Re: [Q] How to load dll's at runtime

[Stephen Vance <vance at deneb dot com>]

Gary V. Vaughan wrote:

> Hi Stephen, thanks for the response!  And your's too Sergey, I have read
> everything from the cygnus web site that seemed relevant now.

[ ... ]

> > Here's a code snippet:
> >
> > void *open_symbol( const char *libname, const char *funcname )
> > {
> >
> >     void *libhandle;
> >
> >     libhandle = dlopen( libname, RTLD_LAZY );
> >     if( libhandle == NULL )
> >         return NULL;
> >
> >     return dlsym( libhandle, funcname );
> > }
> >
> > int (*func)( int, char *); // For example
> >
> > func = open_symbol( "mylib.dll", "myfunc" );
> >
> > (*func)( myint, mycharstar );
> >
> > Invoking dlsym with a NULL libhandle give you a handle to the
> > executable.
>
> dlsym you say?  I am used to using
>
>    libhandle = dlopen (NULL, RTLD_NOW|RTLD_LAZY);
>
> to get an introverted handle.  However I don't seem to be able to get
> either of them to work.  I have followed the execution through with gdb,
> and in either case it fails inside cygwin.dll, with
>
>    error: dlopen: Win32 Error #126
>
> and
>
>    error: dlsym: Win32 Error #87
>
> respectively.

I don't know what the error codes mean.  Givent that the first call fails,
the second is not surprising.  Some problems I have encountered in the past
are:

--  The symbols may not be exported properly using dll_export() if created
with the MS tools.  This is particularly tricky with C++.
--  You may have name mangling issues if you're using C++.
--  The semantics and actual functionality of a NULL library handle vary
between systems.  For example, in HP's shlib library (they don't use dlopen
and friends) NULL is an invalid library handle, but they have a special
constant that can be used for this purpose.
--  PATH on NT, LD_LIBRARY_PATH on most Unices (and I believe Cygwin) and
SHLIB_PATH on HP are not set properly.

> >  On some systems, this means *precisely* the executable
> > (Sun, NT); on others it means the executable and any shared library
> > explicitly linked with it (SGI, HP).  I suspect Cygwin will behave
> > the same way as the regular NT stuff.
>
> Any other clues you might be able to help me with?

The only other suggestions I might have are:

--  Try getting your introverted handle by explicitly passing argv[0].
--  Try using dlerror to tell you the error in English.

>
>
> I fear that there may be no support for the introspective handler within
> b19... but even confirmation of this would be good.
>
> Thanks for your patience.
>
> Gary.

 Good luck.  Dynamic loading stuff can be tricky, particularly when
portability is an issue.

--
Stephen Vance                           |  http://www.deneb.com
Deneb Robotics, Inc.                    |  mailto:vance@deneb.com
5500 New King Street                    |  Phone: (248) 267-9696
Troy, MI 48098-2615                     |  Fax:   (248) 267-8585

What is done well is done quickly enough. -Augustus Caesar



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