3.2.6 Linker Error Messages
  If the linker detects any errors while linking object mod-
  ules, it displays messages indicating the cause and severity of
  the error. If any error or fatal error conditions occur (er-
  rors with severities of E or F), the linker does not produce an
  image file.

  The messages produced by the linker are descriptive, and
  you do not usually need additional information to determine
  the specific error. Some common errors that occur during
  linking are as follows:

  .
        An object module has compilation errors.

        This error occurs when you attempt to link a module
        that has warnings or errors during compilation. You
        can usually link compiled modules for which the com-
        piler generated messages, but you should verify that the
        modules will actually produce the output you expect.
  .
        The input file has a file type other than .OBJ and no file
        type was specified on the command line.

        If you do not specify a file type, the linker assumes the
        file has a file type of .OBJ by default. If the file is not an
        object file and you do not identify it with the appropriate
        file type, the linker signals an error message and does not
        produce an image file.
  .
        You tried to link a nonexistent module.

        The linker signals an error message if you misspell a
        module name on the command line or if the compilation
        contains fatal diagnostics.
  .
        A reference to a symbol name remains unresolved.

        An error occurs when you omit required module or li-
        brary names from the command line and the linker
        cannot locate the definition for a specified global sym-
        bol reference. For example, a main program module
        OCEAN.OBJ calls the subprograms located in object
        modules REEF.OBJ, SHELLS.OBJ, and SEAWEED.OBJ.
        However, the following LINK command does not refer-
        ence the object module SEAWEED.OBJ:
        $ LINK OCEAN, REEF, SHELLS

        This example produces the following error messages:
        %LINK-W-NUDFSYMS, 1 undefined symbol
        %LINK-I-UDFSYMS, SEAWEED
        %LINK-W-USEUNDEF, module "OCEAN" references undefined symbol "SEAWEED"
        %LINK-W-DIAGISUED, completed but with diagnostics

  If an error occurs when you link modules, you can often
  correct the error by reentering the command string and
  specifying the correct modules or libraries.

  See the OpenVMS System Messages and Recovery Procedures
  Reference Manual for a complete list of linker messages.