This code, IOhannes m zmoelnig 2001-09-13, if I am correct, defines a structure of type " _helloworld " and declares an instance of that structure called t_helloworld.

typedef struct _helloworld {
t_object x_obj;
} t_helloworld;

This is where I get confused. The code below defines a function which has one argument, the variable " x " which is declared as a pointer to type t_helloworld.

void helloworld_bang(t_helloworld *x)
{
post("Hello world !!");
}

How is this possible, if t_helloworld is a structure and not a type? Shouldn't the argument be "...(_helloworld *x)"?
I looked at the example code that comes with puredata, and it follows the exact same format. I compiled the examples and they seem to work fine, so I believe that the code is correct; I just don't understand why.

These code snippets came from IOhannes m Zmoeling's awsome guide to writing externals which can be found here:

http://pdstatic.iem.at/externals-HOWTO/node1.html

ok, so what's going on is a C-language idiom:

typedef struct foo { ... } bar;

is equivalent to:

struct foo { ... };
typedef (struct foo) bar;

typedef is creating a typename alias: wherever you would have had to have written "struct foo" before, you can now write just "bar".

so instead of having to write:

void myfunction(struct foo *f) { ... }

you can write:

void myfunction(bar *f) { ... }

and it'll be exactly the same.

It's just a C-language shortcut to save typing "struct " every time.

Personally I find typedef confusing, so I just stick with typing "struct" everywhere, otherwise I forget which things are really structs and which things aren't.

typedefs are useful sometimes, but mostly for "really complicated" types, which "struct foo" isn't.

So in the example which I posted;

typedef struct _helloworld {
t_object x_obj;
} t_helloworld;

the last line defines "t_helloworld" as a shorthand for "struct _helloworld" when declaring variables. Thank you very much Claudius for explaining this; I am a lot less confused when I look at the code!!!