title: How does C code call assembly code (e.g. optimized strlen)?
body: <p>I always read things about how certain functions within the C programming language are optimized by being written in assembly.  Let me apologize if that sentence sounds a little misguided.</p>
  
  <p>So, I'll put it clearly: How is it that when you call some functions like strlen on UNIX/C systems, the actual function you're calling is written in assembly?  Can you write assembly right into C programs somehow or is it an external call situation?  Is it part of the C standard to be able to do this, or is it an operating system specific thing?</p>