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<para>
One of the more useful ways in which you can use multiple
construction environments is to link programs
with different sets of libraries.
</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">
env = Environment()
env.Library('foo', ['f1.c', 'f2.c', 'f3.c'])
</file>
<file name="hello.c">
int main() { printf("Hello, world!\n"); }
</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">
<command>scons</command>
</scons_output>
<para>
On a Windows system,
a build of the above example would look like:
</para>
<scons_output example="ex1">
<command>scons</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>
<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">
env = Environment(LIBS = 'foo', LIBPATH = '.')
env.Library('foo', ['f1.c', 'f2.c', 'f3.c'])
env.Program('prog.c')
</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">
<command>scons</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</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">
env = Environment(LIBS = 'm',
LIBPATH = ['/usr/lib', '/usr/local/lib'])
env.Program('prog.c')
</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">
<command>scons</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</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>
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