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FlexDLL: an implementation of a dlopen-like API for Windows

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Under Windows, DLL (Dynamically-Linked Libraries) are generally used to improve code modularity and sharing. A DLL can be loaded automatically when the program is loaded (if it requires the DLL). The program can also explicitly request Windows to load a DLL at any moment during runtime, using the LoadLibrary function from the Win32 API.

This naturally suggests to use DLLs as a plugin mechanism. For instance, a web server could load extensions modules stored in DLLs at runtime. But Windows does not really make it easy to implement plugins that way. The reason is that when you try to create a DLL from a set of object files, the linker needs to resolve all the symbols, which leads to the very problem solved by FlexDLL:

Windows DLL cannot refer to symbols defined in the main application or in previously loaded DLLs.

Some usual solutions exist, but they are not very flexible. A notable exception is the edll library (its homepage also describes the usual solutions), which follows a rather drastic approach; indeed, edll implements a new dynamic linker which can directly load object files (without creating a Windows DLL).

FlexDLL is another solution to the same problem. Contrary to edll, it relies on the native static and dynamic linkers. Also, it works both with the Microsoft environment (MS linker, Visual Studio compilers) and with Cygwin (GNU linker and compilers, in Cygwin or MinGW mode). Actually, FlexDLL implements mostly the usual dlopen POSIX API, without trying to be fully conformant though (e.g. it does not respect the official priority ordering for symbol resolution). This should make it easy to port applications developped for Unix.


FlexDLL is distributed under the terms of a zlib/libpng open source license. The copyright holder is the Institut National de Recherche en Informatique et en Automatique (INRIA). The project was started when I (= Alain Frisch) was working at INRIA. I'm now working for LexiFi, which is kind enough to let me continue my work on FlexDLL. My office mate at INRIA, Jean-Baptiste Tristan, coined the name FlexDLL.

The runtime support library is written in C. The flexlink wrapper is implemented in the wonderful OCaml language.

Supported toolchains

MSVC: the 32-bit C compiler from Microsoft.

MSVC64: the 64-bit C compiler from Microsoft.

CYGWIN: the 32-bit gcc compiler shipped with Cygwin.

MINGW: the 32-bit gcc compiler from the Mingw64 project, packaged in Cygwin (as i686-w64-mingw32-gcc).

MINGW64: the 64-bit gcc compiler from the Mingw64 project, packaged in Cygwin (as x86_64-w64-mingw32-gcc).

LD: an internal linker to produce .dll (only).


Installation instructions: Simply run the installer and add the resulting directory (e.g. C:\Program Files\flexdll or C:\Program Files (x86)\flexdll) to the PATH. You can also create this directory by hand and unzip the .zip file in it.

Compiling from sources: To compile the code from sources, you'll need a working installation of OCaml, GNU Make, and a C toolchain (compiler + linker) either the one from Microsoft (any version of Visual Studio should work), Cygwin, or Mingw. It is probably a good idea to use a native version of ocamlopt (not the Cygwin port) to compile flexlink. By default, the Makefile will compile support objects for the supported toolchains; you can choose a subset with the CHAINS variable, e.g.: make CHAINS="mingw msvc".


FlexDLL has two components: a wrapper around the static linker, and a tiny runtime library to be linked with the main application. The wrapper must be called in place of the normal linker when you want to produce a DLL or to link the main application. The runtime library relies internally on the native LoadLibrary API to implement a dlopen-like interface.

Let's see a simple example of a plugin. Here is the code for the main program (file dump.c):

    #include <stdlib.h>
    #include "flexdll.h"

    typedef void torun();

    void api(char *msg){ printf("API: %s\n", msg); }

    int main(int argc, char **argv)
      void *sym;
      void *handle;
      int i;
      torun *torun;

      for (i = 1; i < argc; i++) {
        handle = flexdll_dlopen(argv[i], FLEXDLL_RTLD_GLOBAL);

        if (NULL == handle) { printf("error: %s\n", flexdll_dlerror()); exit(2); }

        torun = flexdll_dlsym(handle, "torun");
        if (torun) torun();

This application opens in turn all the DLLs given on its command line, using the FlexDLL function flexdll_dlopen. For each DLL, the program looks for a symbol named torun (which is supposed to refer to a function) and if the symbol is available in the DLL, the function is called. The program also provides a very simple API to its plugin: the api function. The FLEX_RTLD_GLOBAL flag makes all the symbols exported by each DLL available for the DLL to be loaded later.

This main program can be compiled and linked like the commands below (the [...] refers to the directory where FlexDLL is installed).

    # MSVC
    cl /nologo /MD -I[...] -c dump.c
    flexlink -chain msvc -exe -o dump.exe dump.obj

    # MINGW
    i686-w64-mingw32-gcc -I[...] -c dump.c
    flexlink -chain mingw -exe -o dump.exe dump.o

    # CYGWIN
    gcc -I[...] -c dump.c
    flexlink -chain cygwin -exe -o dump.exe dump.o

The compilation step is completely standard, but in order to link the main application, you must call the flexlink tool, which is the wrapper around the linker. The -chain command line switch selects which linker to use, and the -exe option tells the wrapper that it must produce a stand-alone application (not a DLL).

Now we can provide a first plugin (file plug1.c):

    int x = 3;
    void dump_x() { printf("x=%i\n", x); }
    void torun() { api("plug1.torun();"); }

Note that the plugin uses the api symbol from the main application (it would be cleaner to introduce it with an extern declaration). You can compile and link this plugin (into a DLL) with the followind commands:

    # MSVC
    cl /nologo /MD -c plug1.c
    flexlink -chain msvc -o plug1.dll plug1.obj

    # MINGW
    i686-w64-mingw32-gcc -c plug1.c
    flexlink -chain mingw -o plug1.dll plug1.o

    # CYGWIN
    gcc -D_CYGWIN_  -c plug1.c
    flexlink -chain cygwin -o plug1.dll plug1.o

And now you can ask the main program to load the plugin:

    $ ./dump plug1.dll
    API: plug1.torun();

Here is the code for a second plugin (file plug2.c) that refers to symbols (a function and a global variable) defined in the first plugin:

    extern int x;

    void torun() {

      x = 100;

Since the second plugin depends on the first one, you need to load both:

    $ ./dump plug2.dll
    error: Cannot resolve dump_x
    $ ./dump plug1.dll plug2.dll;
    API: plug1.torun();
    API: plug2.torun();

Simple, isn't it? No declspec declaration, no import library to deal with,...

How it works

Object files (.obj/.o) contain relocation information that explain to the linker how some addresses in their code or data sections have to be patched, using the value of some global symbols. When the static linker is invoked to produce a DLL from a set of object files, it assumes that all the relocations can be performed: all the symbols which are used in relocations must be defined in some the objects linked together. FlexDLL drops this constraint following a very simple idea: when a relocation refers to a symbol which is not available, the relocation is turned into a piece of data that will be passed to the runtime support library.

In the example above, the plug1.obj object refers to a symbol api. When this object is turned into a DLL, FlexDLL produce a new temporary object file derived from plug1.obj without the relocation that mention api. Instead, it adds an ``import table'', which is just a piece of data that tells the FlexDLL support library which address has to be patched with the value of a symbol called api to be found somewhere else. You can see the list of such imported symbols by adding the -show-imports option to the flexlink command line:

    $ ../flexlink -chain msvc -o plug1.dll plug1.obj -show-imports
    ** Imported symbols for plug1.obj:

When the flexdll_dlopen function opens this DLL, it will look for an internal symbol that points to the import table, resolve the symbols and patch the code and data segments accordingly. The FlexDLL runtime library must thus maintain a set of symbols together with their concrete value (address). In particular, it knows about the global symbols defined in the main program. Indeed, when you link the main program with flexlink -exe, the wrapper produces a small fresh object file that contains a symbol table, mapping symbol names to their addresses.

    $ ../flexlink -chain msvc -exe -o dump.exe dump.obj -show-exports
    ** Exported symbols:

As you can see, all the global symbols (including those that comes from FlexDLL itself) appear in the global symbol table. FlexDLL knows not only about symbols that comes from the main program, but also about symmbols exported by the DLL it loads. This is needed to implement the flexdll_dlsym function, but also to deal with import tables that mentions symbols defined in previously loaded DLLs (for which the FLEXDLL_RTLD_GLOBAL was used). So the wrapper does not only produce an import table for DLLs, but also an export table:

    $ ../flexlink -chain msvc -o plug1.dll plug1.obj -show-imports -show-exports
    ** Imported symbols for plug1.obj:
    ** Exported symbols:

    $ ../flexlink -chain msvc -o plug2.dll plug2.obj -show-imports -show-exports
    ** Imported symbols for plug2.obj:
    ** Exported symbols:

How does FlexDLL determine which symbols as imported or exported? It uses an algorithm similar to the linker itself. The command line mentions a number of object and library files. In a first pass, the wrapper computed which objects embedded in those libraries will be used. To do that, it looks at which symbols are used, and where they are defined. Then the wrapper consider that all the global (external) symbols are exported. Note that the /export or __declspec(dllexport) directives are not used: all the symbols are exported. (In a future version, FlexDLL will allow to control more precisely which symbols are exported). All the object files (given explicitly, or embedded in a library) that need to import symbols must be rewritten. The flexlink wrapper will produce new tempory object files for them. If you want to understand better how FlexDLL works, you can use the -v and -save-temps command options to tell the wrapper to show you the linker command line and to keep those temporary files alive (by default, there are removed automatically).

Some object files can mention default libraries (they correspond to the /defautlib linker flag, which is often embedded in the object .drectve section). FlexDLL will parse those libraries, but only to see which symbols they define. Those symbols are not considered as being imported by the DLL, but they won't be exported either. A typical case of default libraries are import libraries that behave as interfaces to (normal, non-FlexDLL) DLLs.

Advanced topic: __declspec(dllimport)

C compilers under Windows support a special declaration of external symbols. You can write:

    __declspec(dllimport) extern int mysymbol;

Internally, this declaration has the same effect as declaring:

    extern int *_imp__mysymbol;

and using &x instead of x everywhere in the current unit. In other words, even if your code seems to access x directly, each access actually goes through an extra indirection.

FlexDLL knows about this convention. When a object refers to a symbol of the form _imp__XXX which is not available in the objects that will form the DLL to be created, it resists the temptation of putting an entry for _imp__XXX in the import table. Instead, it adds the equivalent of the following declaration:

    void *_imp__XXX = &XXX;

If the symbol XXX itself is not available, this will in turn produce an entry for XXX in the import table. All these new declarations are put in the same object file that contain the export table, which is global for the DLL to be produced. So, if all the external symbols in a given object files are accessed through this convention, the object file needs not be patched at all.

Note that you can define and use the _imp__XXX symbols by hand, you don't use to use the __declspec(dllimport) notation (this is useful if you use a compiler that doesn't support this notation).

Anyway, there is no compelling reason for adopting this style. A very small advantage might be that there will be fewer relocations at runtime and that more code pages can be shared amongst several instances of the same DLL used by different processes.

Advanced topic: static constructors and the entry point

A Windows DLL can define an optional entry point. When the DLL is loaded, this function is automatically called. (The same function is called when the DLL is unloaded, or when threads are spawned or destroyed.)

Usually, the real entry point is provided by the C runtime library: _cygwin_dll_entry for Cygwin, DllMainCRTStartup for Mingw, _DllMainCRTStartup for MSVC. These functions perform various initialization for the C runtime library, invoke the code that has to be run automatically at load time (e.g for C++: constructors of static objects, or right-hand sides of non-constant initializers for global variables), and then call the function DllMain, which by default does nothing but can be overidden by the program to perform custom initialization of the DLL.

FlexDLL must take control before any custom code (static constructors, DllMain) so as to perform relocations (in case this code refers to symbols found in the main program or previously loaded DLLs). As a consquence, FlexDLL defines its own entry point, which first asks the main program to perform relocations and then calls the regular entry point of the C runtime library. This behavior is implemented in the flexdll_initer.c file and the corresponding object file (whose name depends on the toolchain) is automatically included by flexlink.

It is possible to completely disable the DLL entry point with the -noentry option passed to flexlink. In this case, FlexDLL will perform relocations after the DLL has been opened.


Here is the content of the flexdll.h file:

    #define FLEXDLL_RTLD_GLOBAL 0x0001
    #define FLEXDLL_RTLD_LOCAL  0x0000
    #define FLEXDLL_RTLD_NOEXEC 0x0002

    void *flexdll_dlopen(const char *, int);
#ifndef CYGWIN
    void *flexdll_wdlopen(const wchar_t *, int);
    void *flexdll_dlsym(void *, const char *);
    void flexdll_dlclose(void *);
    char *flexdll_dlerror(void);

    void flexdll_dump_exports(void *);
    void flexdll_dump_relocations(void *);

The flexdll_dl* functions are mostly compatible with their POSIX counterparts. Here is a short explanation of their semantics.

The most important function is flexdll_dlopen. The first argument gives the filename of a DLL to be opened. This DLL must have been produced by the flexlink wrapper. The function resolves the symbols mentioned in the DLL's import table and performs the relocations. The second argument is a mode, made of flags that can be or'ed together. The flag FLEXDLL_RTLD_GLOBAL means that the symbols exported by the opened DLL can be used to resolve relocations for DLLs to be opened later on. The flag FLEXDLL_RTLD_NOEXEC opens the DLL is a special mode, disabling the automatic loaded of dependencies and the FlexDLL resolution pass. This is useful if you want to open a DLL only to check whether it defines some symbol.

The flexdll_dlopen function returns a pointer to an opaque handle that can be used as an argument to the other API functions. If the filename is NULL, the function returns a special handle which refers to the global unit: it includes all the static symbols, plus the symbols from the DLLs openend with the FLEXDLL_RTLD_GLOBAL flag. A given DLL will be opened only once, even if you call the function several times on the same file. The FLEXDLL_RTLD_GLOBAL flag is sticky: if one of the calls mentions it, it will stay forever, even if the corresponding handle is then passed to dlclose (this is because the same handle is actually returned for all the calls). If an error occurs during the call to flexdll_dlopen, the functions returns NULL and the error message can be retrieved using flexdll_dlerror.

The function flexdll_wdlopen is a wide-character version of flexdll_dlopen. The filename argument to flexdll_wdlopen is a wide-character string. flexdll_wdlopen and flexdll_dlopen behave identically otherwise.

The second most important function is flexdll_dlsym which looks for a symbol whose name is the second argument. The first argument can be either a regular handle returned by flexdll_dlopen (the symbol is searched only in the corresponding DLL), the special handle for the global unit as return by a call to flexdll_dlopen(NULL,...) (the symbol is searched amongst the static symbols plus the ones in the DLL openeded with the flag FLEXDLL_RTLD_GLOBAL), or NULL (the symbol is searched only amongst the static symbols). If the symbol cannot be found, the function returns NULL.

The same symbol name can be defined several times. The policy used to choose amongst the various definitions is not specified. This applies both to the automatic resolution that happens when you open a DLL and to the explicit resolution performed by dlsym.

The function flexdll_dlclose must be used with caution. It decrements the reference counter to a given handle and release the DLL from memory when the counter reaches 0. After that time, symbols defined in this DLL are no longer used for resolution. You most probably don't want to close a DLL if you still hold pointers to some of its symbols.

The two functions flexdll_dump_exports and flexdll_dump_relocations are used to display (to the standard output) the internal tables associated to a given DLL handle.

Command line for the flexlink wrapper

      flexlink -o <result.dll> file1.obj file2.obj ... -- <extra linker arguments>

      -o                  Choose the name of the output file
      -exe                Link the main program as an exe file
      -maindll            Link the main program as a dll file
      -noflexdllobj       Do not add the Flexdll runtime object (for exe)
      -noentry            Do not use the Flexdll entry point (for dll)
      -noexport           Do not export any symbol
      -I <dir>            Add a directory where to search for files
      -L <dir>            Add a directory where to search for files
      -l <lib>            Library file
      -chain {msvc|msvc64|cygwin|mingw|mingw64|ld}
                          Choose which linker to use
      -defaultlib <obj>   External object (no export, no import)
      -save-temps         Do not delete intermediate files
      -implib             Do not delete the generated import library
      -outdef             Produce a def file with exported symbols
      -v                  Increment verbosity (can be repeated)
      -show-exports       Show exported symbols
      -show-imports       Show imported symbols
      -dry                Show the linker command line, do not actually run it
      -dump               Only dump the content of object files
      -nocygpath          Do not use cygpath (default for msvc)
      -cygpath            Use cygpath (default for cygwin)
      -no-merge-manifest  Do not merge the manifest (takes precedence over -merge-manifest)
      -merge-manifest     Merge manifest to the dll or exe (if generated)
      -real-manifest      Use the generated manifest (default behavior)
      -default-manifest   Use the default manifest (default.manifest/default_amd64.manifest)
      -export <sym>       Explicitly export a symbol
      -noreexport         Do not reexport symbols imported from import libraries
      -where              Show the FlexDLL directory
      -nounderscore       Normal symbols are not prefixed with an underscore
      -nodefaultlibs      Do not assume any default library
      -builtin            Use built-in linker to produce a dll
      -explain            Explain why library objects are linked
      -subsystem <id>     Set the subsystem (default: console)
      -custom-crt         Use a custom CRT
      -link <option>      Next argument is passed verbatim to the linker
      -D <symbol>         (Ignored)
      -U <symbol>         (Ignored)
      --                  Following arguments are passed verbatim to the linker
      -help               Display this list of options
      --help              Display this list of options

The files given on the command line can be object files (.obj/.o), library files (.a/.lib), or C files (.c). C files will be compiled using the toolchain's C compiler and the resulting object will be used for the actual linking.

The argument for the -l, -I and -L options does not need to be separated with a whitespace from the option (i.e. -LXXX is equivalent to -L XXX).

There is a single set of search directories for all kinds of files. The -I and -L options are synonyms.

As usual, object files included in libraries are linked in only if one of the symbol they export is needed.

The -export option explicitly exports a symbol. If this symbols is found in a library, it will force the corresponding object file to be included.

The -defaultlib option is used to tell flexlink that some object (usually, a library) does not need any relocation (that is, all the symbols it refers to are defined in one of the objects being linked) and that we don't want to re-export the symbols exported by this object. This is usually used for system libraries.

The -cygpath and -nocygpath options control whether flexlink uses the cygpath command or not (default is: no under Msvc, yes underder Cygwin/MinGW/MinGW64 if cygpath can be found in the PATH). When it uses cygpath, flexlink tries to resolve file names directly and otherwise calls cygpath -m (to produce Windows paths from Cygwin paths) if cygpath is available in the path.

The -maindll option is used to build a DLL that behaves as the main program from the point of view of FlexDLL. It cannot have unresolved symbols.

By default, flexlink looks for FlexDLL's object files in the same directory as flexlink.exe itself. It is possible to specify another directory with the FLEXDIR environment variable.

Extra arguments can be passed to flexlink.exe through the environment variable FLEXLINKFLAGS. The arguments coming from this variable are parsed before those coming from the command line.


FlexDLL performs relocations at runtime in the code of the DLL. The good consequence is that there is no indirection: a function call or a reference to a global variable where the target symbol is not in the current DLL be compiled as if it were. This might improve performance, especially because an indirection would consume a register that might be better used for something more interesting. The bad consequence is that the memory pages that contains relocations cannot be shared between different processes.

Bugs, limitations

FlexDLL relies on a parser and generator for COFF files. However, some features are not very well specified, and some well specified features have not been fully implemented. Normally, you should get some assertion failure in these cases. Please report them, so that I can improve FlexDLL.

FlexDLL works for 32 and 64 bits version of Windows. The 32 bits version has been tested under XP and Vista, with the three supported toolchains. The 64 bits version has been tested under Windows Vista x64 with the Microsoft Platform SDK and under Windows 7 64-bit with the Win7 SDK toolchain (no Cygwin).

Real-world examples

Please let me know if you use FlexDLL!

Dynamic loading for OCaml

The initial motivation for FlexDLL was to add dynamic linking of native code to Windows ports of OCaml (Cygwin, MinGW, MSVC). A side-effect was to simplify the dynamic loading of C libraries (e.g. for the toplevel) and to make it work under the Cygwin port, to simplify Makefiles of libraries (now shared between Unix and Windows ports), and to create a native toplevel.