Source

SCons / doc / user / libraries.xml

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<!--

  __COPYRIGHT__

  Permission is hereby granted, free of charge, to any person obtaining
  a copy of this software and associated documentation files (the
  "Software"), to deal in the Software without restriction, including
  without limitation the rights to use, copy, modify, merge, publish,
  distribute, sublicense, and/or sell copies of the Software, and to
  permit persons to whom the Software is furnished to do so, subject to
  the following conditions:

  The above copyright notice and this permission notice shall be included
  in all copies or substantial portions of the Software.

  THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY
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  WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
  NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE
  LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION
  OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION
  WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.

-->

  <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 &b-link-Library;
    instead of &b-link-Program;:

    </para>

    <programlisting>
      Library('foo', ['f1.c', 'f2.c', 'f3.c'])
    </programlisting>

    <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>

    <screen>
      % <userinput>scons -Q</userinput>
      cc -o f1.o -c f1.c
      cc -o f2.o -c f2.c
      cc -o f3.o -c f3.c
      ar rc libfoo.a f1.o f2.o f3.o
      ranlib libfoo.a
    </screen>

    <para>

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

    </para>

    <screen>
      C:\><userinput>scons -Q</userinput>
      cl /Fof1.obj /c f1.c /nologo
      cl /Fof2.obj /c f2.c /nologo
      cl /Fof3.obj /c f3.c /nologo
      lib /nologo /OUT:foo.lib f1.obj f2.obj f3.obj
    </screen>

    <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 Libraries From Source Code or Object Files</title>

      <para>

      The previous example shows building a library from a
      list of source files.
      You can, however, also give the &b-link-Library; call
      object files,
      and it will correctly realize
      In fact, you can arbitrarily mix source code files
      and object files in the source list:

      </para>

      <programlisting>
        Library('foo', ['f1.c', 'f2.o', 'f3.c', 'f4.o'])
      </programlisting>

      <para>

      And SCons realizes that only the source code files
      must be compiled into object files
      before creating the final library:

      </para>

      <screen>
        % <userinput>scons -Q</userinput>
        cc -o f1.o -c f1.c
        cc -o f3.o -c f3.c
        ar rc libfoo.a f1.o f2.o f3.o f4.o
        ranlib libfoo.a
      </screen>

      <para>

      Of course, in this example, the object files
      must already exist for the build to succeed.
      See <xref linkend="chap-nodes"></xref>, below,
      for information about how you can
      build object files explicitly
      and include the built files in a library.

      </para>

    </section>

    <section>
    <title>Building Static Libraries Explicitly:  the &b-StaticLibrary; Builder</title>

      <para>

      The &b-link-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 &b-link-StaticLibrary; function
      instead of &b-Library;:

      </para>

      <programlisting>
        StaticLibrary('foo', ['f1.c', 'f2.c', 'f3.c'])
      </programlisting>

      <para>

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

      </para>

    </section>

    <section>
    <title>Building Shared (DLL) Libraries:  the &b-SharedLibrary; Builder</title>

      <para>

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

      </para>

      <programlisting>
        SharedLibrary('foo', ['f1.c', 'f2.c', 'f3.c'])
      </programlisting>

      <para>

      The output on POSIX:

      </para>

      <screen>
        % <userinput>scons -Q</userinput>
        cc -o f1.os -c f1.c
        cc -o f2.os -c f2.c
        cc -o f3.os -c f3.c
        cc -o libfoo.so -shared f1.os f2.os f3.os
      </screen>

      <para>

      And the output on Windows:

      </para>

      <screen>
        C:\><userinput>scons -Q</userinput>
        cl /Fof1.obj /c f1.c /nologo
        cl /Fof2.obj /c f2.c /nologo
        cl /Fof3.obj /c f3.c /nologo
        link /nologo /dll /out:foo.dll /implib:foo.lib f1.obj f2.obj f3.obj
        RegServerFunc(target, source, env)
        embedManifestDllCheck(target, source, env)
      </screen>

      <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 &cv-link-LIBS; construction variable,
    and by specifying the directory in which
    the library will be found in the 
    &cv-link-LIBPATH; construction variable:

    <!-- In the preceding paragraph, the "$" notation for
         LIBS, LIBPATH etc. is used for the first time.
         Maybe some words of explanation would be nice. -->

    </para>

    <programlisting>
      Library('foo', ['f1.c', 'f2.c', 'f3.c'])
      Program('prog.c', LIBS=['foo', 'bar'], LIBPATH='.')
    </programlisting>

    <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>

    <screen>
      % <userinput>scons -Q</userinput>
      cc -o f1.o -c f1.c
      cc -o f2.o -c f2.c
      cc -o f3.o -c f3.c
      ar rc libfoo.a f1.o f2.o f3.o
      ranlib libfoo.a
      cc -o prog.o -c prog.c
      cc -o prog prog.o -L. -lfoo -lbar
    </screen>

    <para>

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

    </para>

    <screen>
      C:\><userinput>scons -Q</userinput>
      cl /Fof1.obj /c f1.c /nologo
      cl /Fof2.obj /c f2.c /nologo
      cl /Fof3.obj /c f3.c /nologo
      lib /nologo /OUT:foo.lib f1.obj f2.obj f3.obj
      cl /Foprog.obj /c prog.c /nologo
      link /nologo /OUT:prog.exe /LIBPATH:. foo.lib bar.lib prog.obj
      embedManifestExeCheck(target, source, env)
    </screen>

    <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>

    <para>

    Note also that,
    if you only have a single library to link with,
    you can specify the library name in single string,
    instead of a Python list,
    so that:

    </para>

    <programlisting>
      Program('prog.c', LIBS='foo', LIBPATH='.')
    </programlisting>

    <para>

    is equivalent to:

    </para>

    <programlisting>
      Program('prog.c', LIBS=['foo'], LIBPATH='.')
    </programlisting>

    <para>

    This is similar to the way that &SCons;
    handles either a string or a list to
    specify a single source file.

    </para>

  </section>

  <section>
  <title>Finding Libraries:  the &cv-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
    &cv-link-LIBPATH; construction variable.
    &cv-LIBPATH; consists of a list of
    directory names, like so:

    </para>

    <programlisting>
      Program('prog.c', LIBS = 'm',
                        LIBPATH = ['/usr/lib', '/usr/local/lib'])
    </programlisting>

    <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>

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

    <para>

    or a semi-colon on Windows systems:

    </para>

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

    <para>

    (Note that Python requires that the backslash
    separators in a Windows path name
    be escaped within strings.)

    </para>

    <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>

    <screen>
      % <userinput>scons -Q</userinput>
      cc -o prog.o -c prog.c
      cc -o prog prog.o -L/usr/lib -L/usr/local/lib -lm
    </screen>

    <para>

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

    </para>

    <screen>
      C:\><userinput>scons -Q</userinput>
      cl /Foprog.obj /c prog.c /nologo
      link /nologo /OUT:prog.exe /LIBPATH:\usr\lib /LIBPATH:\usr\local\lib m.lib prog.obj
      embedManifestExeCheck(target, source, env)
    </screen>
    <!-- The link command is too wide in the PDF version. 
         There are some other examples of this throughout the document. -->

    <para>

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

    </para>

  </section>
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