What's the default value of int fields of a local struct variable?

Question

What's the default value of int fields of a local struct variable? Will they be zeroed automatically? Or, will they be the same as any other local variables, filled with garbage values?

Answer 1

If the variable has automatic storage — a local variable — and is not explicitly initialized, its state is indefinite. That applies regardless of whether it is a simple variable (e.g. an int) or complex (e.g. a structure or union).

The fields will not reliably be zeroed.

C11 §6.7.9 Initialization ¶10:

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

Answer 2

Note that static variables are automatically initialized to zero, including local static variables.

Consider this example:

typedef struct
{
    int alpha;
    char bravo;
    float charlie;
}  ThreeVarStruct_T;        // Define a structure

int function(void)  // Local Scope
{
    ThreeVarStruct_T        exampleA; // This will hold uninitialized "random" data
    static ThreeVarStruct_T exampleB; // This is guaranteed to be initialized to zero
}