Ignoring Dependencies: the Ignore Method

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 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')
       Ignore(hello, '/usr/include/stdio.h')