pointer-to-pointer (invalid lvalue in unary `&)

[Lucas Machado]

i'm doing some Linux Kernel hacking for a course i'm currently taking.
there is a pointer to a struct (struct example_struct *ex_ptr) in a .c
that i want to access in a system call. i defined a pointer to a
pointer in the .c:

extern struct example_struct **pointer;

and somewhere in the code i tried:

pointer = &ex_ptr;

and i get this error when trying to compile the kernel:

invalid lvalue in unary `&'

and it points to the line: pointer = &ex_ptr;.... any idea why and what
i should do to fix it?

[Leor Zolman]

On 3 Apr 2004 09:49:41 -0800, lm******@eden.r utgers.edu (Lucas Machado)
wrote:

i'm doing some Linux Kernel hacking for a course i'm currently taking.
there is a pointer to a struct (struct example_struct *ex_ptr) in a .c
that i want to access in a system call. i defined a pointer to a
pointer in the .c:

extern struct example_struct **pointer;

and somewhere in the code i tried:

pointer = &ex_ptr;

and i get this error when trying to compile the kernel:

invalid lvalue in unary `&'

and it points to the line: pointer = &ex_ptr;.... any idea why and what
i should do to fix it?

From what you've posted, all looks fine. Can you show /exactly/ what the
definition of ex_ptr looks like? Whatever ex_ptr is, it doesn't seem to
have an address associated with it. If it were just the wrong type for
"pointer", I'd have expected the message to spell that out. So my guess is
that "ex_ptr" is actually something like a constant, as weird as that
sounds...
-leor

--
Leor Zolman --- BD Software --- www.bdsoft.com
On-Site Training in C/C++, Java, Perl and Unix
C++ users: Download BD Software's free STL Error Message Decryptor at:
www.bdsoft.com/tools/stlfilt.html

[Martin Ambuhl]

Lucas Machado wrote:

i'm doing some Linux Kernel hacking for a course i'm currently taking.
there is a pointer to a struct (struct example_struct *ex_ptr) in a .c
that i want to access in a system call. i defined a pointer to a
pointer in the .c:

extern struct example_struct **pointer;

and somewhere in the code i tried:

pointer = &ex_ptr;

and i get this error when trying to compile the kernel:

invalid lvalue in unary `&'

and it points to the line: pointer = &ex_ptr;.... any idea why and what
i should do to fix it?

You have almost certainly misdiagnosed the problem. Look to the lines
before the one for which the error is reported. Simple things like
missing a ';' on the line before may be such a culprit. Notice the use
of your assignment below, which is fine. (If you had an incorrect use
of 'extern', that would probably lead to a linking error instead).

#include <stdio.h>

struct example_struct
{
int f;
} basestruct = { 0};
struct example_struct *ex_ptr = &basestruct;

int main(void)
{
struct example_struct **pointer;
pointer = &ex_ptr;
(*pointer)->f = 3;
printf("basestr uct.f = %d\n", basestruct.f);
printf("ex_ptr->f = %d\n", ex_ptr->f);
printf("(*point er)->f = %d\n", (*pointer)->f);
return 0;
}
basestruct.f = 3
ex_ptr->f = 3
(*pointer)->f = 3

[Martin Ambuhl]

Lucas Machado wrote:

i'm doing some Linux Kernel hacking for a course i'm currently taking.
there is a pointer to a struct (struct example_struct *ex_ptr) in a .c
that i want to access in a system call. i defined a pointer to a
pointer in the .c:

extern struct example_struct **pointer;

and somewhere in the code i tried:

pointer = &ex_ptr;

and i get this error when trying to compile the kernel:

invalid lvalue in unary `&'

and it points to the line: pointer = &ex_ptr;.... any idea why and what
i should do to fix it?

You have almost certainly misdiagnosed the problem. Look to the lines
before the one for which the error is reported. Simple things like
missing a ';' on the line before may be such a culprit. Notice the use
of your assignment below, which is fine. (If you had an incorrect use
of 'extern', that would probably lead to a linking error instead).

#include <stdio.h>

struct example_struct
{
int f;
} basestruct = { 0};
struct example_struct *ex_ptr = &basestruct;

int main(void)
{
struct example_struct **pointer;
pointer = &ex_ptr;
(*pointer)->f = 3;
printf("basestr uct.f = %d\n", basestruct.f);
printf("ex_ptr->f = %d\n", ex_ptr->f);
printf("(*point er)->f = %d\n", (*pointer)->f);
return 0;
}
basestruct.f = 3
ex_ptr->f = 3
(*pointer)->f = 3

[Chris Fogelklou]

"Leor Zolman" <le**@bdsoft.co m> wrote in message
news:re******** *************** *********@4ax.c om...

On 3 Apr 2004 09:49:41 -0800, lm******@eden.r utgers.edu (Lucas Machado)
wrote:

i'm doing some Linux Kernel hacking for a course i'm currently taking.
there is a pointer to a struct (struct example_struct *ex_ptr) in a .c
that i want to access in a system call. i defined a pointer to a
pointer in the .c:

extern struct example_struct **pointer;

and somewhere in the code i tried:

pointer = &ex_ptr;

and i get this error when trying to compile the kernel:

invalid lvalue in unary `&'

and it points to the line: pointer = &ex_ptr;.... any idea why and what
i should do to fix it?

From what you've posted, all looks fine. Can you show /exactly/ what the
definition of ex_ptr looks like? Whatever ex_ptr is, it doesn't seem to
have an address associated with it. If it were just the wrong type for
"pointer", I'd have expected the message to spell that out. So my guess is
that "ex_ptr" is actually something like a constant, as weird as that
sounds...
-leor

--
Leor Zolman --- BD Software --- www.bdsoft.com
On-Site Training in C/C++, Java, Perl and Unix
C++ users: Download BD Software's free STL Error Message Decryptor at:
www.bdsoft.com/tools/stlfilt.html

Ditto! How is ex_ptr defined?

Should be something like:
struct example_struct ex_st;
struct example_struct *ex_ptr;
struct example_struct **pointer;

If you simply want the problem to go away to see if it is a compiler error,
try casting it. That would hide any errors that might occur due to the
compiler only... changing your debugging process from finding a syntax error
to figuring out why your computer is crashing :)

pointer = (struct example_struct **pointer)&ex_p tr;

[Chris Fogelklou]

"Leor Zolman" <le**@bdsoft.co m> wrote in message
news:re******** *************** *********@4ax.c om...

On 3 Apr 2004 09:49:41 -0800, lm******@eden.r utgers.edu (Lucas Machado)
wrote:

i'm doing some Linux Kernel hacking for a course i'm currently taking.
there is a pointer to a struct (struct example_struct *ex_ptr) in a .c
that i want to access in a system call. i defined a pointer to a
pointer in the .c:

extern struct example_struct **pointer;

and somewhere in the code i tried:

pointer = &ex_ptr;

and i get this error when trying to compile the kernel:

invalid lvalue in unary `&'

and it points to the line: pointer = &ex_ptr;.... any idea why and what
i should do to fix it?

From what you've posted, all looks fine. Can you show /exactly/ what the
definition of ex_ptr looks like? Whatever ex_ptr is, it doesn't seem to
have an address associated with it. If it were just the wrong type for
"pointer", I'd have expected the message to spell that out. So my guess is
that "ex_ptr" is actually something like a constant, as weird as that
sounds...
-leor

--
Leor Zolman --- BD Software --- www.bdsoft.com
On-Site Training in C/C++, Java, Perl and Unix
C++ users: Download BD Software's free STL Error Message Decryptor at:
www.bdsoft.com/tools/stlfilt.html

Ditto! How is ex_ptr defined?

Should be something like:
struct example_struct ex_st;
struct example_struct *ex_ptr;
struct example_struct **pointer;

If you simply want the problem to go away to see if it is a compiler error,
try casting it. That would hide any errors that might occur due to the
compiler only... changing your debugging process from finding a syntax error
to figuring out why your computer is crashing :)

pointer = (struct example_struct **pointer)&ex_p tr;

[Richard Heathfield]

Chris Fogelklou wrote:

<snip>

If you simply want the problem to go away to see if it is a compiler
error,
try casting it.
That doesn't seem very fruitful advice. He'd be better off trying to
understand the type system than trying to learn how to circumvent it.
That would hide any errors that might occur due to the
compiler only... changing your debugging process from finding a syntax
error to figuring out why your computer is crashing :)
Finding the syntax error is much easier.
pointer = (struct example_struct **pointer)&ex_p tr;

That cast is badly-formed, and requires a diagnostic.

--
Richard Heathfield : bi****@eton.pow ernet.co.uk
"Usenet is a strange place." - Dennis M Ritchie, 29 July 1999.
C FAQ: http://www.eskimo.com/~scs/C-faq/top.html
K&R answers, C books, etc: http://users.powernet.co.uk/eton

[Richard Heathfield]

Chris Fogelklou wrote:

<snip>

If you simply want the problem to go away to see if it is a compiler
error,
try casting it.
That doesn't seem very fruitful advice. He'd be better off trying to
understand the type system than trying to learn how to circumvent it.
That would hide any errors that might occur due to the
compiler only... changing your debugging process from finding a syntax
error to figuring out why your computer is crashing :)
Finding the syntax error is much easier.
pointer = (struct example_struct **pointer)&ex_p tr;

That cast is badly-formed, and requires a diagnostic.

--
Richard Heathfield : bi****@eton.pow ernet.co.uk
"Usenet is a strange place." - Dennis M Ritchie, 29 July 1999.
C FAQ: http://www.eskimo.com/~scs/C-faq/top.html
K&R answers, C books, etc: http://users.powernet.co.uk/eton

[Chris Fogelklou]

> That doesn't seem very fruitful advice. He'd be better off trying to

understand the type system than trying to learn how to circumvent it.

Yep.

changing your debugging process from finding a syntax
error to figuring out why your computer is crashing :)

Finding the syntax error is much easier

Hence the smiley and the statement that he would have to figure out why the
computer is crashing (sarcasm... oops... my bad :)

pointer = (struct example_struct **pointer)&ex_p tr;

That cast is badly-formed, and requires a diagnostic.

Hmm... It compiles on my system... but from the other posts/responses I've
been participating in, I think my system might be broken. Could you please
elaborate?

typedef struct ui_tag {
uint16 i;
uint16 j;
uint16 k;
uint16 l;
} ui_t, *pui_t;

void main(void)
{
struct ui_tag * ptr_ui;
struct ui_tag ** pptr_ui;
struct ui_tag ui = {0,0,0,0};

ptr_ui = &ui;
pptr_ui = (struct ui_tag **)&ptr_ui;
while(1);

}

[Chris Fogelklou]

> That doesn't seem very fruitful advice. He'd be better off trying to

understand the type system than trying to learn how to circumvent it.

Yep.

changing your debugging process from finding a syntax
error to figuring out why your computer is crashing :)

Finding the syntax error is much easier

Hence the smiley and the statement that he would have to figure out why the
computer is crashing (sarcasm... oops... my bad :)

pointer = (struct example_struct **pointer)&ex_p tr;

That cast is badly-formed, and requires a diagnostic.

Hmm... It compiles on my system... but from the other posts/responses I've
been participating in, I think my system might be broken. Could you please
elaborate?

typedef struct ui_tag {
uint16 i;
uint16 j;
uint16 k;
uint16 l;
} ui_t, *pui_t;

void main(void)
{
struct ui_tag * ptr_ui;
struct ui_tag ** pptr_ui;
struct ui_tag ui = {0,0,0,0};

ptr_ui = &ui;
pptr_ui = (struct ui_tag **)&ptr_ui;
while(1);

}