未初始化时，结构中的值是什么？

Question

Let's say I have a struct:

typedef struct{
   int a;
   float b;
} stuff;

If I have code like:

stuff myStuff;
printf( "%d", myStuff.a ) ;

What would I get? Would the field always be initialized to 0?

Answer 1

In C11, any struct members that are not explicitly initialized (as is the case for you) have indeterminate values. C11 6.7.9/10:

If an object that has automatic storage duration is not initialized explicitly, its value is indeterminate.

Reading such an indeterminate value produces undefined behaviour. C11 6.3.2.1/2:

If the lvalue designates an object of automatic storage duration that could have been declared with the register storage class (never had its address taken), and that object is uninitialized (not declared with an initializer and no assignment to it has been performed prior to use), the behavior is undefined.

Finally, Annex J.2 (Undefined behavior) clarifies this:

The behavior is undefined in the following circumstances:

• The value of an object with automatic storage duration is used while it is indeterminate (6.2.4, 6.7.9, 6.8).

Answer 2

No. The field could have any value at all. Doing something like this is considered a bug since the value is not predictable.

Answer 3

It depends on whether an object of the structure has the static storage duration or it is a local object. If an object of the structure has the static storage duration when all its data members will be initialized by zeroes. otherwise the object will not be initialized.

For example

#include <stdio.h>

typedef struct
{
    int a;
    float b;
} stuff;

stuff s1; // initialized by zeroes

int main( void )
{
    stuff s2;     //indetermined values

    printf( "%d\n", s1.a ) ;  // 0 will be outputed
    printf( "%d\n", s2.a ) ;  // any value can be outputed
}