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Q41004: "UNRESOLVED EXTERNAL"; Can’t Call External from Quick Library

Article: Q41004
Product(s): See article
Version(s): 4.00 4.00b 4.50
Operating System(s): MS-DOS
Keyword(s): ENDUSER | B_BasicCom SR# S890202-70 | mspl13_basic
Last Modified: 20-DEC-1989

Routines in a Quick library are not allowed to call procedures that
are external to that Quick library. If you manually execute LINK with
the /QU option to make a Quick library and you get an "Unresolved
External" error, then the CALL statement in the Quick library that
calls that routine is skipped at run time.

This information applies to the QB.EXE editor supplied with QuickBASIC
Versions 4.00, 4.00b, 4.50, Microsoft BASIC Compiler Versions 6.00 and
6.00b for MS-DOS and OS/2, and Microsoft BASIC PDS Version 7.00 for
MS-DOS and MS OS/2.

A Quick library (which has the filename extension .QLB) is essentially
an executable file, but it cannot be invoked directly from DOS or from
an .EXE program compiled in BASIC. A Quick library can only be used
within the QuickBASIC QB.EXE or BASIC PDS 7.00 QBX.EXE environments.

When you manually execute LINK with the /QU option to attempt to make
a Quick library that tries to CALL a routine that is not in the .OBJ
list or .LIB library list, then the error message "UNRESOLVED
EXTERNAL" is properly displayed at LINK time. Despite the "UNRESOLVED
EXTERNAL" message, the linker still creates a Quick library (.QLB
file). This Quick library runs properly in the QB.EXE or QBX.EXE
environment without giving any error messages. However, the CALL
statement in the Quick library that calls the routine that was
reported as an "UNRESOLVED EXTERNAL" is skipped at run time.

If you attempt to make a Quick library from inside QB.EXE or QBX.EXE
(by using the Make Library command on the Run menu), the error message
"Subprogram not defined" prevents the creation of the Quick library
that has an "UNRESOLVED EXTERNAL."

The following steps illustrate this limitation:

1. Compile the subprogram TEMP5.BAS as follows:

      BC TEMP5.BAS;

   (Compile with the /Fs (far strings) option if you are using BASIC
   PDS 7.00, since the QBX.EXE environment always uses far strings).

2. Make the Quick library TEMP5.QLB as follows in QuickBASIC Version
   4.50:

      LINK TEMP5.OBJ,,,BQLB45.LIB/QU

   (Link with BQLB40.LIB in QuickBASIC Version 4.00; BQLB41.LIB in
   QuickBASIC Version 4.00b; or QBXQLB.LIB in BASIC PDS 7.00).

3. Invoke the QB.EXE or QBX.EXE editor with the TEMP5.QLB Quick
   library as follows:

      QB MAIN/L TEMP5

   (Or QBX MAIN /L TEMP5 if using BASIC PDS Version 7.00)

4. When you run the MAIN.BAS program in QB.EXE, the Quick library
   subroutine TEMP5 attempts to call INMAIN (which is a subprogram
   in the main program), but the CALL INMAIN statement is skipped. The
   program successfully continues.

The CALL INMAIN statement runs successfully if you LINK
MAIN.OBJ+TEMP5.OBJ outside of the editor and run MAIN.EXE from DOS.
(You can create MAIN.OBJ as follows: BC MAIN.BAS;).

The following is MAIN.BAS:

DECLARE SUB temp4 ()
PRINT "main"
CALL temp5
END
SUB inmain STATIC
PRINT "Inside inmain subprogram."
END SUB

The following is TEMP5.BAS, which is made into a Quick library:

SUB temp5 STATIC
PRINT "Inside temp5 subprogram"
CALL inmain  ' This CALL is skipped when run within a Quick Library,
             ' but works fine if you link the .OBJ file into a .EXE.
PRINT "End of temp5 subprogram"
END SUB

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