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<TITLE>3.4 Use of Memory</TITLE>
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<P><FONT SIZE="5">3.4 Use of Memory</FONT><BR>
<P>FunnelWeb is not a memory-stressed program. However,
during its development, problems with the management of
memory seemed to crop up again and again. This section
documents some of these problems and the solutions
that were adopted.
<P>There are three places where memory can be obtained:
the heap, the stack, and from static variables. The
following three sections deal with each of these areas.
<P><BR><FONT SIZE="4"><STRONG>The Heap</STRONG></FONT><BR>
<P>One of the great frustrations of being a user is to
find that a computer program is complaining about lack of
memory when one knows full well that one has allocated at
least ten times as much memory to the program as it would
ever need to do its job. The reason for such error messages
usually has to do with the programmer setting a fixed
"reasonable" limit to a particular data structure
and then locking it up into an array whose bound is
specified by a constant. This malody is particularly common in
old Pascal programs. While the use of arrays can
increase the speed of a program, it also means that the user
cannot increase the capacity of the program without
obtaining the source code and recompiling it.
<P>The alternative is to use the heap for all data
structures that can grow in proportion to the size of the
user's input. This rule has been followed rigorously in
FunnelWeb. This means that as memory spaces increase, users
will be able to hand their version of FunnelWeb more memory
without having to recompile it.
<P><HR WIDTH=30%>
<P>Some problems arose early on the
Macintosh in the use of the heap.
For some obscure reason, many of the heap application
<SAMP>malloc</SAMP> calls were failing. Whatever
it was, it went away when I replaced direct calls to
<SAMP>malloc</SAMP> with calls to a mini package
I wrote (called <SAMP>memory</SAMP>) that allocated large
chunks of memory and then doled out small pieces as required
by the rest of the program.
<P>Having a package to manage all the memory allocation
had two other benefits.
<P>First, only one check was required in the entire
program to see if memory had run out (in the memory
package), and if that failed, the program could be brought
to a screaming halt. This organization was far preferable to
having each piece of code that needed to allocate memory
having to check to see if <SAMP>malloc</SAMP> had failed.
<P>Second, the decision to construct a mini-shell within
FunnelWeb to support regression testing meant that FunnelWeb
proper (the FunnelWeb <SAMP>fw</SAMP> shell command)
could be run many times in any given invocation of
FunnelWeb. As a consequence it was necessary to make sure
that there was no memory leakage
between invocations of FunnelWeb proper. This was
accomplished by reworking the memory package to operate a
watermark system. The user of the package, when requesting
memory, could request "temporary" or
"permanent". If permanent, the memory package forgot
that it had allocated the memory. If temporary, the memory
package places the allocated block on a list. There was then
a function in the memory package that could be called to
deallocate all the temporary memory. All this meant that
so long as all
requests for memory within FunnelWeb proper were for
temporary memory, and that memory was freed at the end of
every run, one could be sure that there was no memory
leakage.
<P><BR><FONT SIZE="4"><STRONG>The Stack</STRONG></FONT><BR>
<P>For a while during the development of FunnelWeb a
particularly nasty bug proved extremely hard to find. The
symptom was that FunnelWeb would crash, sometimes at random,
but more often upon entering a particular function. In the
end, about a day of specific debugging was required before
the problem was tracked down to a stack problem. It turned
out that somehow (either the fault of the Macintosh or the
THINK C language system), the compiler was allocating
just 6K for stack space!!!!!!!
<P>This experience led me immediately to go through the
entire program and eliminate (or remove to the heap) any
automatic variable declarations that used more than one
hundred or so bytes.
<P>The lesson is clearly that C programs that use more
than a few thousand bytes of stack space are risking their
portability. All large data structures should be placed in
the heap.
<P><BR><FONT SIZE="4"><STRONG>Static Variables</STRONG></FONT><BR>
<P>Static variables also proved a problem on the
Macintosh. It turned out that the Macintosh
THINK C compiler did not allow
more than 32K of statics <I>in the entire program</I> . For
a while this restriction was a serious threat to the program
as it was discovered that constant strings were included in
this total! However, some searching revealed a compiler
option that removed the strings from the static category.
<P>Nevertheless, the 32K limit is rather severe. Again, it
seems that for portability reasons, C programs that use a
lot of static variables are risking their portability. As a
result, the FunnelWeb code avoids static variables where
possible in favour of the heap.
<P>
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