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external struct declaration

P: n/a
I have a Lex file containing definitions of 2 structures like:

%{
struct a {...};
struct b
{
struct a i;
...
};
struct a x[10];
struct b y[10];
....
%}

In main program I have external declarations

/* main.c */

extern struct a;
extern struct b;
extern struct a x[];
extern struct b y[];

When I try to compile and run I am getting

'Warning:Useless keyword or type name in external declaration'
and
'Invalid use of undefined type : struct a'

Is there anything wrong in the external declarations
of the structure?
Nov 14 '05 #1
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5 Replies


P: n/a
anonymous <iu*********@yahoo.co.in> wrote:
I have a Lex file containing definitions of 2 structures like: %{
[Hint for those not used to lex: the '%{' starts a section of
C code which goes unchanged into the C source file created from
the lex file, so the question isn't really OT.]
struct a {...};
This makes a new structure type 'a' known within that (lex) file
(but only within that file, nowhere else).
struct b
{
struct a i;
...
};
And that of the new structure type 'b'.
struct a x[10];
struct b y[10];
And here you define two arrays of structures of type 'a' and 'b'.
Everything fine so far.
In main program I have external declarations /* main.c */ extern struct a;
You can use 'extern' only when there's a variable or function name
following it. What you write here is basically like having a line like

extern int;

which simply makes doesn't make sense. That explains the first warning
'Warning:Useless keyword or type name in external declaration'
Even worse, when the compiler reads that file it has no idea at all
what 'struct a' is or means. It doesn't know that 'struct a' has been
(or may only will be) declared in a different source file. What you
need to do is either declare again here what 'struct a' means - but
that's a rather bad idea, because it's too easy to have the decla-
rations get out of sync - or to create an include file, into which
you put what 'struct a' is, and then include it from every file that
needs to know what 'struct a' means. If you do that the second warning
'Invalid use of undefined type : struct a'
for the line
extern struct a x[];


will disappear.
Regards, Jens
--
\ Jens Thoms Toerring ___ Je***********@physik.fu-berlin.de
\__________________________ http://www.toerring.de
Nov 14 '05 #2

P: n/a
[...]
Even worse, when the compiler reads that file it has no idea at all
what 'struct a' is or means. It doesn't know that 'struct a' has been
(or may only will be) declared in a different source file.

[...]

But when I create the executable, both the .o files are used:

gcc -o lex.yy.o main.o

so, the definitions must be visible - right?
Nov 14 '05 #3

P: n/a
anonymous <iu*********@yahoo.co.in> wrote:
[...]
Even worse, when the compiler reads that file it has no idea at all
what 'struct a' is or means. It doesn't know that 'struct a' has been
(or may only will be) declared in a different source file. [...]

But when I create the executable, both the .o files are used: gcc -o lex.yy.o main.o so, the definitions must be visible - right?


But then you're already linking. But what you got was an error
message from the compiler. And the compiler only deals with
single files at a time (if you don't count included files, but
they simple are more or less pasted in the source). So, when
the compiler sees your yy.c file (or however it's called) it
finds the definition of the structure and creates an .o file from
it without problems. But when it gets to main.c (the one where you
also use the structure), it has forgotten everything it did see in
the other file (typically, it's a completely new invocation of the
compiler) and so it has no idea what that structure is supposed to
be and complains, and you don't even get a main.o file. The easiest
way to make the type "struct a" visible to both files is to put the
definition into an additional include file that then gets included
from both files.
Regards, Jens
--
\ Jens Thoms Toerring ___ Je***********@physik.fu-berlin.de
\__________________________ http://www.toerring.de
Nov 14 '05 #4

P: n/a

In article <2g************@uni-berlin.de>, Je***********@physik.fu-berlin.de writes:
anonymous <iu*********@yahoo.co.in> wrote:
/* main.c */
extern struct a;
You can use 'extern' only when there's a variable or function name
following it. What you write here is basically like having a line like

extern int;

which simply makes doesn't make sense. That explains the first warning
'Warning:Useless keyword or type name in external declaration'


Good so far.
Even worse, when the compiler reads that file it has no idea at all
what 'struct a' is or means.
Incomplete struct declarations are fine in C, provided you don't try
to use them as complete types, and in fact they're useful for
declaring opaque types in at least one context - function prototypes.

If main.c doesn't need to complete structs a and b (if it doesn't
need to do anything with their members, and doesn't need to take
their size, eg using the sizeof operator), then:

struct a;
struct b;

would be fine in main.c, to introduce the names "a" and "b" to the
struct namespace.

The only use I can think of offhand for this "bare" struct tag
declaration is before a function prototype which uses a pointer to
the struct, as in

struct a;
extern void foo(struct a *);

because there a previously-unseen struct tag (if you omitted the
"struct a;" line) will declare the structure only in prototype scope,
which is basically useless.
It doesn't know that 'struct a' has been
(or may only will be) declared in a different source file.


Which is fine, since it doesn't need to know whether any other TU
("source file") has declared struct a. That's irrelevant to the
behavior of main.c.

In this case, the OP probably wants to use struct a as a complete
type - if main.c is going to operate on that that array of struct a,
it needs to be complete - but there are APIs that use incomplete
structure definitions to create and pass opaque types safely.

--
Michael Wojcik mi************@microfocus.com

Advertising Copy in a Second Language Dept.:
Tapestry of the encounting and the farewell, the birth and the death.
You can hear the human being's song running through the 100 years.
-- Squaresoft
Nov 14 '05 #5

P: n/a
Groovy hepcat Je***********@physik.fu-berlin.de was jivin' on 16 May
2004 10:51:23 GMT in comp.lang.c.
Re: external struct declaration's a cool scene! Dig it!
anonymous <iu*********@yahoo.co.in> wrote:
extern struct a;


You can use 'extern' only when there's a variable or function name
following it. What you write here is basically like having a line like

extern int;

which simply makes doesn't make sense. That explains the first warning


Actually, as the compiler (probably) sees it, the errant line
declares an external object of type struct. Hence this warning:
'Warning:Useless keyword or type name in external declaration'


"struct" (without a tag) is indeed a useless keyword or type name.

--

Dig the even newer still, yet more improved, sig!

http://alphalink.com.au/~phaywood/
"Ain't I'm a dog?" - Ronny Self, Ain't I'm a Dog, written by G. Sherry & W. Walker.
I know it's not "technically correct" English; but since when was rock & roll "technically correct"?
Nov 14 '05 #6

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