How do I access a pointer member inside a structure in C?

Question

struct sample
{
    int a;
    char b;
    float c;
    int *al;
    union un
    {
        int a;
        char c;
        float f;
    }*ptr;
}test;

How do I access the structure member 'al' and the union members a,c,f?

I would like to use 'test' and 'ptr' when accessing them al and a,c,f respec. How does that work? — Arun, Mar 03 '14 at 08:09

Lee Duhem · Answer 1 · 2014-03-03T08:14:31.093

3

No difference than others:

access al
```
test.al
```
If you want the value of al, you could get it by *(test.al).
access a, c, f
```
test.ptr->a;
test.ptr->c;
test.ptr->f;
```

edited Mar 03 '14 at 08:14

answered Mar 03 '14 at 08:08

Lee Duhem

14,695
3
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47

Still have to dereference test.al. test->al won't work, obviously, because you're not dereferencing test, so *(test.al) would make it clearer. – ciphermagi Mar 03 '14 at 08:10
@ciphermagi `test.al` is the variable itself, `*(test.al)` is the value of that variable. Anyway, you have a point. Answer updated. – Lee Duhem Mar 03 '14 at 08:13
4

I'm not convinced that `*(test.al)` is clearer than `*test.al`. – Jonathan Leffler Mar 03 '14 at 08:16
2

@JonathanLeffler I guess that depends on how familiar with C operator precedence you are – Lee Duhem Mar 03 '14 at 08:18

Mads Gadeberg · Answer 2 · 2014-03-03T08:37:31.400

The thing is that you need to dereference the pointer.

Normally we would do this to dereference the union.

test.*ptr.a.

The problem with this is that the compiler will execute the dotnotation before the dereferencing symbol, The compiler will therefore dereference the field in the union, instead of the union it self.

To solve this issue we can put ´*ptr´in parentheses to force deferencing the union before accesing the field. Like this.

test.(*ptr).a

For an easier syntax this can also be written as

test.ptr->a

How do I access a pointer member inside a structure in C?

2 Answers2