Built-in functions allow you to directly access hardware and
  machine instructions to perform operations that are cumbersome,
  slow, or impossible in pure C.

  These functions are very efficient because they are built into the
  VSI C compiler.  This means that a call to one of these functions
  does not result in a reference to a function in the C run-time
  library or in your programs.  Instead, the compiler generates the
  machine instructions necessary to carry out the function directly
  at the call site.  Because most of these built-in functions closely
  correspond to single VAX, Alpha, or Itanium machine instructions,
  the result is small, fast code.

  Some of these functions (such as those that operate on strings or
  bits) are of general interest.  Others (such as the functions
  dealing with process context) are of interest if you are writing
  device drivers or other privileged software.  Some of the functions
  are privileged and unavailable to user mode programs.

  Be sure to include the <builtins.h> header file in your source
  program to access these built-in functions.

  VSI C supports the #pragma builtins preprocessor directive for
  compatibility with VAX C, but it is not required.

  Some of the built-in functions have optional arguments or allow a
  particular argument to have one of many different types.  To
  describe different valid combinations of arguments, the description
  of each built-in function may list several different prototypes for
  the function.  As long as a call to a built-in function matches one
  of the prototypes listed, the call is valid.  Furthermore, any
  valid call to a built-in function acts as if the corresponding
  prototype was in scope, so the compiler performs the argument
  checking and argument conversions specified by that prototype.

  The majority of the built-in functions are named after the machine
  instruction that they generate.  For more information on these
  built-in functions, see the documentation on the corresponding
  machine instruction.  In particular, see that reference for the
  structure of queue entries manipulated by the queue built-in
  functions.