu;

void func(stu **pst);

int main() {
    stu *pst;
    pst = (stu*)malloc(sizeof(stu));
    pst->a = 10;
    func(&pst);
}

void func(stu **pstu) {
    /* how to access the variable a */
}

1) Want to access the structure variable a by passing the pointer address, in above function func.

2) In following case how it will behave example:

typedef struct {
    int a;
} stu, *pstdu;

void func(pstdu *pst);

int main() {
    stu *pst;
    pst = (stu*)malloc(sizeof(stu));
    pst->a = 10;
    func(&pst);
}

void func(pstdu *pstu) {
   /* how to access the variable a */
}

Answer 1

You need to dereference the first pointer, then use the pointer-to-member operator:

(*pstu)->a

The parenthesis are required because the pointer-to-member operator has higher precedence than the dereference operator.

This is the same for both cases because stu ** and pstdu * represent the same type. As mentioned in the comments, it's considered bad practice to have a pointer in a typedef because it can hide that fact that a pointer is in use and can become confusing.