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Re: Suggestion: Detect inconsistent structure definitions
- From: Daniel Jacobowitz <drow at mvista dot com>
- To: Zack Weinberg <zack at codesourcery dot com>
- Cc: gdb at sources dot redhat dot com
- Date: Wed, 13 Mar 2002 19:07:08 -0500
- Subject: Re: Suggestion: Detect inconsistent structure definitions
- References: <20020313182221.GE8197@codesourcery.com>
On Wed, Mar 13, 2002 at 10:22:22AM -0800, Zack Weinberg wrote:
> Consider the following two source files:
>
> -- a.c --
> struct A {
> int a;
> int b;
> };
>
> struct A a = { 1, 2 };
>
> -- b.c --
> struct A {
> char a;
> char b;
> };
>
> extern struct A a;
>
> int main(void) {
> if (a.a == 1 && b.a == 2)
> return 0;
> else
> return 1;
> }
>
> --
> It is obvious that the complete program consisting of these two files
> is buggy: the declarations of struct A do not match. However, the
> program will compile, link, and execute with no complaints, just an
> unexpected return value.
>
> When the program is large and complicated, this sort of bug can be
> near-impossible to find, especially when the structure type
> declaration _is_ properly isolated in a header file, but other headers
> (possibly from third-party libraries) have issued inconsistent
> typedefs/#defines for the aggregate's member types. I just spent two
> days chasing exactly this problem in an INN installation.
>
> The compiler and linker do not have enough information to detect the
> bug, but gdb does; each object file's debug info will contain a type
> declaration for struct A, and they won't match. With stabs, it's
> obvious just doing
It's not clear that GDB does. Consider a slightly modified example:
> -- a.c --
> struct A {
> int a;
> int b;
> };
>
> static struct A a = { 1, 2 };
>
> -- b.c --
> struct A {
> char a;
> char b;
> };
>
> static struct A a = {3, 4};
>
> int main(void) {
> if (a.a == 1 && b.a == 2)
> return 0;
> else
> return 1;
> }
>
> --
OK, that's a somewhat degenerate case, but my point holds. When do we
have enough information to know that two references are 'supposed' to
be of the same type, rather than an implementation-private type? And in
stabs, at least, no debug information appears to be emitted for
'extern' statements, so we don't know if a file referenced the type it
had a different definition of or not.
Perhaps a command to show types with multiple different definitions?
Please file a GNATS PR so that this idea doesn't get forgotten.
--
Daniel Jacobowitz Carnegie Mellon University
MontaVista Software Debian GNU/Linux Developer