Q40188: FRE(-2) Returns Fixed Lowest Value Even After Stack Shrinks

Article: Q40188
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 B_BasicCom | mspl13_basic
Last Modified: 20-SEP-1990

The FRE(-2) function returns in bytes the amount of unused stack space
available for the program.

However, when the stack-space allocation decreases, the value returned
by FRE(-2) stays fixed at the smallest size that was unused by the
stack up to that point of program execution. In other words, the
values successively returned by FRE(-2) never increase -- they either
decrease or stay the same.

Microsoft has confirmed this to be a problem in QuickBASIC in 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) versions 7.00
and 7.10 for MS-DOS and MS OS/2 (buglist7.00, buglist7.10). We are
researching this problem and will post new information here as it
becomes available.

The sample program below demonstrates this behavior. The example given
is a recursive subprogram that reports the value of FRE(-2) on the way
down and back up through 10 levels of subprogram recursion. As the
stack space increases with the level of recursion, the value of
FRE(-2) decreases as expected. However, as the stack space decreases
with lower levels of recursion, the value reported by FRE(-2) stays
"bottomed out" at the lowest value reached during recursion.

The only way to reset the "bottomed-out" value returned by FRE(-2) is
to end the program or execute a CLEAR statement. Note: CLEAR erases
all variables and closes all files. In some cases, as shown in Example
2, adding an INPUT statement before the FRE(-2) allows FRE(-2) to go
back up. Adding INPUT does not help Example 1.

Example 1
---------

The following code example demonstrates a case in which FRE(-2) goes
down and stays down:

   DECLARE SUB test (num)
   CLS
   PRINT "Before any calls, FRE(-2):"; FRE(-2)
   CALL test(0)
   END
   SUB test (num)
     PRINT "Call number"; num; "on the way DOWN, FRE(-2):"; FRE(-2)
     IF num < 9 THEN
       CALL test(num + 1)
     ELSE
       PRINT "Bottom of recursion reached."
     END IF
     PRINT "Call number"; num; "on the way UP, FRE(-2):"; FRE(-2)
   END SUB

Example 2
---------

The following example demonstrates a case in which adding an INPUT
statement lets the value returned by FRE(-2) go back up:

   DECLARE SUB foo ()
   DEFINT A-Z
   CLS
   'INPUT k   ' Adding this INPUT statement makes FRE(-2) go back up.
   PRINT FRE(-2)
   a$ = "Test string"
   CALL foo
   PRINT FRE(-2)
   END

   SUB foo
   SHARED a$
   PRINT "hello "; a$
   END SUB

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