Accessing a pointer through a nested structure

struct x {
  int *u;
};

struct y {
  struct x *z;
};

int main()
{
  static y b;
  static int g=7;
  b.z->u=&g;
}

The statement b.z->u=&g gives a segmentation error. If I remove the static in front of int g:

int g=7;开发者_开发技巧
b.z->u=&g;  

The code executes properly.

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b.z is not yet initialized. You initialize b with:

static y b;

but its member field z is still a null pointer (pointing so some random location). So accessing its member u results in an segmentation error as you are accessing some random memory.

I suppose that like this should work (did not try):

static y b;
static x c;
static int g=7;
b.z = &c;
b.z->u=&g;

Why your second example works, I do not know. I suspect it is due to 'luck'...

---

Because b.z hasn't been set to point anywhere useful. It's currently just a NULL pointer.^*

You need to do something along the lines of:

b.z = malloc(sizeof(*b.z));

first (i.e. create an actual object).

Remember to free this at some point.

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_{* Note that it's only NULL because b is declared as static. If b were non-static, it would be pointing at somewhere random in memory.}

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It will give you a segmentation error, because what you're doing is undefined behavior. You're accessing the z pointer of the structure without initializing it (that is, without giving it a memory space to point to). The fact that it is not giving you a segmentation error in the case the variable is not static is not important, as the whole access to an uninitialized pointer is wrong.

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You never allocated memory for your b.z. Your b.z contains an uninitialized garbage value, which is why trying to dereference b.z (in b.z->u) causes a segmentation fault.

P.S. You declared your objects static, which means that b.z initially contains a null value (not an "uninitialized garbage value" as I stated above). Nevertheless, dereferencing a null pointer is also undefined.