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<section id="sect-mergeflags"
         xmlns="http://www.scons.org/dbxsd/v1.0"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://www.scons.org/dbxsd/v1.0 http://www.scons.org/dbxsd/v1.0/scons.xsd">
<title>Merging Options into the Environment: the &MergeFlags; Function</title>

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 <para>

 &SCons; construction environments have a &MergeFlags; method
 that merges a dictionary of values into the construction environment.
 &MergeFlags; treats each value in the dictionary
 as a list of options such as one might pass to a command
 (such as a compiler or linker).
 &MergeFlags; will not duplicate an option
 if it already exists in the construction environment variable.

 </para>

 <para>

 &MergeFlags; tries to be intelligent about merging options.
 When merging options to any variable
 whose name ends in <varname>PATH</varname>,
 &MergeFlags; keeps the leftmost occurrence of the option,
 because in typical lists of directory paths,
 the first occurrence "wins."
 When merging options to any other variable name,
 &MergeFlags; keeps the rightmost occurrence of the option,
 because in a list of typical command-line options,
 the last occurrence "wins."

 </para>

 <scons_example name="mergeflags_MergeFlags1">
   <file name="SConstruct" printme="1">
env = Environment()
env.Append(CCFLAGS = '-option -O3 -O1')
flags = { 'CCFLAGS' : '-whatever -O3' }
env.MergeFlags(flags)
print env['CCFLAGS']
   </file>
 </scons_example>

 <scons_output example="mergeflags_MergeFlags1" suffix="1">
    <scons_output_command>scons -Q</scons_output_command>
 </scons_output>

 <para>

 Note that the default value for &cv-link-CCFLAGS;
 <!--
 [TODO: for when we make CLVar public]
 is a <varname>CLVar</varname>,
 -->
 is an internal &SCons; object
 which automatically converts
 the options we specified as a string into a list.

 </para>

 <scons_example name="mergeflags_MergeFlags2">
   <file name="SConstruct" printme="1">
env = Environment()
env.Append(CPPPATH = ['/include', '/usr/local/include', '/usr/include'])
flags = { 'CPPPATH' : ['/usr/opt/include', '/usr/local/include'] }
env.MergeFlags(flags)
print env['CPPPATH']
   </file>
 </scons_example>

 <scons_output example="mergeflags_MergeFlags2" suffix="1">
    <scons_output_command>scons -Q</scons_output_command>
 </scons_output>

 <para>

 Note that the default value for &cv-link-CPPPATH;
 <!--
 [TODO: for when we make CLVar public]
 is a Python list, not a <varname>CLVar</varname>,
 -->
 is a normal Python list,
 so we must specify its values as a list
 in the dictionary we pass to the &MergeFlags; function.

 </para>

 <para>

 If &MergeFlags; is passed anything other than a dictionary,
 it calls the &ParseFlags; method to convert it into a dictionary.

 </para>

 <scons_example name="mergeflags_MergeFlags3">
   <file name="SConstruct" printme="1">
env = Environment()
env.Append(CCFLAGS = '-option -O3 -O1')
env.Append(CPPPATH = ['/include', '/usr/local/include', '/usr/include'])
env.MergeFlags('-whatever -I/usr/opt/include -O3 -I/usr/local/include')
print env['CCFLAGS']
print env['CPPPATH']
   </file>
 </scons_example>

 <scons_output example="mergeflags_MergeFlags3" suffix="1">
    <scons_output_command>scons -Q</scons_output_command>
 </scons_output>

 <para>

 In the combined example above,
 &ParseFlags; has sorted the options into their corresponding variables
 and returned a dictionary for &MergeFlags; to apply
 to the construction variables
 in the specified construction environment.

 </para>

</section>
