Add lib-msvc-compat module.

ChangeLog

@@ -1,3 +1,13 @@
+2009-04-01  Simon Josefsson  <simon@josefsson.org>
+
+	* modules/lib-msvc-compat: New module.  Thanks to Bruno Haible
+	<bruno@clisp.org>, Ralf Wildenhues <Ralf.Wildenhues@gmx.de>, and
+	Eric Blake <ebb9@byu.net> for review.
+	* MODULES.html.sh: Add lib-msvc-compat.
+	* doc/gnulib.texi: Link to new section.
+	* m4/ld-output-def.m4: New file.
+	* doc/ld-output-def.texi: New file.
+
 2009-04-01  Simon Josefsson  <simon@josefsson.org>
 
 	Rename ld-version-script to lib-symbol-versions.  Suggested by

MODULES.html.sh

@@ -3132,6 +3132,7 @@ func_all_modules ()
   func_module include_next
   func_module ldd
   func_module lib-ignore
+  func_module lib-msvc-compat
   func_module lib-symbol-versions
   func_module link-warning
   func_module no-c++

doc/gnulib.texi

@@ -5823,6 +5823,7 @@ This list of functions is sorted according to the header that declares them.
 * Searching for Libraries::
 * Exported Symbols of Shared Libraries::
 * LD Version Scripts::
+* Visual Studio Compatibility::
 * Supporting Relocation::
 * func::
 * warnings::
@@ -5915,6 +5916,8 @@ generated automatically.
 
 @include ld-version-script.texi
 
+@include ld-output-def.texi
+
 @include relocatable-maint.texi
 
 @include func.texi

doc/ld-output-def.texi

@@ -0,0 +1,64 @@
+@node Visual Studio Compatibility
+@section Visual Studio Compatibility
+@cindex DEF files
+@cindex LD DEF files
+
+The @code{lib-msvc-compat} module detects whether the linker supports
+@code{--output-def} when building a library.  That parameter is used
+to generate a DEF file for a shared library (DLL).  DEF files are
+useful for developers that use Visual Studio to develop programs that
+links to your library.  See the GNU LD manual for more information.
+
+There are other ways to create a DEF file, but we believe they are all
+sub-optimal to using @code{--output-def} during the build process.
+The variants we have considered include:
+
+@itemize @bullet
+@item Use DUMPBIN /EXPORTS.
+The tool does not generate DEF files directly, so its output needs to
+be post processed manually.  The tool is documented to potentially not
+work with non-MS development tools
+(@url{http://support.microsoft.com/kb/131313/en-us}), which is the
+case when MinGW is used to build the library.
+
+@item Use IMPDEF.
+There is a tool called IMPDEF
+(@url{http://sei.pku.edu.cn/~caodg/course/c/reference/win32/tools/dlltool.html})
+that can generate DEF files.  However, it is not part of a standard
+Visual Studio installation.  Further, it is documented as being an
+unreliable process.
+
+@item Use DLLTOOL.
+The dlltool is part of the MinGW suite, and thus not part of a
+standard Visual Studio installation.  The documentation for the IMPDEF
+tool claims that DLLTOOL is the wrong tool for this job.  Finally,
+DLLTOOL does not generate DEF files directly, so it requires
+post-processing of the output.
+
+@end itemize
+
+If you are using libtool to build your shared library, here is how to
+use this module.  Import @code{lib-msvc-compat} to your project, and
+then add the following lines to the @code{Makefile.am} that builds the
+library:
+
+@smallexample
+if HAVE_LD_OUTPUT_DEF
+libfoo_la_LDFLAGS += -Wl,--output-def,libfoo-$(SOVERSION).def
+defexecdir = $(bindir)
+defexec_DATA = libfoo-$(SOVERSION).def
+DISTCLEANFILES += $(defexec_DATA)
+endif
+@end smallexample
+
+The @code{SOVERSION} variable needs to be defined.  It should be the
+shared library version number used in the DLL filename.  For Windows
+targets you compute this value from the values you pass to Libtool's
+@code{-version-info}.  Assuming you have variables @code{LT_CURRENT}
+and @code{LT_AGE} defined for the @code{CURRENT} and @code{AGE}
+libtool version integers, you compute @code{SOVERSION} as follows:
+
+@smallexample
+SOVERSION=`expr $@{LT_CURRENT@} - $@{LT_AGE@}`
+AC_SUBST(SOVERSION)
+@end smallexample

m4/ld-output-def.m4

@@ -0,0 +1,29 @@
+# ld-output-def.m4 serial 2
+dnl Copyright (C) 2008, 2009 Free Software Foundation, Inc.
+dnl This file is free software; the Free Software Foundation
+dnl gives unlimited permission to copy and/or distribute it,
+dnl with or without modifications, as long as this notice is preserved.
+
+dnl From Simon Josefsson
+
+# gl_LD_OUTPUT_DEF()
+# -------------
+# Check if linker supports -Wl,--output-def and define automake
+# conditional HAVE_LD_OUTPUT_DEF if it is.
+AC_DEFUN([gl_LD_OUTPUT_DEF],
+[
+  AC_CACHE_CHECK([if gcc/ld supports -Wl,--output-def],
+    [gl_cv_ld_output_def],
+    [if test "$enable_shared" = no; then
+       gl_cv_ld_output_def="not needed, shared libraries are disabled"
+     else
+       gl_ldflags_save=$LDFLAGS
+       LDFLAGS="-Wl,--output-def,conftest.def"
+       AC_LINK_IFELSE([AC_LANG_PROGRAM([])],
+                   [gl_cv_ld_output_def=yes],
+                   [gl_cv_ld_output_def=no])
+       rm -f conftest.def
+       LDFLAGS="$gl_ldflags_save"
+     fi])
+  AM_CONDITIONAL([HAVE_LD_OUTPUT_DEF], test "x$gl_cv_ld_output_def" = "xyes")
+])

modules/lib-msvc-compat

@@ -0,0 +1,14 @@
+Description:
+Macros to test whether LD support --output-def.
+
+Files:
+m4/ld-output-def.m4
+
+configure.ac:
+gl_LD_OUTPUT_DEF
+
+License:
+unlimited
+
+Maintainer:
+Simon Josefsson