Q45687: "Out of String Space" Concatenating Variable-Length String

Article: Q45687
Product(s): See article
Version(s): 1.00 1.01 1.02 2.00 2.01 3.00 4.00 4.00b 4.50
Operating System(s): MS-DOS
Keyword(s): ENDUSER | SR# S890606-8 B_BasicCom | mspl13_basic
Last Modified: 28-DEC-1989

For variable-length string concatenation to execute successfully, even
if only 1 byte is to be added, there must be enough available memory
[reported by FRE("")] for the length of a copy of the existing string,
plus the length of the string being added, plus its new 4-byte string
descriptor. Otherwise, you will get an "Out of String Space" error,
caused by the temporary old string remaining in memory during the
string concatenation. The old string is deallocated only after a
successful concatenation.

This information applies to QuickBASIC Versions 1.00, 1.01, 1.02,
2.00, 2.01, 3.00, 4.00, 4.00b, and 4.50, and to BASIC Compiler
Versions 6.00 and 6.00b for MS-DOS and OS/2.

This information also applies to Microsoft BASIC PDS Version 7.00 when
using BC.EXE without the /Fs compiler switch. Inside the QBX.EXE
environment and when using /Fs compiler switch, BASIC 7.00 uses a 64K
segment for temporary string storage and procedure-level strings (that
is, strings declared inside subroutines). To see how much temporary
string space you have when using far strings, use FRE("StringLiteral"),
where StringLiteral is any constant string (for example, "Hello"). For
more information about far strings, read Chapter 11 in the "Microsoft
BASIC 7.0: Programmer's Guide" for BASIC PDS Version 7.00.

When you concatenate a variable-length string with another string in
BASIC, a new 4-byte string descriptor is created. A copy of the
original string is moved to a new location in string space and the
string to be concatenated is appended to it. The length field in the
new string descriptor is updated to reflect the length of the
concatenated string. Only then is the memory used by the original
string and its descriptor released (deallocated).

New strings are allocated above existing strings and deallocated
strings in DGROUP. When deallocated strings fragment string space to
the point where a new string fills the last space at the top of
DGROUP, BASIC automatically performs string space compaction (garbage
collection) to make free string space contiguous again. Passing a
string argument (such as "", the null string) to the FRE("") function
always forces string space compaction before reporting the amount of
string space available in DGROUP.

For example, assume FRE("") returns 20,004. An attempt to add a single
byte to an existing string of 20,000 bytes would fail because 20,005
bytes are needed for the concatenation to be successful:

   Existing string   = 20,000 bytes
   String to add     =      1 byte longer
   String descriptor =      4 bytes
                    -----------
   Memory needed       20,005 bytes

Because only 20,004 bytes of memory are available, an "Out of String
Space" message will be generated.

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