Sometimes it makes sense to not rebuild a program, even if a dependency file changes. In this case, you would tell SCons specifically to ignore a dependency as follows:
hello = Program('hello.c') Ignore(hello, 'hello.h')
% scons -Q hello cc -c -o hello.o hello.c cc -o hello hello.o % scons -Q hello scons: `hello' is up to date. % edit hello.h [CHANGE THE CONTENTS OF hello.h] % scons -Q hello scons: `hello' is up to date.
Now, the above example is a little contrived, because it's hard to imagine a real-world situation where you wouldn't want to rebuild hello if the hello.h file changed. A more realistic example might be if the hello program is being built in a directory that is shared between multiple systems that have different copies of the stdio.h include file. In that case, SCons would notice the differences between the different systems' copies of stdio.h and would rebuild hello each time you change systems. You could avoid these rebuilds as follows:
hello = Program('hello.c', CPPPATH=['/usr/include']) Ignore(hello, '/usr/include/stdio.h')
Ignore can also be used to prevent a generated file from being built
by default. This is due to the fact that directories depend on
their contents. So to ignore a generated file from the default build,
you specify that the directory should ignore the generated file.
Note that the file will still be built if the user specifically
requests the target on scons command line, or if the file is
a dependency of another file which is requested and/or is built
hello_obj=Object('hello.c') hello = Program(hello_obj) Ignore('.',[hello,hello_obj])
% scons -Q scons: `.' is up to date. % scons -Q hello cc -o hello.o -c hello.c cc -o hello hello.o % scons -Q hello scons: `hello' is up to date.