"extern struct foobar" linux compilation warning

[Rick Anderson]

All,

I am receiving the following compilation error on LINUX
(but not Solaris, HPUX, WIN32, etc):

compiling osr.c
LBFO.h(369): warning #64: declaration does not declare anything
extern struct foobar;
^

This is apparently a forward reference to a data structure
for the purposes of declaring a pointer to this opaque
structure.

Any ideas how to get rid of this warning???

Thanks in advance!
Rick

[Rick Anderson]

BTW: This is the "icc" (intel compiler) *not*
gcc...

It appears -fms_extensions compiler option is not supported.

FYI

Thanks!
Rick

[Kevin Bracey]

In message <Xn************ *************** *******@148.87. 1.53>
Rick Anderson <Ri************ **@oracle.com> wrote:

I am receiving the following compilation error on LINUX
(but not Solaris, HPUX, WIN32, etc):

compiling osr.c
LBFO.h(369): warning #64: declaration does not declare anything
extern struct foobar;
^

This is apparently a forward reference to a data structure
for the purposes of declaring a pointer to this opaque
structure.

Any ideas how to get rid of this warning???

Remove the "extern". It's meaningless unless you're actually declaring an
object, which you're not. You're merely naming a structure type.

--
Kevin Bracey, Principal Software Engineer
Tematic Ltd Tel: [phone]
182-190 Newmarket Road Fax: [phone]
Cambridge, CB5 8HE, United Kingdom WWW: http://www.tematic.com/

[Michael Mair]

Rick Anderson wrote:

All,

I am receiving the following compilation error on LINUX
(but not Solaris, HPUX, WIN32, etc):

compiling osr.c
LBFO.h(369): warning #64: declaration does not declare anything
extern struct foobar;
^
extern struct foobar baz;

would make sense.

struct foobar;

can make sense.
"extern" goes with objects, not with types.

This is apparently a forward reference to a data structure
for the purposes of declaring a pointer to this opaque
structure.
Is it?

If it is really necessary to get something opaque,
use void *:

typedef void * FooBarHandle;

The user gets only the handle and even a look at the
typedef does not tell how and where information is stored.
If the user can be trusted to not circumvent your access
macros and functions (and your data structures are not
top secret), the header file giving the type definition
for "struct foobar" should be included.
Any ideas how to get rid of this warning???

Heal the code.
Cheers
Michael
--
E-Mail: Mine is an /at/ gmx /dot/ de address.

[Kevin Bracey]

In message <39************ *@individual.ne t>
Michael Mair <Mi**********@i nvalid.invalid> wrote:

If it is really necessary to get something opaque,
use void *:

typedef void * FooBarHandle;

The user gets only the handle and even a look at the
typedef does not tell how and where information is stored.
If the user can be trusted to not circumvent your access
macros and functions (and your data structures are not
top secret), the header file giving the type definition
for "struct foobar" should be included.

I beg to differ. I'd prefer

typedef struct foobar *FooBarHandle;

leaving the contents of struct foobar opaque. That gives you a bit more type
safety - someone can't accidentally give you a BarHandle instead of a
FooBarHandle.

--
Kevin Bracey, Principal Software Engineer
Tematic Ltd Tel: [phone]
182-190 Newmarket Road Fax: [phone]
Cambridge, CB5 8HE, United Kingdom WWW: http://www.tematic.com/

[Michael Mair]

Kevin Bracey wrote:

In message <39************ *@individual.ne t>
Michael Mair <Mi**********@i nvalid.invalid> wrote:

If it is really necessary to get something opaque,
use void *:

typedef void * FooBarHandle;

The user gets only the handle and even a look at the
typedef does not tell how and where information is stored.
If the user can be trusted to not circumvent your access
macros and functions (and your data structures are not
top secret), the header file giving the type definition
for "struct foobar" should be included.

I beg to differ. I'd prefer

typedef struct foobar *FooBarHandle;

leaving the contents of struct foobar opaque. That gives you a bit more type
safety - someone can't accidentally give you a BarHandle instead of a
FooBarHandle.

You are right, at least for the application at hand.

I was thinking of completely opaque information where you do not
even want the people to know what kind of type may be underlying,
i.e. if the type in question is an array, structure or whatever
-- in this case, one obviously does not want type safety.
Cheers
Michael
--
E-Mail: Mine is an /at/ gmx /dot/ de address.

[CBFalconer]

Kevin Bracey wrote:

Michael Mair <Mi**********@i nvalid.invalid> wrote:

If it is really necessary to get something opaque,
use void *:

typedef void * FooBarHandle;

The user gets only the handle and even a look at the
typedef does not tell how and where information is stored.
If the user can be trusted to not circumvent your access
macros and functions (and your data structures are not
top secret), the header file giving the type definition
for "struct foobar" should be included.

I beg to differ. I'd prefer

typedef struct foobar *FooBarHandle;

leaving the contents of struct foobar opaque. That gives you a
bit more type safety - someone can't accidentally give you a
BarHandle instead of a FooBarHandle.

Exactly what I was mulling how to express, when I decided to look
onwards and see what others had said about it. void* allows you to
receive and pass pointers of unknown types and purpose onward
freely, but this is not such a case.

--
Chuck F (cb********@yah oo.com) (cb********@wor ldnet.att.net)
Available for consulting/temporary embedded and systems.
<http://cbfalconer.home .att.net> USE worldnet address!

[Ian Pilcher]

Kevin Bracey wrote:

I beg to differ. I'd prefer

typedef struct foobar *FooBarHandle;

leaving the contents of struct foobar opaque. That gives you a bit more type
safety - someone can't accidentally give you a BarHandle instead of a
FooBarHandle.

Definitely better than a void *, but really not necessary. As long as

struct foobar;

is declared, pointers to struct foobar can be freely passed around, but
not dereferenced.

--
=============== =============== =============== =============== ============
Ian Pilcher i.*******@comca st.net
=============== =============== =============== =============== ============

[Rick Anderson]

Ian Pilcher <i.*******@comc ast.net> wrote in
news:QN******** ************@co mcast.com:

Kevin Bracey wrote:
Definitely better than a void *, but really not necessary. As long as

struct foobar;

is declared, pointers to struct foobar can be freely passed around,
but not dereferenced.

That works great - thanks!
Rick

[Kevin Bracey]

In message <QN************ ********@comcas t.com>
Ian Pilcher <i.*******@comc ast.net> wrote:

Kevin Bracey wrote:

I beg to differ. I'd prefer

typedef struct foobar *FooBarHandle;

leaving the contents of struct foobar opaque. That gives you a bit more
type safety - someone can't accidentally give you a BarHandle instead of
a FooBarHandle.

Definitely better than a void *, but really not necessary. As long as

struct foobar;

is declared, pointers to struct foobar can be freely passed around, but
not dereferenced.

Ah, but making it a typedef hides the fact the handle is a struct from the
basic API. One might want (for whatever reason) to change the handle from an
int to a structure or vice-versa.

--
Kevin Bracey, Principal Software Engineer
Tematic Ltd Tel: [phone]
182-190 Newmarket Road Fax: [phone]
Cambridge, CB5 8HE, United Kingdom WWW: http://www.tematic.com/