dereferencing pointer to incomplete type

dereferencing pointer to incomplete type

[Anshuman S. Rawat]

Hi,
I got the following error while compiling some module -
 
Arping.xs:153: dereferencing pointer to incomplete type
 make: *** [Arping.o] Error 1
 
 
 Now line 153 in the source is - 
 
 memcpy(enet_src, src_mac->ether_addr_octet,6);
 
 where
 
 u_char enet_src[6] = {0x00, 0x00, 0x00, 0x00, 0x00,0x00};
 
 struct ether_addr *src_mac;
 
 
 struct ether_addr
 {
     u_char  ether_addr_octet[6];
 };
 
 are the definitions.
 
 I haven't been able to figure out what the problem is.
Any help will be appreciated.
 
 Thanks.
Anshuman
 

Re: dereferencing pointer to incomplete type

[Anshuman S. Rawat]

> On Wed, Jun 16, 2004 at 03:56:37PM -0400, Anshuman S. Rawat wrote:
> > Arping.xs:153: dereferencing pointer to incomplete type
> >  make: *** [Arping.o] Error 1
> >  
> >  memcpy(enet_src, src_mac->ether_addr_octet,6);
> ...
> >  struct ether_addr
> >  {
> >      u_char  ether_addr_octet[6];
> >  };
> 
> Hi, I think you should define that struct _before_ you use it. This
> error is exactly what you get when you define it later.

The struct is defined before it is used through a header file.
I just wrote it afterwards here to show what it was.

Thanks.
-Anshuman


> 
> HTH,
> Christoph
> 

Re: dereferencing pointer to incomplete type

[Christoph Bussenius]

On Wed, Jun 16, 2004 at 03:56:37PM -0400, Anshuman S. Rawat wrote:
> Arping.xs:153: dereferencing pointer to incomplete type
>  make: *** [Arping.o] Error 1
>  
>  memcpy(enet_src, src_mac->ether_addr_octet,6);

...

>  struct ether_addr
>  {
>      u_char  ether_addr_octet[6];
>  };

Hi, I think you should define that struct _before_ you use it. This
error is exactly what you get when you define it later.

HTH,
Christoph

-- 
``There's no dark side of the moon, really
Matter of fact, it's all dark''

--Pink Floyd

Re: dereferencing pointer to incomplete type

[Glynn Clements]

Anshuman S. Rawat wrote:

> > > Arping.xs:153: dereferencing pointer to incomplete type

In the absence of any other errors, this usually means that you are
dereferencing a pointer to a structure or union type, but the compiler
doesn't have the definition of the structure.

This is normally only relevant to struct/union types, as it is
possible to refer to undefined struct/union types without a syntax
error, i.e.

	struct foo *p;
	union bar *q;

Attempting to use any other unknown type, e.g.

	foo *p;

would result in a syntax error (e.g. "parse error before '*' token").

However, any operation which requires the actual definition of those
structures (i.e. accessing fields with -> or performing pointer
arithmetic) will generate the above "incomplete type" error unless the
definitions are known.

> > >  make: *** [Arping.o] Error 1
> > >  
> > >  memcpy(enet_src, src_mac->ether_addr_octet,6);
> > ...
> > >  struct ether_addr
> > >  {
> > >      u_char  ether_addr_octet[6];
> > >  };
> > 
> > Hi, I think you should define that struct _before_ you use it. This
> > error is exactly what you get when you define it later.
> 
> The struct is defined before it is used through a header file.

The error message indicates that the compiler doesn't know the
definition of "struct ether_addr", which suggests that it hasn't
actually been defined (e.g. because the relevant header file
(net/ethernet.h) hasn't been included).

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

Re: dereferencing pointer to incomplete type

[Micha Feigin]

On Wed, Jun 16, 2004 at 03:56:37PM -0400, Anshuman S. Rawat wrote:
> Hi,
> I got the following error while compiling some module -
>  
> Arping.xs:153: dereferencing pointer to incomplete type
>  make: *** [Arping.o] Error 1
>  
>  
>  Now line 153 in the source is - 
>  
>  memcpy(enet_src, src_mac->ether_addr_octet,6);
>  
>  where
>  
>  u_char enet_src[6] = {0x00, 0x00, 0x00, 0x00, 0x00,0x00};
>  
>  struct ether_addr *src_mac;
>  
>  
>  struct ether_addr
>  {
>      u_char  ether_addr_octet[6];
>  };
>  
>  are the definitions.
>  
>  I haven't been able to figure out what the problem is.
> Any help will be appreciated.
>  

Try as a test to add the struct definition just before the call to
memcpy and see if that resolves the error. Could be that for some
reason things aren't included properly.

>  Thanks.
> Anshuman
>  
> 
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Re: dereferencing pointer to incomplete type

[Anshuman S. Rawat]

> > Arping.xs:153: dereferencing pointer to incomplete type
> >  make: *** [Arping.o] Error 1
> >
> >  Now line 153 in the source is - 
> >  
> >  memcpy(enet_src, src_mac->ether_addr_octet,6);
> >  where
> >  u_char enet_src[6] = {0x00, 0x00, 0x00, 0x00, 0x00,0x00};
> >  struct ether_addr *src_mac;
> >  struct ether_addr
> >  {
> >      u_char  ether_addr_octet[6];
> >  };
> 
> Try as a test to add the struct definition just before the call to
> memcpy and see if that resolves the error. Could be that for some
> reason things aren't included properly.

I already tried this. It didn't help.

-Anshuman 

Re: dereferencing pointer to incomplete type

[Micha Feigin]

On Wed, Jun 16, 2004 at 07:02:04PM -0400, Anshuman S. Rawat wrote:
> > > Arping.xs:153: dereferencing pointer to incomplete type
> > >  make: *** [Arping.o] Error 1
> > >
> > >  Now line 153 in the source is - 
> > >  
> > >  memcpy(enet_src, src_mac->ether_addr_octet,6);
> > >  where
> > >  u_char enet_src[6] = {0x00, 0x00, 0x00, 0x00, 0x00,0x00};
> > >  struct ether_addr *src_mac;
> > >  struct ether_addr
> > >  {
> > >      u_char  ether_addr_octet[6];
> > >  };
> > 
> > Try as a test to add the struct definition just before the call to
> > memcpy and see if that resolves the error. Could be that for some
> > reason things aren't included properly.
> 
> I already tried this. It didn't help.
> 

In this case its probably something else.

Are you sure that u_char is properly defined? what happens if you add
before the these definitions?

typedef unsigned char * u_char;

or write:

memcpy((void *)enet_src, (void *)(src_mac->ether_addr_octet), 6);

or just to make sure we are looking at the right thing to do

memcpy(enet_src, enet_src, 6);

BTW, what is the .xs file sufix?

> -Anshuman 
> 
> 
> -
> To unsubscribe from this list: send the line "unsubscribe linux-c-programming" in
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Re: dereferencing pointer to incomplete type

[Christoph Bussenius]

> BTW, what is the .xs file sufix?

Hi Micha,

XS is an interface to write modules for Perl in C.

Regards,
Christoph

-- 
``There's no dark side of the moon, really
Matter of fact, it's all dark''

--Pink Floyd

Re: dereferencing pointer to incomplete type

[Glynn Clements]

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);

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.

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

Re: dereferencing pointer to incomplete type

[Micha Feigin]

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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