| <!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"> |
| <html><head><meta http-equiv="Content-Type" content="text/html; charset=ANSI_X3.4-1968"><title>Building and Using DLLs</title><link rel="stylesheet" type="text/css" href="docbook.css"><meta name="generator" content="DocBook XSL Stylesheets Vsnapshot"><link rel="home" href="cygwin-ug-net.html" title="Cygwin User's Guide"><link rel="up" href="programming.html" title="Chapter 4. Programming with Cygwin"><link rel="prev" href="gdb.html" title="Debugging Cygwin Programs"><link rel="next" href="windres.html" title="Defining Windows Resources"></head><body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="3" align="center">Building and Using DLLs</th></tr><tr><td width="20%" align="left"><a accesskey="p" href="gdb.html">Prev</a> </td><th width="60%" align="center">Chapter 4. Programming with Cygwin</th><td width="20%" align="right"> <a accesskey="n" href="windres.html">Next</a></td></tr></table><hr></div><div class="sect1"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a name="dll"></a>Building and Using DLLs</h2></div></div></div><p>DLLs are Dynamic Link Libraries, which means that they're linked |
| into your program at run time instead of build time. There are three |
| parts to a DLL:</p><div class="itemizedlist"><ul class="itemizedlist compact" style="list-style-type: disc; "><li class="listitem"><p> the exports </p></li><li class="listitem"><p> the code and data </p></li><li class="listitem"><p> the import library </p></li></ul></div><p>The code and data are the parts you write - functions, |
| variables, etc. All these are merged together, like if you were |
| building one big object files, and put into the dll. They are not |
| put into your .exe at all.</p><p>The exports contains a list of functions and variables that the |
| dll makes available to other programs. Think of this as the list of |
| "global" symbols, the rest being hidden. Normally, you'd create this |
| list by hand with a text editor, but it's possible to do it |
| automatically from the list of functions in your code. The |
| <code class="filename">dlltool</code> program creates the exports section of |
| the dll from your text file of exported symbols.</p><p>The import library is a regular UNIX-like |
| <code class="filename">.a</code> library, but it only contains the tiny bit of |
| information needed to tell the OS how your program interacts with |
| ("imports") the dll. This information is linked into your |
| <code class="filename">.exe</code>. This is also generated by |
| <code class="filename">dlltool</code>.</p><div class="sect2"><div class="titlepage"><div><div><h3 class="title"><a name="dll-build"></a>Building DLLs</h3></div></div></div><p>This page gives only a few simple examples of gcc's DLL-building |
| capabilities. To begin an exploration of the many additional options, |
| see the gcc documentation and website, currently at |
| <a class="ulink" href="http://gcc.gnu.org/" target="_top">http://gcc.gnu.org/</a> |
| </p><p>Let's go through a simple example of how to build a dll. |
| For this example, we'll use a single file |
| <code class="filename">myprog.c</code> for the program |
| (<code class="filename">myprog.exe</code>) and a single file |
| <code class="filename">mydll.c</code> for the contents of the dll |
| (<code class="filename">mydll.dll</code>).</p><p>Fortunately, with the latest gcc and binutils the process for building a dll |
| is now pretty simple. Say you want to build this minimal function in mydll.c:</p><pre class="screen"> |
| #include <stdio.h> |
|
|
| int |
| hello() |
| { |
| printf ("Hello World!\n"); |
| } |
| </pre><p>First compile mydll.c to object code:</p><pre class="screen">gcc -c mydll.c</pre><p>Then, tell gcc that it is building a shared library:</p><pre class="screen">gcc -shared -o mydll.dll mydll.o</pre><p> |
| That's it! To finish up the example, you can now link to the |
| dll with a simple program: |
| </p><pre class="screen"> |
| int |
| main () |
| { |
| hello (); |
| } |
| </pre><p> |
| Then link to your dll with a command like: |
| </p><pre class="screen">gcc -o myprog myprog.c -L./ -lmydll</pre><p>However, if you are building a dll as an export library, |
| you will probably want to use the complete syntax:</p><pre class="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_libs} \ |
| -Wl,--no-whole-archive ${dependency_libs}</pre><p> |
| The name of your library is <code class="literal">${module}</code>, prefixed with |
| <code class="literal">cyg</code> for the DLL and <code class="literal">lib</code> for the |
| import library. Cygwin DLLs use the <code class="literal">cyg</code> prefix to |
| differentiate them from native-Windows MinGW DLLs, see |
| <a class="ulink" href="http://mingw.org" target="_top">the MinGW website</a> for more details. |
| <code class="literal">${old_libs}</code> are all |
| your object files, bundled together in static libs or single object |
| files and the <code class="literal">${dependency_libs}</code> are import libs you |
| need to link against, e.g |
| <strong class="userinput"><code>'-lpng -lz -L/usr/local/special -lmyspeciallib'</code></strong>. |
| </p></div><div class="sect2"><div class="titlepage"><div><div><h3 class="title"><a name="dll-link"></a>Linking Against DLLs</h3></div></div></div><p>If you have an existing DLL already, you need to build a |
| Cygwin-compatible import library. If you have the source to compile |
| the DLL, see <a class="xref" href="dll.html#dll-build" title="Building DLLs">the section called “Building DLLs”</a> for details on having |
| <code class="filename">gcc</code> build one for you. If you do not have the |
| source or a supplied working import library, you can get most of |
| the way by creating a .def file with these commands (you might need to |
| do this in <code class="filename">bash</code> for the quoting to work |
| correctly):</p><pre class="screen"> |
| echo EXPORTS > foo.def |
| nm foo.dll | grep ' T _' | sed 's/.* T _//' >> foo.def |
| </pre><p>Note that this will only work if the DLL is not stripped. |
| Otherwise you will get an error message: "No symbols in |
| foo.dll".</p><p>Once you have the <code class="filename">.def</code> file, you can create |
| an import library from it like this:</p><pre class="screen"> |
| dlltool --def foo.def --dllname foo.dll --output-lib foo.a |
| </pre></div></div><div class="navfooter"><hr><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="gdb.html">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="programming.html">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="windres.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">Debugging Cygwin Programs </td><td width="20%" align="center"><a accesskey="h" href="cygwin-ug-net.html">Home</a></td><td width="40%" align="right" valign="top"> Defining Windows Resources</td></tr></table></div></body></html> |
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