第1节 QGIS源码获取和编译(QGIS2.2)Cygwin

FlexDLL: an implementation of a dlopen-like API for Windows Introduction ------------ Under Windows, DLL ([Dynamically-Linked Libraries](http://en.wikipedia.org/wiki/Dynamic-link_library)) 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`](https://msdn.microsoft.com/en-us/library/windows/desktop/ms684175(v=vs.85).aspx) 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](http://edll.sourceforge.net/) 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`](http://www.opengroup.org/onlinepubs/009695399/functions/dlopen.html) 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. About ----- FlexDLL is distributed under the terms of a zlib/libpng open source [license](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](http://www.lexifi.com), 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](http://ocaml.org) 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). Download -------- - [Source releases](https://github.com/alainfrisch/flexdll/releases). - [Binary release 0.35 (self-extracting installer)](http://alain.frisch.fr/flexdll/flexdll-0.35-setup.exe). - [Binary release 0.35 (.zip file)](http://alain.frisch.fr/flexdll/flexdll-bin-0.35.zip). - [Development version](https://github.com/alainfrisch/flexdll). - [Changelog](CHANGES). **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"`**. Overview -------- 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(); } exit(0); } ```` 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() { api("plug2.torun();"); dump_x(); x = 100; dump_x(); } ```` 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(); x=3 x=100 ```` Simple, isn't it? No **`declspec`** declaration, no import library to deal with,... How it works ------------ Object files (.obj/.o)