foward typedef structures - c

Question

gcc 4.4.4 c89
I have this in my header file.
port.h
struct struct_tag;
int initialize_ports(struct_tag *port);
In my implemenation file I have this:
port.c
typedef struct struct_tag {
int port_id;
} Port_t;
And in my driver.h file, I have the following:
#include "port.h"
int initialize_ports(struct_tag *port)
{
port = malloc(sizeof *port);
/* do checking here */
}
I have forward declared the structure, as I want to hide the internal elements.
However, I am getting the following error on my initialize_ports in the header file:
expected ‘)’ before ‘*’ token
I am just wondering how can I forward declare and be able to pass the structure as a parameter?
Many thanks for any advice,

Answer 1

You should use:
int initialize_ports(struct struct_tag *port);
^^^^^^
Also, forward declarations give you an incomplete type which you don't know the size of. If you need to allocate a struct struct_tag you need to include the full definition for it. Alternatively you could use some create_struct_tag() function if you want to make it fully opaque.

Answer 2

As other answers have noted, you could change struct_tag to struct struct_tag in the prototype. Another way of getting your code to compile is to write
typedef struct struct_tag struct_tag;
in place of your existing struct struct_tag; (i.e. combine the typedef with the forward definition). That then does allow you to write
int initialize_ports(struct_tag *port)
without compile failures. However, this is still not quite what you want, because the caller can neither allocate a local variable of this type, nor malloc() one - because they don't know the size.
Other answers have suggested that you should open up the definition of the structure. That's generally not the right answer - because it removes the abstraction layer you're trying to create. Much better to have functions (in the port.c, i.e. the library that does know about the internals) such as:
struct_tag *create_port(...);
void free_port(struct_tag *port)
i.e. to create and free the structures - and indeed for other operations (such as reading from / writing to the structure) too.

Answer 3

You'll get an error as you don't KNOW the size of "port" as all it has to go on is the forward declaration.
In summary you are best off not using a forward declaration here unless you also set a constant value that is the sizeof "struct_tag" ... You would most likely be best off just fully declaring it.

Answer 4

The sizeof operator is evaluated at compile time not runtime, so at the line:
port = malloc(sizeof *port);
the compiler has no information regarding the size of the structure.
Solutions include:
fully define the type in the header file.
define initialize_ports() in port.c after the struct is fully defined.
have initialize_ports() call a function defined in ports.c to get the size of Port_t at run-time.
In any case you should not define initialize_ports() in the header file driver.h unless your compiler supports the inline or _inline keyword and you use it. Such usage would however render the code non ISO C compliant, and therefore less portable, however due to C++'s standard support for the keyword, you are likely to find it as an extension in most C tool-chains that include C++ compilation, so long as you do not use excessively strict compliance options.
However the error message you are getting is for a different reason. Unlike C++ in C struct_tag alone does not represent a type (if it did, you'd not have needed the typedef!), you must use the struct keyword.