path: root/doc/user/libraries.in

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  <para>

  It's often useful to organize large software projects
  by collecting parts of the software into one or more libraries.
  &SCons; makes it easy to create libraries
  and to use them in the programs.

  </para>

  <section>
  <title>Building Libraries</title>

    <para>

    You build your own libraries by specifying &Library;
    instead of &Program;:

    </para>

    <scons_example name="ex1" printme="1">
      <file name="SConstruct" printme="1">
      Library('foo', ['f1.c', 'f2.c', 'f3.c'])
      </file>
      <file name="f1.c">
      void f1() { printf("f1.c\n"); }
      </file>
      <file name="f2.c">
      void f2() { printf("f2.c\n"); }
      </file>
      <file name="f3.c">
      void f3() { printf("f3.c\n"); }
      </file>
    </scons_example>

    <para>

    &SCons; uses the appropriate library prefix and suffix for your system.
    So on POSIX or Linux systems,
    the above example would build as follows
    (although &ranlib may not be called on all systems):

    </para>

    <scons_output example="ex1" os="posix">
      <command>scons -Q</command>
    </scons_output>

    <para>

    On a Windows system,
    a build of the above example would look like:

    </para>

    <scons_output example="ex1" os="win32">
      <command>scons -Q</command>
    </scons_output>

    <para>

    The rules for the target name of the library
    are similar to those for programs:
    if you don't explicitly specify a target library name,
    &SCons; will deduce one from the
    name of the first source file specified,
    and &SCons; will add an appropriate
    file prefix and suffix if you leave them off.

    </para>

    <section>
    <title>Building Static Libraries Explicitly</title>

      <para>

      The &Library; function builds a traditional static library.
      If you want to be explicit about the type of library being built,
      you can use the synonym &StaticLibrary; function
      instead of &Library:

      </para>

      <scons_example name="StaticLibrary" printme="1">
        <file name="SConstruct" printme="1">
        StaticLibrary('foo', ['f1.c', 'f2.c', 'f3.c'])
        </file>
      </scons_example>

      <para>

      There is no functional difference between the
      &StaticLibrary; and &Library; functions.

      </para>

    </section>

    <section>
    <title>Building Shared (DLL) Libraries</title>

      <para>

      If you want to build a shared library (on POSIX systems)
      or a DLL file (on Windows systems),
      you use the &SharedLibrary; function:

      </para>

      <scons_example name="SharedLibrary" printme="1">
        <file name="SConstruct" printme="1">
        SharedLibrary('foo', ['f1.c', 'f2.c', 'f3.c'])
        </file>
        <file name="f1.c">
        void f1() { printf("f1.c\n"); }
        </file>
        <file name="f2.c">
        void f2() { printf("f2.c\n"); }
        </file>
        <file name="f3.c">
        void f3() { printf("f3.c\n"); }
        </file>
      </scons_example>

      <para>

      The output on POSIX:

      </para>

      <scons_output example="SharedLibrary" os="posix">
        <command>scons -Q</command>
      </scons_output>

      <para>

      And the output on Windows:

      </para>

      <scons_output example="SharedLibrary" os="win32">
        <command>scons -Q</command>
      </scons_output>

      <para>

      Notice again that &SCons; takes care of
      building the output file correctly,
      adding the <literal>-shared</literal> option
      for a POSIX compilation,
      and the <literal>/dll</literal> option on Windows.

      </para>

    </section>

  </section>

  <section>
  <title>Linking with Libraries</title>

    <para>

    Usually, you build a library
    because you want to link it with one or more programs.
    You link libraries with a program by specifying
    the libraries in the &LIBS; construction variable,
    and by specifying the directory in which
    the library will be found in the 
    &LIBPATH; construction variable:

    </para>

    <scons_example name="ex2">
      <file name="SConstruct" printme="1">
      Library('foo', ['f1.c', 'f2.c', 'f3.c'])
      Program('prog.c', LIBS='foo', LIBPATH='.')
      </file>
      <file name="f1.c">
      int main() { printf("Hello, world!\n"); }
      </file>
      <file name="f2.c">
      int main() { printf("Hello, world!\n"); }
      </file>
      <file name="f3.c">
      int main() { printf("Hello, world!\n"); }
      </file>
      <file name="prog.c">
      int main() { printf("Hello, world!\n"); }
      </file>
    </scons_example>

    <para>

    Notice, of course, that you don't need to specify a library
    prefix (like <literal>lib</literal>)
    or suffix (like <literal>.a</literal> or <literal>.lib</literal>).
    &SCons; uses the correct prefix or suffix for the current system.

    </para>

    <para>

    On a POSIX or Linux system,
    a build of the above example would look like:

    </para>

    <scons_output example="ex2" os="posix">
      <command>scons -Q</command>
    </scons_output>

    <para>

    On a Windows system,
    a build of the above example would look like:

    </para>

    <scons_output example="ex2" os="win32">
      <command>scons -Q</command>
    </scons_output>

    <para>

    As usual, notice that &SCons; has taken care
    of constructing the correct command lines
    to link with the specified library on each system.

    </para>

  </section>

  <section>
  <title>Finding Libraries:  the &LIBPATH; Construction Variable</title>

    <para>

    By default, the linker will only look in
    certain system-defined directories for libraries.
    &SCons; knows how to look for libraries
    in directories that you specify with the
    &LIBPATH; construction variable.
    &LIBPATH; consists of a list of
    directory names, like so:

    </para>

    <scons_example name="ex3">
      <file name="SConstruct" printme="1">
      Program('prog.c', LIBS = 'm',
                        LIBPATH = ['/usr/lib', '/usr/local/lib'])
      </file>
      <file name="prog.c">
      int main() { printf("prog.c\n"); }
      </file>
    </scons_example>

    <para>

    Using a Python list is preferred because it's portable
    across systems.  Alternatively, you could put all of
    the directory names in a single string, separated by the
    system-specific path separator character:
    a colon on POSIX systems:

    </para>

    <sconstruct>
      LIBPATH = '/usr/lib:/usr/local/lib'
    </sconstruct>

    <para>

    or a semi-colon on Windows systems:

    </para>

    <sconstruct>
      LIBPATH = 'C:\lib;D:\lib'
    </sconstruct>

    <para>

    When the linker is executed,
    &SCons; will create appropriate flags
    so that the linker will look for
    libraries in the same directories as &SCons;.
    So on a POSIX or Linux system,
    a build of the above example would look like:

    </para>

    <scons_output example="ex3" os="posix">
      <command>scons -Q</command>
    </scons_output>

    <para>

    On a Windows system,
    a build of the above example would look like:

    </para>

    <scons_output example="ex3" os="win32">
      <command>scons -Q</command>
    </scons_output>

    <para>

    Note again that &SCons; has taken care of
    the system-specific details of creating
    the right command-line options.

    </para>

  </section>