Question

I want to hide the nature of a public struct for an API inside my .h/.c couple, so I declare only the typedef in the .h and I complete the declaration in the .c like this:

foo.h

typedef struct filters_s filters_t;

/* some public functions declaration using filters_t */
(...)

foo.c

typedef struct filters_s filter_node_t;

struct filters_s
{
  filter_node_t *children[96];

  (...)
}

As you can see, filters_s is in fact the root node of a tree so internally, I’m using filter_node_t but externally, I don’t want to expose the “tree” nature of the struct.
So, my “problem” is that ideally I’d like to have also another name for the struct like filter_node_s, but I don’t know if it’s possible.

Answer 1

If you want to hide the implementation of the structure then you need a opaque pointer to the structure. There you would pass this pointer to a function that will get or modify the data of the structure.

The declaration will be in the *.h header file. And the definition will be in the *.c file.

Something like this in the *.h (header file):

typedef struct tag_device device_t;

Then in the *.c (implementation file):

struct tag_device {
size_t id;
char *name;
};

void set_data(device_t *dev, size_t id, char *name)
{
dev->id = id;
dev->name = strdup(*name);
}

Then in your *.c (driver file)

device_t *device = malloc(sizeof *device)

set_data(device, 1, "device02345");

I have just typed this in, so it might not be perfect as I haven’t checked for errors. Always remember to free memory after you finish with it.

Hope this helps