<sect1 id="dll"><title>Building and Using DLLs</title><para>DLLs are Dynamic Link Libraries, which means that they're linkedinto your program at run time instead of build time.  There are threeparts to a DLL:</para><itemizedlist spacing="compact"><listitem><para> the exports </para></listitem><listitem><para> the code and data </para></listitem><listitem><para> the import library </para></listitem></itemizedlist><para>The code and data are the parts you write - functions,variables, etc.  All these are merged together, like if you werebuilding one big object files, and put into the dll.  They are notput into your .exe at all.</para><para>The exports contains a list of functions and variables that thedll makes available to other programs.  Think of this as the list of"global" symbols, the rest being hidden.  Normally, you'd create thislist by hand with a text editor, but it's possible to do itautomatically from the list of functions in your code.  The<filename>dlltool</filename> program creates the exports section ofthe dll from your text file of exported symbols.</para><para>The import library is a regular UNIX-like<filename>.a</filename> library, but it only contains the tiny bit ofinformation needed to tell the OS how your program interacts with("imports") the dll.  This information is linked into your<filename>.exe</filename>.  This is also generated by<filename>dlltool</filename>.</para><sect2 id="dll-build"><title>Building DLLs</title><para>OK, let's go through a simple example of how to build a dll.For this example, we'll use a single file<filename>myprog.c</filename> for the program(<filename>myprog.exe</filename>) and a single file<filename>mydll.c</filename> for the contents of the dll(<filename>mydll.dll</filename>).</para><para>Fortunately, with the latest gcc and binutils the process for building a dllis now pretty simple. Say you want to build this minimal function in mydll.c:</para><screen>#include &lt;stdio.h&gt;inthello(){  printf ("Hello World!\n");}  </screen><para>First compile mydll.c to object code:</para><screen>gcc -c mydll.c</screen><para>Then, tell gcc that it is building a shared library:</para><screen>gcc -shared -o mydll.dll mydll.o</screen><para>That's it! To finish up the example, you can now link to thedll with a simple program:</para><screen>int main (){  hello ();}  </screen><para>Then link to your dll with a command like:</para><screen>gcc -o myprog myprog.ca -L./ -lmydll</screen><para>However, if you are building a dll as an export library,you will probably want to use the complete syntax:</para><screen>gcc -shared -o cyg${module}.dll \    -Wl,--out-implib=lib${module}.dll.a \    -Wl,--export-all-symbols \    -Wl,--enable-auto-import \    -Wl,--whole-archive ${old_lib} \    -Wl,--no-whole-archive ${dependency_libs}</screen><para>Where ${module} is the name of your DLL, ${old_lib} are allyour object files, bundled together in static libs or single objectfiles and the ${dependency_libs} are import libs you need tolink against, e.g '-lpng -lz -L/usr/local/special -lmyspeciallib'.</para></sect2><sect2 id="dll-link"><title>Linking Against DLLs</title><para>If you have an existing DLL already, you need to build aCygwin-compatible import library.  If you have the source to compilethe DLL, see <Xref Linkend="dll-build"> for details on having <filename>gcc</filename> build one for you.  If you do not have thesource or a supplied working import library, you can get most ofthe way by creating a .def file with these commands (you might need todo this in <filename>bash</filename> for the quoting to workcorrectly):</para><screen>echo EXPORTS > foo.defnm foo.dll | grep ' T _' | sed 's/.* T _//' >> foo.def</screen><para>Note that this will only work if the DLL is not stripped.Otherwise you will get an error message: "No symbols infoo.dll".</para><para>Once you have the <filename>.def</filename> file, you can createan import library from it like this:</para><screen>dlltool --def foo.def --dllname foo.dll --output-lib foo.a</screen></sect2></sect1>