c - foward typedef structures -

gcc 4.4.4 c89 

i have in header file.

port.h

struct struct_tag;  int initialize_ports(struct_tag *port); 

in implemenation file have this:

port.c

typedef struct struct_tag {     int port_id; } port_t; 

and in driver.h file, have following:

#include "port.h" int initialize_ports(struct_tag *port) {     port = malloc(sizeof *port);     /* checking here */ } 

i have forward declared structure, want hide internal elements.

however, getting following error on initialize_ports in header file:

expected ‘)’ before ‘*’ token 

i wondering how can forward declare , able pass structure parameter?

many advice,

as other answers have noted, could change struct_tag struct struct_tag in prototype. way of getting code compile write

typedef struct struct_tag struct_tag; 

in place of existing struct struct_tag; (i.e. combine typedef forward definition). allow write

int initialize_ports(struct_tag *port) 

without compile failures. however, still not quite want, because caller can neither allocate local variable of type, nor malloc() 1 - because don't know size.

other answers have suggested should open definition of structure. that's not right answer - because removes abstraction layer you're trying create. better have functions (in port.c, i.e. library does know internals) such as:

struct_tag *create_port(...); void free_port(struct_tag *port) 

i.e. create , free structures - , indeed other operations (such reading / writing structure) too.