Q68164: CLOSE Doesn’t Release Far Heap Used by First OPEN "COM" Buffer

Article: Q68164
Product(s): See article
Version(s): 4.00 4.00b 4.50
Operating System(s): MS-DOS
Keyword(s): ENDUSER | SR# S901012-19 B_BasicCom | mspl13_basic
Last Modified: 15-JAN-1991

When you use the OPEN "COMn:" statement, the far heap memory allocated
for the communications receive and transmit buffers is not deallocated
when the file is closed. However, this communications memory area
allocated in the far heap is reused if the COM port is reopened. [Far
heap memory usage is indicated by the FRE(-1) function.]

Note that the file's memory overhead portion consumed in DGROUP (the
default data segment) is completely released by the CLOSE statement.
[DGROUP memory is reported by the FRE("") function in QuickBASIC 4.x
and in BASIC Compiler 6.00/6.00b, but reported by the STACK function
in BASIC PDS 7.00/7.10.]

This information applies to Microsoft QuickBASIC versions 4.00, 4.00b,
and 4.50; to Microsoft BASIC Compiler versions 6.00 and 6.00b for
MS-DOS; and to Microsoft BASIC Professional Development System (PDS)
versions 7.00 and 7.10 for MS-DOS.

By default, the communications transmit and receive buffers are set at
512 bytes. The transmit and receive buffer sizes can be changed by
parameters in the OPEN "COMn:" string. For example,

   TB2048 = Increases the transmit buffer size to 2048 bytes.
   RB2048 = Increases the receive buffer size to 2048 bytes.

The following code example duplicates this far heap consumption
behavior:

PRINT "Far Heap before open:"; FRE(-1)
OPEN "COM1:9600,N,8,1,RB4096" FOR RANDOM AS #1
PRINT "Far Heap after open:"; FRE(-1)
CLOSE #1
PRINT "Far Heap after close:"; FRE(-1)

The output from this program shows that after the OPEN statement, the
far heap is correctly reduced by 4096 plus some additional memory
needed to maintain a file. But after the CLOSE, only a few bytes
(equal to the file overhead release in DGROUP memory) get reallocated
to the far heap.

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