Question

This code is just a situation that I found in my actual code which is very big so I’m giving this.here in this code the structure “struct node” is not defined it is defined in another c source file.

my c source code:

/* test.c */

  1 #include<stdio.h>
  2 #include "test2.h"
  3
  4 void f(struct node * k)
  5 {
  6   
  7 }

my header file :

/* test2.h */

  1 extern void f(struct node * k);

When I compile this code with gcc to create an object file:

gcc -w -c test.c

I get:

test.c:6: error: conflicting types for 'f'
test2.h:1: error: previous declaration of 'f' was here

I have given the complete prototype of function f(). Why I am getting this error?

Another thing is that when I don’t include the header file test2.h in test.c and explicitly declare the function prototype in test.c, it compiles successfully. Code is below:

 /* test.c */

      1 #include<stdio.h>
      2 void f(struct node *k);
      3
      4 void f(struct node * k)
      5 {
      6
      7 }

gcc -c -w test.c

No error.

Can you please explain why this time I am not getting an error?

Answer 1

This has nothing to do with the extern keyword on the prototype (although it’s not necessary).

You need a forward declaration for struct node:

/* test2.h */

struct node;  // <== this

extern void f(struct node * k);

Without that forward declaration, the struct node inside the function prototype is local to that particular function prototype.

So when the compiler sees the definition of f() it also sees another different local declaration for a struct node and thinks that you have a conflicting declaration for f().

See C99 6.9.1 “Scopes of identifiers”:

For each different entity that an identifier designates, the
identifier is visible (i.e., can be used) only within a region of
program text called its scope. Different entities designated by the
same identifier either have different scopes, or are in different name
spaces. There are four kinds of scopes: function, file, block, and
function prototype. (A function prototype is a declaration of a
function that declares the types of its parameters.)

…

If the declarator or type specifier that declares the identifier
appears within the list of parameter declarations in a function
prototype (not part of a function definition), the identifier has
function prototype scope, which terminates at the end of the function
declarator.

GCC 4.6.1 gives a nice warning about this for me:

C:\temp\test.c:3:22: warning: 'struct node' declared inside parameter list [enabled by default]
C:\temp\test.c:3:22: warning: its scope is only this definition or declaration, which is probably not what you want [enabled by default]

C++ makes function prototype scope apply only to the parameter names (C++03 3.3.3), so you won’t see this problem if you compile the code as C++.