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4.17 ld and WIN32 (cygwin/mingw)

This section describes some of the win32 specific ld issues. See Command Line Options for detailed description 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
  • –exclude-modules-for-implib
  • –version-script

When auto-export is in operation, ld will export all the non-local (global and common) symbols it finds in a DLL, with the exception of a few symbols known to belong to the system’s runtime and libraries. As it will often not be desirable to export all of a DLL’s symbols, which may include private functions that are not part of any public interface, the command-line options listed above may be used to filter symbols out from the list for exporting. The ‘--output-def’ option can be used in order to see the final list of exported symbols with all exclusions taken into effect.

If ‘--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=0x20000000

_bar = bar
another_foo = abc.dll.afoo
var1 DATA
doo = foo == foo2
eoo DATA == var1

This example defines a DLL with a non-default base address and seven symbols in the export table. The third exported symbol _bar is an alias for the second. The fourth symbol, another_foo is resolved by "forwarding" to another module and treating it as an alias for afoo exported from the DLL ‘abc.dll’. The final symbol var1 is declared to be a data object. The ‘doo’ symbol in export library is an alias of ‘foo’, which gets the string name in export table ‘foo2’. The ‘eoo’ symbol is an data export symbol, which gets in export table the name ‘var1’.

The optional LIBRARY <name> command indicates the internal name of the output DLL. If ‘<name>’ does not include a suffix, the default library suffix, ‘.DLL’ is appended.

When the .DEF file is used to build an application, rather than a library, the NAME <name> command should be used instead of LIBRARY. If ‘<name>’ does not include a suffix, the default executable suffix, ‘.EXE’ is appended.

With either LIBRARY <name> or NAME <name> the optional specification BASE = <number> may be used to specify a non-default base address for the image.

If neither LIBRARY <name> nor NAME <name> is specified, or they specify an empty string, the internal name is the same as the filename specified on the command line.

The complete specification of an export symbol is:

  ( (  ( <name1> [ = <name2> ] )
     | ( <name1> = <module-name> . <external-name>))
  [ @ <integer> ] [NONAME] [DATA] [CONSTANT] [PRIVATE] [== <name3>] ) *

Declares ‘<name1>’ as an exported symbol from the DLL, or declares ‘<name1>’ as an exported alias for ‘<name2>’; or declares ‘<name1>’ as a "forward" alias for the symbol ‘<external-name>’ in the DLL ‘<module-name>’. Optionally, the symbol may be exported by the specified ordinal ‘<integer>’ alias. The optional ‘<name3>’ is the to be used string in import/export table for the symbol.

The optional keywords that follow the declaration indicate:

NONAME: Do not put the symbol name in the DLL’s export table. It will still be exported by its ordinal alias (either the value specified by the .def specification or, otherwise, the value assigned by the linker). The symbol name, however, does remain visible in the import library (if any), unless PRIVATE is also specified.

DATA: The symbol is a variable or object, rather than a function. The import lib will export only an indirect reference to foo as the symbol _imp__foo (ie, foo must be resolved as *_imp__foo).

CONSTANT: Like DATA, but put the undecorated foo as well as _imp__foo into the import library. Both refer to the read-only import address table’s pointer to the variable, not to the variable itself. This can be dangerous. If the user code fails to add the dllimport attribute and also fails to explicitly add the extra indirection that the use of the attribute enforces, the application will behave unexpectedly.

PRIVATE: Put the symbol in the DLL’s export table, but do not put it into the static import library used to resolve imports at link time. The symbol can still be imported using the LoadLibrary/GetProcAddress API at runtime or by by using the GNU ld extension of linking directly to the DLL without an import library.

See ld/deffilep.y in the binutils sources for the full specification of other DEF file statements

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 information.

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 achieve 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, especially 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 three 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.

3. Symbol aliases can only be resolved using an import lib. This is critical when linking against OS-supplied dll’s (eg, the win32 API) in which symbols are usually exported as undecorated aliases of their stdcall-decorated assembly names.

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 many 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.

aligned common symbols

As a GNU extension to the PE file format, it is possible to specify the desired alignment for a common symbol. This information is conveyed from the assembler or compiler to the linker by means of GNU-specific commands carried in the object file’s ‘.drectve’ section, which are recognized by ld and respected when laying out the common symbols. Native tools will be able to process object files employing this GNU extension, but will fail to respect the alignment instructions, and may issue noisy warnings about unknown linker directives.

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