title: How does SCons build content signature about a function?
body: <p>Recently I used SCons to build project. But it rebuild everytime.. After reading .sconsign.dblite file, I found that SCons will create a content signature for a function. Like this:</p>
  <pre><code>TargetA.txt: 99bb021f789f7bb1c23935daffecfb8e 1598528466 22769
          31aada685dfb3c4e5d1e474ba5cc251d [buildfunction(target, source, env)]
  </code></pre>
  <p>It create a signature for function <code>buildfunction(target, source, env)</code> which created file <code>TargetA.txt</code>. But I couldn't find the relative code. I think it's offered by SCons. Can anyone give me a help?</p>