autobook_255.html

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<DT>non Windows host
<DD>It seems obvious, but we must also be careful not to contaminate the
code when it is compiled on a machine which doesn't need to jump through
the DLL hoops.
</DL>
<P>

The changes to <TT>`hello.c'</TT> are no different to what would be required
on a Unix machine.  I have declared the <CODE>greet</CODE> variable to
allow the caller to override the default greeting:
</P><P>

<TABLE width=100%><tr><td>&nbsp;</td><td class=example bgcolor=#6688aa><br><pre>#if HAVE_CONFIG_H
#  include &#60;config.h&#62;
#endif

#include &#60;stdio.h&#62;

#include "hello.h"

const char *greet = "Hello";

int
hello (const char *who)
{
    printf("%s, %s!\n", greet, who);
    return 0;
}
</pre></td></tr></table></P><P>

Again, since the DLL specific changes have been encapsulated in the
<TT>`hello.h'</TT> file, enhancements to <TT>`main.c'</TT> are unsurprising too:
</P><P>

<TABLE width=100%><tr><td>&nbsp;</td><td class=example bgcolor=#6688aa><br><pre>#if HAVE_CONFIG_H
#  include &#60;config.h&#62;
#endif

#include "hello.h"

int
main (int argc, const char *const argv[])
{
    if (argc &#62; 1)
      {
        greet = argv[1];
      }
    return hello("World");
}
</pre></td></tr></table></P><P>

The final thing to be aware of is to be careful about ensuring that
<SAMP>`LIBHELLO_DLL_IMPORT'</SAMP> is defined when we link an executable against
the <TT>`libhello'</TT> DLL, but not defined if we link it against the
static archive.  It is impossible to automate this completely,
particularly when the executable in question is from another package and
is using the installed <TT>`hello.h'</TT> header.  In that case it is the
responsibility of the author of that package to probe the system with
<CODE>configure</CODE> to decide whether it will be linking with the
DLL or the static archive, and defining <SAMP>`LIBHELLO_DLL_IMPORT'</SAMP>
as appropriate.
</P><P>

Things are a little simpler when everything is under the control of a
single package, but even then it isn't quite possible to tell for sure
whether Libtool is going to build a DLL or only a static  library.
For example, if some dependencies are dropped for being static, Libtool
may disregard <SAMP>`-no-undefined'</SAMP> (see section <A HREF="autobook_88.html#SEC88">11.2.1 Creating Libtool Libraries with Automake</A>).  One possible solution is:
</P><P>

<OL>
<LI>
Define a function in the library that invokes <SAMP>`return 1'</SAMP> from a
DLL.   Fortunately that's easy to accomplish thanks to
<SAMP>`-DDLL_EXPORT'</SAMP>, in this case, by adding the following to
<TT>`hello.c'</TT>:
<P>

<TABLE width=100%><tr><td>&nbsp;</td><td class=example bgcolor=#6688aa><br><pre>#if defined WIN32 &#38;&#38; defined DLL_EXPORT
char
libhello_is_dll (void)
{
  return 1;
}
#endif /* WIN32 &#38;&#38; DLL_EXPORT */
</pre></td></tr></table></P><P>

<LI>
Link a program with the library, and check whether it is a DLL by
seeing if the link succeeded.
<P>

<LI>
To get cross builds to work, you must, in the same vein, test whether
linking a program which calls <SAMP>`libhello_is_dll'</SAMP> succeeds to tell
whether or not to define <SAMP>`LIBHELLO_DLL_IMPORT'</SAMP>.
</OL>
<P>

As an example of building the <TT>`hello'</TT> binary we can add the
following code to <TT>`configure.in'</TT>, just before the call
to <SAMP>`AC_OUTPUT'</SAMP>:
</P><P>

<TABLE><tr><td>&nbsp;</td><td class=smallexample><FONT SIZE=-1><pre># ----------------------------------------------------------------------
# Win32 objects need to tell the header whether they will be linking
# with a dll or static archive in order that everything is imported
# to the object in the same way that it was exported from the
# archive (extern for static, __declspec(dllimport) for dlls)
# ----------------------------------------------------------------------
LIBHELLO_DLL_IMPORT=
case "$host" in
*-*-cygwin* | *-*-mingw* )
  if test X"$enable_shared" = Xyes; then
    AC_TRY_LINK_FUNC([libhello_is_dll],
                     [LIBHELLO_DLL_IMPORT=-DLIBHELLO_DLL_IMPORT])
  fi
  ;;
esac
AC_SUBST(LIBHELLO_DLL_IMPORT)
</FONT></pre></td></tr></table></P><P>

And we must also arrange for the flag to be passed while compiling any
objects which will end up in a binary which links with the dll.  For
this simple example, only <TT>`main.c'</TT> is affected, and we can add the
following rule to the end of <TT>`Makefile.am'</TT>:
</P><P>

<TABLE><tr><td>&nbsp;</td><td class=smallexample><FONT SIZE=-1><pre>main.o: main.c
        $(COMPILE) @LIBHELLO_DLL_IMPORT@ -c main.c
</FONT></pre></td></tr></table></P><P>

In a more realistic project, there would probably be dozens of files
involved, in which case it would probably be easier to move them all
to a separate subdirectory, and give them a <TT>`Makefile.am'</TT> of their
own which could include:
</P><P>

<TABLE><tr><td>&nbsp;</td><td class=smallexample><FONT SIZE=-1><pre>CPPFLAGS        = @LIBHELLO_DLL_IMPORT@
</FONT></pre></td></tr></table></P><P>

Now, lets put all this into practice, and check that it works:
</P><P>

<TABLE width=100%><tr><td>&nbsp;</td><td class=example bgcolor=#6688aa><br><pre>$ make
cd . &#38;&#38; aclocal
cd . &#38;&#38; automake --foreign Makefile
cd . &#38;&#38; autoconf
...
checking for gcc option to produce PIC ... -DDLL_EXPORT
checking if gcc PIC flag  -DDLL_EXPORT works... yes
...
checking whether to build shared libraries... yes
...
gcc -DHAVE_CONFIG_H -I. -I. -I. -g -O2 -Wp,-MD,.deps/hello.pp \
-c  -DDLL_EXPORT -DPIC hello.c -o .libs/hello.lo
gcc -DHAVE_CONFIG_H -I. -I. -I. -g -O2 -Wp,-MD,.deps/hello.pp \
-c hello.c -o hello.o &#62;/dev/null 2&#62;&#38;1
...
gcc -DHAVE_CONFIG_H -I. -I. -I.     -g -O2 -DLIBHELLO_DLL_IMPORT \
-c main.c
...
gcc -g -O2 -o ./libs/hello main.o .libs/libimp-hello-0-0-0.a \
-Wl,--rpath -Wl,/usr/local/lib
creating hello
...
$ ./hello
Hello, World!
$ ./hello Howdy
Howdy, World!
</pre></td></tr></table></P><P>

The recipe also works if I use only the static archives:
</P><P>

<TABLE width=100%><tr><td>&nbsp;</td><td class=example bgcolor=#6688aa><br><pre>$ make clean
...
$ ./configure --disable-shared
...
checking whether to build shared libraries... no
...
$ make
...
gcc -DHAVE_CONFIG_H -I. -I. -I. -f -O2 -Wp,-MD,.deps/hello.pp \
-c hello.c -o hello.o
...
ar cru ./libs/libhello.a  hello.o
...
gcc -DHAVE_CONFIG_H -I. -I. -I.     -g -O2 -c main.c
...
gcc -g -O2 -o hello main.o ./.libs/libhello.a
$ ./hello
Hello, World!
$ ./hello "G'Day"
G'day, World!
</pre></td></tr></table></P><P>

And just to be certain that I am really testing a new statically linked
executable:
</P><P>

<TABLE><tr><td>&nbsp;</td><td class=smallexample><FONT SIZE=-1><pre>$ ldd ./hello
hello.exe       -&#62; /tmp/hello.exe
cygwin1.dll     -&#62; /usr/bin/cygwin1.dll
kernel32.dll    -&#62; /WINNT/system32/kernel32.dll
ntdll.dll       -&#62; /WINNT/system32/ntdll.dll
advapi32.dll    -&#62; /WINNT/system32/advapi32.dll
user32.dll      -&#62; /WINNT/system32/user32.dll
gdi32.dll       -&#62; /WINNT/system32/gdi32.dll
rpcrt4.dll      -&#62; /WINNT/system32/rpcrt4.dll
</FONT></pre></td></tr></table></P><P>

<A NAME="Runtime Loading of DLLs"></A>
</TR></TABLE>
<BR>  
<FONT SIZE="-1">
This document was generated
by <I>Gary V. Vaughan</I> on <I>September, 12  2004</I>
using <A HREF="http://www.mathematik.uni-kl.de/~obachman/Texi2html
"><I>texi2html</I></A>

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