Question

When trying to discover something in the code of some open source C projects, I often see this kind of typedef:

typedef struct _StructA StructA;
typedef struct _LinusTorvalds LinusTorvalds;

Why not directly define and use StructA, just StructA, instead of defining _StructA then “typedef” it?
What’s the point of this “_” technique?

Answer 1

In C, a struct must be used as:

struct _StructA my_variable;

…i.e., you must type struct not only when you define the struct, but also every time you use it. The typedef eliminates the typing of struct when you use it (but not when you define it: there is no way to eliminate that). With a typedef then, declaring an instance of the struct becomes:

StructA my_variable;

Now, that doesn’t completely explain the _StructA/StructA business in the defining of the struct/typedef, as you can do this:

typedef struct {
    ...
} StructA;

…here, we take an anonymous struct and immediately feed it to typedef. But using this method, you can’t use the struct inside itself, as it doesn’t (yet) have a name. For that, you need something like:

typedef struct _Foo {
    int data;
    struct _Foo *next;
} Foo;