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4.8 ld and WIN32 (cygwin/mingw)

This section describes some of the win32 specific ld issues. See Command Line Options for detailed decription of the command line options mentioned here.

import libraries
The standard Windows linker creates and uses so-called import libraries, which contains information for linking to dll's. They are regular static archives and are handled as any other static archive. The cygwin and mingw ports of ld have specific support for creating such libraries provided with the --out-implib command line option.
exporting DLL symbols
The cygwin/mingw ld has several ways to export symbols for dll's.
using auto-export functionality
By default ld exports symbols with the auto-export functionality, which is controlled by the following command line options:
  • –export-all-symbols [This is the default]
  • –exclude-symbols
  • –exclude-libs

If, however, --export-all-symbols is not given explicitly on the command line, then the default auto-export behavior will be disabled if either of the following are true:

  • A DEF file is used.
  • Any symbol in any object file was marked with the __declspec(dllexport) attribute.

using a DEF file
Another way of exporting symbols is using a DEF file. A DEF file is an ASCII file containing definitions of symbols which should be exported when a dll is created. Usually it is named <dll name>.def and is added as any other object file to the linker's command line. The file's name must end in .def or .DEF.
               gcc -o <output> <objectfiles> <dll name>.def

Using a DEF file turns off the normal auto-export behavior, unless the --export-all-symbols option is also used.

Here is an example of a DEF file for a shared library called xyz.dll:

               LIBRARY "xyz.dll" BASE=0x10000000
               _bar = bar

This example defines a base address and three symbols. The third symbol is an alias for the second. For the complete format specification see ld/deffilep.y in the binutils sources.

While linking a shared dll, ld is able to create a DEF file with the --output-def <file> command line option.

Using decorations
Another way of marking symbols for export is to modify the source code itself, so that when building the DLL each symbol to be exported is declared as:
               __declspec(dllexport) int a_variable
               __declspec(dllexport) void a_function(int with_args)

All such symbols will be exported from the DLL. If, however, any of the object files in the DLL contain symbols decorated in this way, then the normal auto-export behavior is disabled, unless the --export-all-symbols option is also used.

Note that object files that wish to access these symbols must not decorate them with dllexport. Instead, they should use dllimport, instead:

               __declspec(dllimport) int a_variable
               __declspec(dllimport) void a_function(int with_args)

This complicates the structure of library header files, because when included by the library itself the header must declare the variables and functions as dllexport, but when included by client code the header must declare them as dllimport. There are a number of idioms that are typically used to do this; often client code can omit the __declspec() declaration completely. See --enable-auto-import and automatic data imports for more imformation.

automatic data imports
The standard Windows dll format supports data imports from dlls only by adding special decorations (dllimport/dllexport), which let the compiler produce specific assembler instructions to deal with this issue. This increases the effort necessary to port existing Un*x code to these platforms, especially for large c++ libraries and applications. The auto-import feature, which was initially provided by Paul Sokolovsky, allows one to omit the decorations to archieve a behavior that conforms to that on POSIX/Un*x platforms. This feature is enabled with the --enable-auto-import command-line option, although it is enabled by default on cygwin/mingw. The --enable-auto-import option itself now serves mainly to suppress any warnings that are ordinarily emitted when linked objects trigger the feature's use.

auto-import of variables does not always work flawlessly without additional assistance. Sometimes, you will see this message

"variable '<var>' can't be auto-imported. Please read the documentation for ld's --enable-auto-import for details."

The --enable-auto-import documentation explains why this error occurs, and several methods that can be used to overcome this difficulty. One of these methods is the runtime pseudo-relocs feature, described below.

For complex variables imported from DLLs (such as structs or classes), object files typically contain a base address for the variable and an offset (addend) within the variable–to specify a particular field or public member, for instance. Unfortunately, the runtime loader used in win32 environments is incapable of fixing these references at runtime without the additional information supplied by dllimport/dllexport decorations. The standard auto-import feature described above is unable to resolve these references.

The --enable-runtime-pseudo-relocs switch allows these references to be resolved without error, while leaving the task of adjusting the references themselves (with their non-zero addends) to specialized code provided by the runtime environment. Recent versions of the cygwin and mingw environments and compilers provide this runtime support; older versions do not. However, the support is only necessary on the developer's platform; the compiled result will run without error on an older system.

--enable-runtime-pseudo-relocs is not the default; it must be explicitly enabled as needed.

direct linking to a dll
The cygwin/mingw ports of ld support the direct linking, including data symbols, to a dll without the usage of any import libraries. This is much faster and uses much less memory than does the traditional import library method, expecially when linking large libraries or applications. When ld creates an import lib, each function or variable exported from the dll is stored in its own bfd, even though a single bfd could contain many exports. The overhead involved in storing, loading, and processing so many bfd's is quite large, and explains the tremendous time, memory, and storage needed to link against particularly large or complex libraries when using import libs.

Linking directly to a dll uses no extra command-line switches other than -L and -l, because ld already searches for a number of names to match each library. All that is needed from the developer's perspective is an understanding of this search, in order to force ld to select the dll instead of an import library.

For instance, when ld is called with the argument -lxxx it will attempt to find, in the first directory of its search path,

          cygxxx.dll (*)

before moving on to the next directory in the search path.

(*) Actually, this is not cygxxx.dll but in fact is <prefix>xxx.dll, where <prefix> is set by the ld option --dll-search-prefix=<prefix>. In the case of cygwin, the standard gcc spec file includes --dll-search-prefix=cyg, so in effect we actually search for cygxxx.dll.

Other win32-based unix environments, such as mingw or pw32, may use other <prefix>es, although at present only cygwin makes use of this feature. It was originally intended to help avoid name conflicts among dll's built for the various win32/un*x environments, so that (for example) two versions of a zlib dll could coexist on the same machine.

The generic cygwin/mingw path layout uses a bin directory for applications and dll's and a lib directory for the import libraries (using cygwin nomenclature):

          	libxxx.dll.a   (in case of dll's)
          	libxxx.a       (in case of static archive)

Linking directly to a dll without using the import library can be done two ways:

1. Use the dll directly by adding the bin path to the link line

          gcc -Wl,-verbose  -o a.exe -L../bin/ -lxxx

However, as the dll's often have version numbers appended to their names (cygncurses-5.dll) this will often fail, unless one specifies -L../bin -lncurses-5 to include the version. Import libs are generally not versioned, and do not have this difficulty.

2. Create a symbolic link from the dll to a file in the lib directory according to the above mentioned search pattern. This should be used to avoid unwanted changes in the tools needed for making the app/dll.

          ln -s bin/cygxxx.dll lib/[cyg|lib|]xxx.dll[.a]

Then you can link without any make environment changes.

          gcc -Wl,-verbose  -o a.exe -L../lib/ -lxxx

This technique also avoids the version number problems, because the following is perfectly legal

          	libxxx.dll.a -> ../bin/cygxxx-5.dll

Linking directly to a dll without using an import lib will work even when auto-import features are exercised, and even when --enable-runtime-pseudo-relocs is used.

Given the improvements in speed and memory usage, one might justifiably wonder why import libraries are used at all. There are two reasons:

1. Until recently, the link-directly-to-dll functionality did not work with auto-imported data.

2. Sometimes it is necessary to include pure static objects within the import library (which otherwise contains only bfd's for indirection symbols that point to the exports of a dll). Again, the import lib for the cygwin kernel makes use of this ability, and it is not possible to do this without an import lib.

So, import libs are not going away. But the ability to replace true import libs with a simple symbolic link to (or a copy of) a dll, in most cases, is a useful addition to the suite of tools binutils makes available to the win32 developer. Given the massive improvements in memory requirements during linking, storage requirements, and linking speed, we expect that many developers will soon begin to use this feature whenever possible.

symbol aliasing
adding additional names
Sometimes, it is useful to export symbols with additional names. A symbol foo will be exported as foo, but it can also be exported as _foo by using special directives in the DEF file when creating the dll. This will affect also the optional created import library. Consider the following DEF file:
               LIBRARY "xyz.dll" BASE=0x61000000
               _foo = foo

The line _foo = foo maps the symbol foo to _foo.

Another method for creating a symbol alias is to create it in the source code using the "weak" attribute:

               void foo () { /* Do something.  */; }
               void _foo () __attribute__ ((weak, alias ("foo")));

See the gcc manual for more information about attributes and weak symbols.

renaming symbols
Sometimes it is useful to rename exports. For instance, the cygwin kernel does this regularly. A symbol _foo can be exported as foo but not as _foo by using special directives in the DEF file. (This will also affect the import library, if it is created). In the following example:
               LIBRARY "xyz.dll" BASE=0x61000000
               _foo = foo

The line _foo = foo maps the exported symbol foo to _foo.

Note: using a DEF file disables the default auto-export behavior, unless the --export-all-symbols command line option is used. If, however, you are trying to rename symbols, then you should list all desired exports in the DEF file, including the symbols that are not being renamed, and do not use the --export-all-symbols option. If you list only the renamed symbols in the DEF file, and use --export-all-symbols to handle the other symbols, then the both the new names and the original names for the renamed symbols will be exported. In effect, you'd be aliasing those symbols, not renaming them, which is probably not what you wanted.

weak externals
The Windows object format, PE, specifies a form of weak symbols called weak externals. When a weak symbol is linked and the symbol is not defined, the weak symbol becomes an alias for some other symbol. There are three variants of weak externals: As a GNU extension, weak symbols that do not specify an alternate symbol are supported. If the symbol is undefined when linking, the symbol uses a default value.