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Q29923: FUNCTIONs and Subprograms Can Affect FIELDed Arrays in COMMON

Article: Q29923
Product(s): See article
Version(s): 4.00 4.00b 4.50
Operating System(s): MS-DOS
Keyword(s): ENDUSER | buglist4.00 buglist4.00b buglist4.50 | mspl13_basic
Last Modified: 22-JAN-1990

An array that is in both a FIELD and a COMMON statement can be
adversely affected if you pass it as a parameter to a FUNCTION or
subprogram procedure.

The program below has two FIELD statements that result in two arrays
pointing to the same memory location. However, after a call to a
FUNCTION that should not affect the arrays, they no longer point to
the same place in memory. Compiling with debug (BC /D) does not help.

Microsoft has confirmed this to be a problem in QuickBASIC Versions
4.00, 4.00b, and 4.50, in Microsoft BASIC Compiler Versions 6.00 and
6.00b for MS-DOS and MS OS/2 (buglist6.00, buglist6.00b), and in
Microsoft BASIC Professional Development System (PDS) Version 7.00 for
MS-DOS and MS OS/2 (buglist7.00). We are researching this problem and
will post new information here as it becomes available.

The program works correctly in QuickBASIC Version 3.00 if the FUNCTION
is made into a subprogram. (QuickBASIC Version 3.00 does not support
FUNCTION procedures.)

The following is an example using a FUNCTION, but the problem also
occurs if the FUNCTION is made into a subprogram:

DECLARE FUNCTION Nothing% (any$)
DEFINT A-Z
COMMON SHARED /GLOBAL/ field1$(), field2$()
DIM field1$(20), field2$(20)
CLS
OPEN "anyfile.tmp" FOR RANDOM SHARED AS #1 LEN = 10
FIELD #1, 10 AS field1$(1)
FIELD #1, 10 AS field2$(1)

'** NOTE: At this point, field1$(1) and field2$(1) should point to    **
'** the same storage location (buffer space) as verified below.       **

LSET field1$(1) = "ABC"
PRINT field1$(1); field2$(1)
LSET field2$(1) = "DEF"
PRINT field1$(1); field2$(1)

x = Nothing(field1$(1))     '** prints field1$ but should not change it. **
'x = Nothing((field1$(1)))  '** passes by value -- this works **

'** Now notice that field1$(1) no longer points to the same storage as  **
'** its counterpart, field2$(1), as verified below.                     **

LSET field1$(1) = "ABC"
PRINT field1$(1); field2$(1)
LSET field2$(1) = "GHI"
PRINT field1$(1); field2$(1)
CLOSE
END

FUNCTION Nothing (any$)
        x$ = any$       '** so won't affect value of any$ **
        PRINT x$
END FUNCTION

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