The WEAK_EXTERNAL attribute specifies that a variable or routine
  is  not  critical  to  the linking operation.  To resolve a weak
  reference, the linker searches only  the  named  input  modules.
  You  can  specify  an identifier with this attribute to indicate
  the name by which the  corresponding  object  is  known  to  the
  linker.

  Syntax:

     [WEAK_EXTERNAL]

  or,

     [WEAK_EXTERNAL(identifier)]

  or,

     [WEAK_EXTERNAL('string-literal')]

  The 'identifier' is the name of the  identifier  passed  to  the
  linker.  If you omit the identifier, the name of the variable is
  used.

  The 'string-literal' is the name of the string literal  that  is
  passed to the linker unmodified.

  Compilation units cannot  have  the  EXTERNAL  or  WEAK_EXTERNAL
  attribute.