Why does it redefine a pointer to a struct?

[fl]

Hi,
I find the following definition from a company provided header file. The sample application software still uses FIR16, not *FIR16_handle.

The typedef line really puzzles me. Could you explain it to me? It would be better to give a small example.


Thanks,




typedef struct {
long *coeff_ptr; /* Pointer to Filter coefficient */
long * dbuffer_ptr; /* Delay buffer ptr */
int cbindex; /* Circular Buffer Index */
int order; /* Order of the Filter */
int input; /* Latest Input sample */
int output; /* Filter Output */
void (*init)(void *); /* Ptr to Init funtion */
void (*calc)(void *); /* Ptr to calc fn */
}FIR16;

/*---------------------------------------------------------------
Define a Handles for the Filter Modules
-----------------------------------------------------------------*/
typedef FIR16 *FIR16_handle;

 

[Joe Pfeiffer]

Hi,
I find the following definition from a company provided header
file. The sample application software still uses FIR16, not
*FIR16_handle.

The typedef line really puzzles me. Could you explain it to me? It
would be better to give a small example.
typedef struct {

}FIR16;

/*---------------------------------------------------------------
Define a Handles for the Filter Modules
-----------------------------------------------------------------*/
typedef FIR16 *FIR16_handle;

We just had quite a thread on typedefs! Reviewing that wouldn't be a
bad idea. Anyway, some programmers prefer to hide the syntax for
structs and pointers by using typedefs.

So if you were to declare

FIR16 john;

you'd be declaring an instance of the struct up there, and you could get
at its fields by saying things like

john.input = 42;

You can also declare a pointer to this struct by saying something like

FIR16 *paul;

and use it with things like

paul = malloc(sizeof *paul);
paul->input = 42;

Also, because of the second typedef, a FIR16_handle is another name for
a pointer to a FIR16, so you could say things like

FIR16_handle george;

george = malloc(sizeof *george);
george->input = 42;

It's a little unusual (in my experience) to see a handle typedef and a
struct definition both in a header file. Most of the time when I see
somebody want to call something a handle, they want to make it opaque.
In these cases, all that would appear in your header file would be

typedef FIR16 *FIR16_handle;

the struct definition would be internal to the library you're using, and
the only way to access elements of the struct would be through
library-supplied functions.