Re: dereferencing pointer to incomplete type

[Micha Feigin <michf@post.tau.ac.il>]

On Thu, Jun 17, 2004 at 11:24:51AM +0100, Glynn Clements wrote:
> 
> Micha Feigin wrote:
> 
> > > > > Arping.xs:153: dereferencing pointer to incomplete type
> 
> > In this case its probably something else.
> 
> I doubt it.
> 
> > Are you sure that u_char is properly defined?
> 
> If it wasn't, attempts to define variables or structure fields of type
> u_char would result in a parse error.
> 
> An "incomplete type" error has to refer to a struct, union or enum
> (ANSI C forbids forward references to enum types, but gcc allows it). 
> These are the only cases where the compiler can know that something is
> a type without knowing the type's definition.
> 
> I'm quite certain that the problem is due to the fact that the
> compiler knows that src_mac is a pointer to "struct ether_addr" but
> doesn't know the definition of "struct ether_addr".
> 
> As the error message says:
> 
> 	Arping.xs:153: dereferencing pointer to incomplete type
> 
> Assuming that the poster is correct in stating that line 153 is:
> 
> 	memcpy(enet_src, src_mac->ether_addr_octet,6);
> 

Thus my suggestion to change src_mac->ether_addr_octet to enet_src, it
obviously does the wrong thing but should kill the error message if we
are looking at the right line.

> the only dereference in that line is "src_mac->ether_addr_octet", and
> the pointer is "src_mac". So, the message is stating that src_mac has
> "incomplete" type, which means that the compiler knows that it is a
> struct, but doesn't know its definition (in particular, it doesn't
> know the offset of the ether_addr_octet field, so it cannot compile
> the expression).
> 
> For the record, the compiler wouldn't be concerned if enet_src had
> incomplete type, because it isn't being dereferenced. The following
> source file compiles without any errors or warnings (even with "-ansi
> -pedantic -Wall"):
> 
> 	#include <string.h>
> 	#include <net/ethernet.h>
> 	
> 	extern struct foo *enet_src;
> 	extern struct ether_addr *src_mac;
> 	
> 	void foo(void)
> 	{
> 		memcpy(enet_src, src_mac->ether_addr_octet,6);
> 	}
> 
> In summary, I strongly suspect that the .xs file isn't producing the C
> code which the author expects; specifically, that the definition for
> "struct ether_addr" isn't being seen.
> 

That seems that way, but when I suggested adding the definition in the
file just before the call the OP said that he tried that and it didn't
solve the problem.

> -- 
> Glynn Clements <glynn.clements@virgin.net>
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