SideEffect(side_effect, target): A side effect is an indirect target that is a side effect of building other targets. For example, a Windows PDB file is created as a side effect of building the .obj files for a static library. Calling declares side_effect as a side effect of building target. Both side_effect and target can be a list, a file name, or a node. If a target is a side effect of multiple build commands, SCons will ensure that only one set of commands is executed at a time. Consequently, you only need to use this method for side-effect targets that are built as a result of multiple build commands.

It should be noted that side effects are not automatically cleaned by scons -c. To do that, please see Clean.

For example, if there are two commands that write information to a shared log file, the two commands should not be run in parallel:

f1 = Command('file1.out', 'file1.in', './build --log log.txt $SOURCE $TARGET')  
f2 = Command('file2.out', 'file2.in', './build --log log.txt $SOURCE $TARGET')

The SideEffect() method can be used to say that these two commands should not be run at the same time. It can be specified a number of ways:

SideEffect('log.txt', f1 + f2)

or equivalently:

SideEffect('log.txt', ['file1.out', 'file2.out'])

Side effects accumulate, so this sequence is equivalent as well:

SideEffect('log.txt', 'file1.out')  
SideEffect('log.txt', 'file2.out')

As is this sequence:

SideEffect('log.txt', f1)  
SideEffect('log.txt', f2)

The SideEffect() method returns a list of the side-effect files, so this will also work:

s = SideEffect('log.txt', f1) 
SideEffect(s, f2)

The file need not exist (or ever be created); by specifying a dummy side-effect file, this method still prevents parallel builds:

f1 = Command('file1.out', 'file1.in', './build $SOURCE $TARGET')  
f2 = Command('file2.out', 'file2.in', './build $SOURCE $TARGET')  
SideEffect('a.dummy.file', f1 + f2)

TODO: Add an example of a Builder that contains a SideEffect() call.