<!-- doc/src/sgml/dfunc.sgml -->

<sect2 id="dfunc">
 <title>Compiling and Linking Dynamically-loaded Functions</title>

 <para>
  Before you are able to use your
  <productname>PostgreSQL</productname> extension functions written in
  C, they must be compiled and linked in a special way to produce a
  file that can be dynamically loaded by the server.  To be precise, a
  <firstterm>shared library</firstterm> needs to be
  created.<indexterm><primary>shared library</></indexterm>

 </para>

 <para>
  For information beyond what is contained in this section
  you should read the documentation of your
  operating system, in particular the manual pages for the C compiler,
  <command>cc</command>, and the link editor, <command>ld</command>.
  In addition, the <productname>PostgreSQL</productname> source code
  contains several working examples in the
  <filename>contrib</filename> directory.  If you rely on these
  examples you will make your modules dependent on the availability
  of the <productname>PostgreSQL</productname> source code, however.
 </para>

 <para>
  Creating shared libraries is generally analogous to linking
  executables: first the source files are compiled into object files,
  then the object files are linked together.  The object files need to
  be created as <firstterm>position-independent code</firstterm>
  (<acronym>PIC</acronym>),<indexterm><primary>PIC</></> which
  conceptually means that they can be placed at an arbitrary location
  in memory when they are loaded by the executable.  (Object files
  intended for executables are usually not compiled that way.)  The
  command to link a shared library contains special flags to
  distinguish it from linking an executable (at least in theory
  &mdash; on some systems the practice is much uglier).
 </para>

 <para>
  In the following examples we assume that your source code is in a
  file <filename>foo.c</filename> and we will create a shared library
  <filename>foo.so</filename>.  The intermediate object file will be
  called <filename>foo.o</filename> unless otherwise noted.  A shared
  library can contain more than one object file, but we only use one
  here.
 </para>

<!--
  Note: Reading GNU Libtool sources is generally a good way of
  figuring out this information.  The methods used within PostgreSQL
  source code are not necessarily ideal.
-->

  <variablelist>
   <varlistentry>
    <term>
     <systemitem class="osname">FreeBSD</>
     <indexterm><primary>FreeBSD</><secondary>shared library</></>
    </term>
    <listitem>
     <para>
      The compiler flag to create <acronym>PIC</acronym> is
      <option>-fpic</option>.  To create shared libraries the compiler
      flag is <option>-shared</option>.
<programlisting>
gcc -fpic -c foo.c
gcc -shared -o foo.so foo.o
</programlisting>
      This is applicable as of version 3.0 of
      <systemitem class="osname">FreeBSD</>.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term>
     <systemitem class="osname">HP-UX</>
     <indexterm><primary>HP-UX</><secondary>shared library</></>
    </term>
    <listitem>
     <para>
      The compiler flag of the system compiler to create
      <acronym>PIC</acronym> is <option>+z</option>.  When using
      <application>GCC</application> it's <option>-fpic</option>. The
      linker flag for shared libraries is <option>-b</option>.  So:
<programlisting>
cc +z -c foo.c
</programlisting>
      or:
<programlisting>
gcc -fpic -c foo.c
</programlisting>
      and then:
<programlisting>
ld -b -o foo.sl foo.o
</programlisting>
      <systemitem class="osname">HP-UX</> uses the extension
      <filename>.sl</filename> for shared libraries, unlike most other
      systems.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term>
     <systemitem class="osname">Linux</>
     <indexterm><primary>Linux</><secondary>shared library</></>
    </term>
    <listitem>
     <para>
      The compiler flag to create <acronym>PIC</acronym> is
      <option>-fpic</option>.  On some platforms in some situations
      <option>-fPIC</option> must be used if <option>-fpic</option>
      does not work.  Refer to the GCC manual for more information.
      The compiler flag to create a shared library is
      <option>-shared</option>.  A complete example looks like this:
<programlisting>
cc -fpic -c foo.c
cc -shared -o foo.so foo.o
</programlisting>
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term>
     <systemitem class="osname">macOS</>
     <indexterm><primary>macOS</><secondary>shared library</></>
    </term>
    <listitem>
     <para>
      Here is an example.  It assumes the developer tools are installed.
<programlisting>
cc -c foo.c
cc -bundle -flat_namespace -undefined suppress -o foo.so foo.o
</programlisting>
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term>
     <systemitem class="osname">NetBSD</>
     <indexterm><primary>NetBSD</><secondary>shared library</></>
    </term>
    <listitem>
     <para>
      The compiler flag to create <acronym>PIC</acronym> is
      <option>-fpic</option>.  For <acronym>ELF</acronym> systems, the
      compiler with the flag <option>-shared</option> is used to link
      shared libraries.  On the older non-ELF systems, <literal>ld
      -Bshareable</literal> is used.
<programlisting>
gcc -fpic -c foo.c
gcc -shared -o foo.so foo.o
</programlisting>
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term>
     <systemitem class="osname">OpenBSD</>
     <indexterm><primary>OpenBSD</><secondary>shared library</></>
    </term>
    <listitem>
     <para>
      The compiler flag to create <acronym>PIC</acronym> is
      <option>-fpic</option>.  <literal>ld -Bshareable</literal> is
      used to link shared libraries.
<programlisting>
gcc -fpic -c foo.c
ld -Bshareable -o foo.so foo.o
</programlisting>
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term>
     <systemitem class="osname">Solaris</>
     <indexterm><primary>Solaris</><secondary>shared library</></>
    </term>
    <listitem>
     <para>
      The compiler flag to create <acronym>PIC</acronym> is
      <option>-KPIC</option> with the Sun compiler and
      <option>-fpic</option> with <application>GCC</>.  To
      link shared libraries, the compiler option is
      <option>-G</option> with either compiler or alternatively
      <option>-shared</option> with <application>GCC</>.
<programlisting>
cc -KPIC -c foo.c
cc -G -o foo.so foo.o
</programlisting>
      or
<programlisting>
gcc -fpic -c foo.c
gcc -G -o foo.so foo.o
</programlisting>
     </para>
    </listitem>
   </varlistentry>

  </variablelist>

 <tip>
  <para>
   If this is too complicated for you, you should consider using
   <ulink url="http://www.gnu.org/software/libtool/">
   <productname>GNU Libtool</productname></ulink>,
   which hides the platform differences behind a uniform interface.
  </para>
 </tip>

 <para>
  The resulting shared library file can then be loaded into
  <productname>PostgreSQL</productname>.  When specifying the file name
  to the <command>CREATE FUNCTION</command> command, one must give it
  the name of the shared library file, not the intermediate object file.
  Note that the system's standard shared-library extension (usually
  <literal>.so</literal> or <literal>.sl</literal>) can be omitted from
  the <command>CREATE FUNCTION</command> command, and normally should
  be omitted for best portability.
 </para>

 <para>
  Refer back to <xref linkend="xfunc-c-dynload"> about where the
  server expects to find the shared library files.
 </para>

<!--
Under AIX, object files are compiled normally but building the shared
library requires a couple of steps.  First, create the object file:
.nf
cc <other flags> -c foo.c
.fi
You must then create a symbol \*(lqexports\*(rq file for the object
file:
.nf
mkldexport foo.o `pwd` &gt; foo.exp
.fi
Finally, you can create the shared library:
.nf
ld <other flags> -H512 -T512 -o foo.so -e _nostart \e
   -bI:.../lib/postgres.exp -bE:foo.exp foo.o \e
   -lm -lc 2>/dev/null
.fi
  -->

</sect2>
