142 lines
4.7 KiB
Plaintext
142 lines
4.7 KiB
Plaintext
<sect1 id="dll"><title>Building and Using DLLs</title>
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<para>DLLs are Dynamic Link Libraries, which means that they're linked
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into your program at run time instead of build time. There are three
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parts to a DLL:</para>
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<itemizedlist spacing="compact">
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<listitem><para> the exports </para></listitem>
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<listitem><para> the code and data </para></listitem>
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<listitem><para> the import library </para></listitem>
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</itemizedlist>
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<para>The code and data are the parts you write - functions,
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variables, etc. All these are merged together, like if you were
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building one big object files, and put into the dll. They are not
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put into your .exe at all.</para>
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<para>The exports contains a list of functions and variables that the
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dll makes available to other programs. Think of this as the list of
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"global" symbols, the rest being hidden. Normally, you'd create this
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list by hand with a text editor, but it's possible to do it
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automatically from the list of functions in your code. The
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<filename>dlltool</filename> program creates the exports section of
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the dll from your text file of exported symbols.</para>
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<para>The import library is a regular UNIX-like
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<filename>.a</filename> library, but it only contains the tiny bit of
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information needed to tell the OS how your program interacts with
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("imports") the dll. This information is linked into your
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<filename>.exe</filename>. This is also generated by
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<filename>dlltool</filename>.</para>
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<sect2 id="dll-build"><title>Building DLLs</title>
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<para>This page gives only a few simple examples of gcc's DLL-building
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capabilities. To begin an exploration of the many additional options,
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see the gcc documentation and website, currently at
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<ulink url="http://gcc.gnu.org/">http://gcc.gnu.org/</ulink>
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</para>
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<para>Let's go through a simple example of how to build a dll.
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For this example, we'll use a single file
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<filename>myprog.c</filename> for the program
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(<filename>myprog.exe</filename>) and a single file
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<filename>mydll.c</filename> for the contents of the dll
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(<filename>mydll.dll</filename>).</para>
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<para>Fortunately, with the latest gcc and binutils the process for building a dll
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is now pretty simple. Say you want to build this minimal function in mydll.c:</para>
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<screen>
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#include <stdio.h>
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int
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hello()
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{
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printf ("Hello World!\n");
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}
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</screen>
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<para>First compile mydll.c to object code:</para>
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<screen>gcc -c mydll.c</screen>
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<para>Then, tell gcc that it is building a shared library:</para>
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<screen>gcc -shared -o mydll.dll mydll.o</screen>
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<para>
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That's it! To finish up the example, you can now link to the
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dll with a simple program:
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</para>
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<screen>
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int
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main ()
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{
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hello ();
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}
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</screen>
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<para>
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Then link to your dll with a command like:
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</para>
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<screen>gcc -o myprog myprog.c -L./ -lmydll</screen>
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<para>However, if you are building a dll as an export library,
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you will probably want to use the complete syntax:</para>
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<screen>gcc -shared -o cyg${module}.dll \
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-Wl,--out-implib=lib${module}.dll.a \
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-Wl,--export-all-symbols \
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-Wl,--enable-auto-import \
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-Wl,--whole-archive ${old_libs} \
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-Wl,--no-whole-archive ${dependency_libs}</screen>
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<para>
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The name of your library is <literal>${module}</literal>, prefixed with
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<literal>cyg</literal> for the DLL and <literal>lib</literal> for the
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import library. Cygwin DLLs use the <literal>cyg</literal> prefix to
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differentiate them from native-Windows MinGW DLLs, see
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<ulink url="http://mingw.org">the MinGW website</ulink> for more details.
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<literal>${old_libs}</literal> are all
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your object files, bundled together in static libs or single object
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files and the <literal>${dependency_libs}</literal> are import libs you
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need to link against, e.g
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<userinput>'-lpng -lz -L/usr/local/special -lmyspeciallib'</userinput>.
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</para>
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</sect2>
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<sect2 id="dll-link"><title>Linking Against DLLs</title>
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<para>If you have an existing DLL already, you need to build a
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Cygwin-compatible import library. If you have the source to compile
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the DLL, see <xref linkend="dll-build"></xref> for details on having
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<filename>gcc</filename> build one for you. If you do not have the
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source or a supplied working import library, you can get most of
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the way by creating a .def file with these commands (you might need to
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do this in <filename>bash</filename> for the quoting to work
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correctly):</para>
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<screen>
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echo EXPORTS > foo.def
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nm foo.dll | grep ' T _' | sed 's/.* T _//' >> foo.def
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</screen>
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<para>Note that this will only work if the DLL is not stripped.
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Otherwise you will get an error message: "No symbols in
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foo.dll".</para>
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<para>Once you have the <filename>.def</filename> file, you can create
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an import library from it like this:</para>
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<screen>
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dlltool --def foo.def --dllname foo.dll --output-lib foo.a
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</screen>
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</sect2>
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</sect1>
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