lvalue cast

[Martijn]

Hi,

I recently updated my version of gcc (to gcc version 3.4.2
(mingw-special) ), and I get a warning I am not too sure about how to best
solve:

warning: use of cast expressions as lvalues is deprecated

The code that invokes this warning has to do with PIDLs (pointer to item id
list, which are a Windows thing and not too important really). The problem
is that I have a pointer to a structure (typedef'd to ITEMIDLIST) and in
order to access some data I have to move over a certain amount of bytes (the
pointer really points to an array with arbitrary-sized memory blocks, and
the structure is just a way to interface those blocks), so this is what I
did:

ITEMIDLIST* pidl;
int offset;

...

(char*)pidl += offset;

What can I do to fix the warning? Preferebly without the use of an extra
variable (e.g. a char*).

Thanks,

--
Martijn
http://www.sereneconcepts.nl

[SM Ryan]

"Martijn" <su************ *********@hot-remove-mail.com> wrote:
# Hi,
#
# I recently updated my version of gcc (to gcc version 3.4.2
# (mingw-special) ), and I get a warning I am not too sure about how to best
# solve:
#
# warning: use of cast expressions as lvalues is deprecated
#
# The code that invokes this warning has to do with PIDLs (pointer to item id
# list, which are a Windows thing and not too important really). The problem
# is that I have a pointer to a structure (typedef'd to ITEMIDLIST) and in
# order to access some data I have to move over a certain amount of bytes (the
# pointer really points to an array with arbitrary-sized memory blocks, and
# the structure is just a way to interface those blocks), so this is what I
# did:
#
# ITEMIDLIST* pidl;
# int offset;
#
# ...
#
# (char*)pidl += offset;
#
# What can I do to fix the warning? Preferebly without the use of an extra
# variable (e.g. a char*).

To cast an lvalue V of type S to an lvalue of type T,
you can do something like

*(T*)&V

lvalue S V
pointer to S &V
pointer to T (T*)&V
lvalue T *(T*)&V

--
SM Ryan http://www.rawbw.com/~wyrmwif/
Leave it to the Catholics to destroy existence.

[Richard Tobin]

In article <43************ ***********@new s.xs4all.nl>,
Martijn <su************ *********@hot-remove-mail.com> wrote:

ITEMIDLIST* pidl;
int offset;

...

(char*)pidl += offset;

You could do "pidl = (ITEMIDLIST*)(( (char *)pidl) + offset)".

Someone else will probably address thet portability issues with this
sort of thing. I assume you are aware of alignment issues.

-- Richard

[pete]

Martijn wrote:

Hi,

I recently updated my version of gcc (to gcc version 3.4.2
(mingw-special) ), and I get a warning I am not too sure about how to best
solve:

warning: use of cast expressions as lvalues is deprecated

The code that invokes this warning has to do with PIDLs (pointer to item id
list, which are a Windows thing and not too important really). The problem
is that I have a pointer to a structure (typedef'd to ITEMIDLIST) and in
order to access some data I have to move over a certain amount of bytes (the
pointer really points to an array with arbitrary-sized memory blocks, and
the structure is just a way to interface those blocks), so this is what I
did:

ITEMIDLIST* pidl;
int offset;

...

(char*)pidl += offset;

What can I do to fix the warning?
Preferebly without the use of an extra variable (e.g. a char*).

pidl = (ITEMIDLIST *)((char *)pidl + offset);

--
pete

[Keith Thompson]

"Martijn" <su************ *********@hot-remove-mail.com> writes:

I recently updated my version of gcc (to gcc version 3.4.2
(mingw-special) ), and I get a warning I am not too sure about how to best
solve:

warning: use of cast expressions as lvalues is deprecated
That's an odd warning. The use of cast expressions as lvalues isn't
just deprecated; it's illegal (a constraint violation). It's been
illegal at least since C90; I don't know whether it might have been
allowed before that.

[...] ITEMIDLIST* pidl;
int offset;

...

(char*)pidl += offset;

What can I do to fix the warning? Preferebly without the use of an extra
variable (e.g. a char*).

I think this should do it:

pidl = (ITEMIDLIST*)(( char*)pidl + offset);

But you might find it cleaner to step through the array using a char*,
and convert the char* value to the appropriate type when you use it.

--
Keith Thompson (The_Other_Keit h) ks***@mib.org <http://www.ghoti.net/~kst>
San Diego Supercomputer Center <*> <http://users.sdsc.edu/~kst>
We must do something. This is something. Therefore, we must do this.

[Martijn]

Keith Thompson wrote:

"Martijn" <su************ *********@hot-remove-mail.com> writes:

[snipped]

ITEMIDLIST* pidl;
int offset;

...

(char*)pidl += offset;

What can I do to fix the warning? Preferebly without the use of an
extra variable (e.g. a char*).

I think this should do it:

pidl = (ITEMIDLIST*)(( char*)pidl + offset);

Thanks everyone for the response. The thing that threw me of was +=. I of
course intended to cast just the right hand of the = sign, but using +=
kind-of made that rvalue an lvalue. Now I feel kinda silly askin' this,
especially with the experience I've had programming C ...

[snipped a little more]

Thanks again,

--
Martijn
http://www.sereneconcepts.nl

[pete]

Martijn wrote:

The thing that threw me of was +=.

(x) += (y)
means exactly
(x) = (x) + (y)
for any values of x and y.

If you have
unsigned short us = USHRT_MAX;
then
++us;
yields undefined behavior, if USHRT_MAX == INT_MAX.

--
pete

[Christian Bau]

In article <11************ *@corp.supernew s.com>,
SM Ryan <wy*****@tang o-sierra-oscar-foxtrot-tango.fake.org> wrote:

"Martijn" <su************ *********@hot-remove-mail.com> wrote:
# Hi,
#
# I recently updated my version of gcc (to gcc version 3.4.2
# (mingw-special) ), and I get a warning I am not too sure about how to best
# solve:
#
# warning: use of cast expressions as lvalues is deprecated
#
# The code that invokes this warning has to do with PIDLs (pointer to item id
# list, which are a Windows thing and not too important really). The problem
# is that I have a pointer to a structure (typedef'd to ITEMIDLIST) and in
# order to access some data I have to move over a certain amount of bytes (the
# pointer really points to an array with arbitrary-sized memory blocks, and
# the structure is just a way to interface those blocks), so this is what I
# did:
#
# ITEMIDLIST* pidl;
# int offset;
#
# ...
#
# (char*)pidl += offset;
#
# What can I do to fix the warning? Preferebly without the use of an extra
# variable (e.g. a char*).

To cast an lvalue V of type S to an lvalue of type T,
you can do something like

*(T*)&V

lvalue S V
pointer to S &V
pointer to T (T*)&V
lvalue T *(T*)&V

That has the distinct disadvantage of invoking undefined behavior if you
use V later (with a few exceptions). The original would hopefully have
defined behavior in the gcc language (which is not quite the same as the
C language).

Consider an implementation where sizeof (ITEMIDLIST *) != sizeof (char
*). Your code is likely to cause trouble. With gcc I would hope that it
does something reasonable in that case.

[Christian Bau]

In article <ln************ @nuthaus.mib.or g>,
Keith Thompson <ks***@mib.or g> wrote:

"Martijn" <su************ *********@hot-remove-mail.com> writes:

I recently updated my version of gcc (to gcc version 3.4.2
(mingw-special) ), and I get a warning I am not too sure about how to best
solve:

warning: use of cast expressions as lvalues is deprecated

That's an odd warning. The use of cast expressions as lvalues isn't
just deprecated; it's illegal (a constraint violation). It's been
illegal at least since C90; I don't know whether it might have been
allowed before that.

It is legal and deprectated in the language implemented by the gcc
compiler with default settings, which is not quite the same as C.

[Richard Tobin]

In article <ln************ @nuthaus.mib.or g>,
Keith Thompson <ks***@mib.or g> wrote:

That's an odd warning. The use of cast expressions as lvalues isn't
just deprecated; it's illegal (a constraint violation).
Presumably the gcc extension is deprecated.
illegal at least since C90; I don't know whether it might have been
allowed before that.

It was allowed in some drafts of ANSI C.

-- Richard