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Re: [RFC] [PATCH] libltdl error reporting
Gary V. Vaughan
Re: [RFC] [PATCH] libltdl error reporting
Thu, 10 Jun 2010 13:55:24 +0700
On 10 Jun 2010, at 12:33, Peter O'Gorman wrote:
> As I am sure many are aware, libltdl's error reporting is pretty dumb,
> lt_dlerror() regularly reports things like "file not found" where the actual
> problem might be something completely different, and a reasonable error
> string may be readily available from dlerror().
> When I looked at the manual, and read the description for lt_dlerror() I saw
> "Return a human readable string describing the most recent error that
> occurred from any of libltdl's functions. Return NULL if no errors have
> occurred since initialization or since it was last called." and I thought
> "Well, that's stupid - that's not what dlerror() does". So, I went to see
> what POSIX says - it's also wrong -
> http://www.opengroup.org/onlinepubs/9699919799/functions/dlerror.html, I will
> file a bug with them later.
> At least glibc and Mac OS X reset the dlerror() string to NULL every time a
> dl*() api is called (I will check what other systems do in the next few
> days). This is so programmers can do:
> dlsym(handle, "foo");
> if ((err = dlerror()) != NULL) printf(stderr,"dlsym error: %s\n",err);
> without dlerror() returning some other error that occurred long before the
> call to dlsym().
Interesting. IIRC, Thomas chose the semantics and API of libltdl to closely
match the Unix dlfcn.h calls so that it could be used as a direct replacement
by a client with very little editing. If glibc and OS X (which to my mind
probably covers 80% of the users of libltdl) is already doing something saner
than the documentation might have lead us to believe, then your change looks
like a net win to me!
Aside: I'm leaning away from upholding the 'drop-in-with-minimum-edits'
philosophy for my rewrite, since the dlfcn.h API seems like a pretty bad design
to me. After all, all people really need to do is call functions with a known
name and known signature which happen to be in another library. I'm seriously
contemplating using a *much* smaller and cleaner API, which ends up with client
code more along the lines of:
static jmp_buf exception;
my_get_dso_func (const char *dsopath, const char *symname)
if (! setjmp (exception))
/* setjmp returns 0 on first invocation */
return lt_dlsym (dsopath, symname);
/* but if lt_dlsym throws an exception, we end up here */
fputs ("myapp: my_get_dso_func failed", stderr);
With that as a starting point, it's easy enough to maintain an error stack in
an exception struct that wraps around setjmp/longjmp, and to pretty up the code
above with some macros to make it look more like the common throw/catch syntax.
At that point, with libltdl2 doing the book-keeping, the entire API is just:
typedef void *lt_exception;
void *lt_dlsym (const char *dsopath, const char *symname);
void lt_dlclose (const char *dsopath);
plus some glue to make catching an error thrown from libltdl2 and for unwinding
the error stack inside it as easy as possible. WDYT?
> Unfortunately, it looks like libltdl attempts to save error state across
> calls to lt_*() functions, and generally does not do it well.
> This patch resets the error every time a public ltdl function is called, and
> when setting an error tests if it is already set and doesn't overwrite it if
> it is. The fist error "wins".
> The patch is fairly simple, and undoubtedly still imperfect, but test results
> are good, one test required a small patch to stop segfaulting, another should
> have unexpectedly passed, but doesn't, I think it is another error in the
> test, but haven't looked too closely yet.
> If something like this does get in, I'd prefer to not change the libltld
> soname, I don't believe better error reporting will cause client applications
> to break. I could be wrong, of course :)
> Patch attached (or course, a final patch will have to change the manual as
> well) - thoughts?
And add some test coverage. I like it. And it gives us something to tide us
over until I've finished libltdl2 in 2012 ;)
Gary V. Vaughan (address@hidden)
Re: [RFC] [PATCH] libltdl error reporting, Bob Friesenhahn, 2010/06/10
Re: [RFC] [PATCH] libltdl error reporting, Peter O'Gorman, 2010/06/14