Problem with dlopen()

[Juanjo Garcia-2]

This function does some nonstandard things, but the most annoying one
is imposing that the library to be loaded has to have a file
extension.

If I invoke dlopen("/path/to/temp/dir/temporary_file", ...) then the
routine fails with an error like "File or directory does not exist".
It is probably not that text, but something similar output by
dlerror(). If I rename the file as "temporary_file.foo" then dlopen()
succeeds.

Using strace reveals that in the first case dlopen() is adding the
extension ".dll" and complaining that the file does not exist. In
addition to this, the file has to be given executable permissions
using chmod() or dlopen will fail.

The reason I found this bug is that ECL (Embeddable Common Lisp)
compiles lisp files to DLLs and uses nonstandard names for that. With
mingw32 and Microsoft Visual Studio C++ we never had a problem, but
cygwin broke when we used our code.

Juanjo

--
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c/ Serrano, 113b, Madrid 28006 (Spain)
http://juanjose.garciaripoll.googlepages.com

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[Corinna Vinschen-2]

On Nov  1 20:21, Juan Jose Garcia-Ripoll wrote:

> This function does some nonstandard things, but the most annoying one
> is imposing that the library to be loaded has to have a file
> extension.
>
> If I invoke dlopen("/path/to/temp/dir/temporary_file", ...) then the
> routine fails with an error like "File or directory does not exist".
> It is probably not that text, but something similar output by
> dlerror(). If I rename the file as "temporary_file.foo" then dlopen()
> succeeds.
>
> Using strace reveals that in the first case dlopen() is adding the
> extension ".dll" and complaining that the file does not exist. In
> addition to this, the file has to be given executable permissions
> using chmod() or dlopen will fail.
>
> The reason I found this bug is that ECL (Embeddable Common Lisp)
> compiles lisp files to DLLs and uses nonstandard names for that. With
> mingw32 and Microsoft Visual Studio C++ we never had a problem, but
> cygwin broke when we used our code.

I'm somewhat puzzled by this, for two reasons:

- dlopen() uses LoadLibrary() under the hood.  With Mingw32 or VC++ you
  have to do the same, apparently.  LoadLibrary appends the .dll
  extension by itself(*) if the given filename has no extension.  So, if
  you had a shared lib called "foo", and you called LoadLibrary ("foo"),
  then you should have seen en error in MingW32/VC++.  Only by adding a
  trailing dot, you can avoid automative adding of the .dll suffix, same
  as in Cygwin's dlopen() function.

- If the library is not executable, the LoadLibrary function returns
  an "access denied" error.  You should have this observed, too.

Try this:

  $ cat > x.c << EOF
  #include <windows.h>
  #include <stdio.h>

  int main (int argc, char **argv)
  {
    HMODULE m = LoadLibrary (argv[1]);
    if (!m)
      printf ("%s: %lu\n", argv[1], GetLastError ());
    else
      printf ("Success\n");
    return 0;
  }
  EOF
  $ gcc x.c -o x
  $ cp /bin/cygncurses-9.dll ./xdll.dll
  $ ls -l xdll.dll
  -rwxr-xr-x 1 corinna vinschen 167936 2009-11-02 14:01 xdll.dll
  $ ./x xdll
  Success
  $ mv xdll.dll xdll
  $ ./x xdll
  xdll: 126
  $ net helpmsg 126

  The specified module could not be found.
 
  $ ./x xdll.        # Note the extra dot!
  Success
  $ chmod -x xdll
  $ ls -l xdll
  -rw-r--r-- 1 corinna vinschen 167936 2009-11-02 14:01 xdll
  $ ./x xdll.
  xdll.: 5
  $ net helpmsg 5

  Access is denied.

  $ mv xdll xdll.dll # Even with a dll suffix, you need execute permissions
  $ ./x xdll
  xdll: 5
  $ ./x xdll.dll
  xdll.dll: 5


Corinna

(*) http://msdn.microsoft.com/en-us/library/ms684175%28VS.85%29.aspx


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Corinna Vinschen                  Please, send mails regarding Cygwin to
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