Question

I have a program that defines a typedef struct within an existing typedef struct, and I am wondering why I am getting a compilation error.

Here is the program:

typedef struct Outer
{
    typedef struct Inner
    {
        int b;
    }INNER;


    INNER inner;
    int a; 

}OUTER;

int main()
{ 
    OUTER obj;
    obj.a = 10;
    obj.inner.b=8;
    return 0;
}   

on compilation gives following error ::

test.c:3:5: error:expected specifier-qualifier-list before ‘typedef’
test.c: In function ‘main’:
test.c:17:5: error: ‘OUTER’ has no member named ‘a’
test.c:18:5: error: ‘OUTER’ has no member named ‘inner’  

but, when I changed the program to

typedef struct Outer
{
    struct Inner
    {
        int b;
    };

    struct Inner inner;
    int a; 

 }OUTER;

 int main()
 {
    OUTER obj;
    obj.a = 10;
    obj.inner.b=8;
    return 0;
 }   

it compiles successfully.

Why is typedef not allowed with inner structures?

Answer 1

C does not allow a storage-class specifier (typedef, but also static or extern) in the declaration of a structure member. This is specified in the syntax of structures and unions declaration in 6.7.2.1p1 in C99.

/* The compiler will issue a diagnostic for this declaration */ 
struct s {    
    static int a;
};

You can compare 6.7.2.1p1 syntax with the syntax of a declaration where the declarator is not a function parameter or a structure / union member in 6.7p1 and see that in this case the storage-class specifiers are allowed.