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FUNNELWEB MANUAL WEB PAGE
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Copyright (c) Ross N. Williams 1992,1999. All rights reserved.
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Permission is granted to redistribute and use this manual in
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modifications since the manual's original release by Ross N.
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Williams (www.ross.net) consist of translations or other
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transformations that alter only the manual's form, not its
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content. THIS MANUAL IS PROVIDED "AS IS" AND WITHOUT ANY
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<TITLE>3.4 Use of Memory</TITLE>
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<A HREF="compile.html"><FONT COLOR="#FFFFFF">1 Compile</FONT></A><BR>
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<A HREF="implement.html"><FONT COLOR="#FFFFFF">3 Implement</FONT></A><BR>
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<P><FONT SIZE="5">3.4 Use of Memory</FONT><BR>
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<P>FunnelWeb is not a memory-stressed program. However,
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during its development, problems with the management of
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memory seemed to crop up again and again. This section
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documents some of these problems and the solutions
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<P>There are three places where memory can be obtained:
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the heap, the stack, and from static variables. The
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following three sections deal with each of these areas.
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<P><BR><FONT SIZE="4"><STRONG>The Heap</STRONG></FONT><BR>
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<P>One of the great frustrations of being a user is to
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find that a computer program is complaining about lack of
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memory when one knows full well that one has allocated at
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least ten times as much memory to the program as it would
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ever need to do its job. The reason for such error messages
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usually has to do with the programmer setting a fixed
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"reasonable" limit to a particular data structure
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and then locking it up into an array whose bound is
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specified by a constant. This malody is particularly common in
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old Pascal programs. While the use of arrays can
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increase the speed of a program, it also means that the user
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cannot increase the capacity of the program without
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obtaining the source code and recompiling it.
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<P>The alternative is to use the heap for all data
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structures that can grow in proportion to the size of the
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user's input. This rule has been followed rigorously in
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FunnelWeb. This means that as memory spaces increase, users
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will be able to hand their version of FunnelWeb more memory
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without having to recompile it.
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<P>Some problems arose early on the
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Macintosh in the use of the heap.
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For some obscure reason, many of the heap application
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<SAMP>malloc</SAMP> calls were failing. Whatever
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it was, it went away when I replaced direct calls to
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<SAMP>malloc</SAMP> with calls to a mini package
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I wrote (called <SAMP>memory</SAMP>) that allocated large
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chunks of memory and then doled out small pieces as required
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by the rest of the program.
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<P>Having a package to manage all the memory allocation
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had two other benefits.
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<P>First, only one check was required in the entire
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program to see if memory had run out (in the memory
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package), and if that failed, the program could be brought
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to a screaming halt. This organization was far preferable to
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having each piece of code that needed to allocate memory
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having to check to see if <SAMP>malloc</SAMP> had failed.
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<P>Second, the decision to construct a mini-shell within
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FunnelWeb to support regression testing meant that FunnelWeb
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proper (the FunnelWeb <SAMP>fw</SAMP> shell command)
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could be run many times in any given invocation of
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FunnelWeb. As a consequence it was necessary to make sure
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that there was no memory leakage
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between invocations of FunnelWeb proper. This was
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accomplished by reworking the memory package to operate a
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watermark system. The user of the package, when requesting
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memory, could request "temporary" or
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"permanent". If permanent, the memory package forgot
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that it had allocated the memory. If temporary, the memory
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package places the allocated block on a list. There was then
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a function in the memory package that could be called to
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deallocate all the temporary memory. All this meant that
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requests for memory within FunnelWeb proper were for
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temporary memory, and that memory was freed at the end of
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every run, one could be sure that there was no memory
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<P><BR><FONT SIZE="4"><STRONG>The Stack</STRONG></FONT><BR>
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<P>For a while during the development of FunnelWeb a
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particularly nasty bug proved extremely hard to find. The
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symptom was that FunnelWeb would crash, sometimes at random,
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but more often upon entering a particular function. In the
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end, about a day of specific debugging was required before
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the problem was tracked down to a stack problem. It turned
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out that somehow (either the fault of the Macintosh or the
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THINK C language system), the compiler was allocating
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just 6K for stack space!!!!!!!
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<P>This experience led me immediately to go through the
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entire program and eliminate (or remove to the heap) any
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automatic variable declarations that used more than one
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<P>The lesson is clearly that C programs that use more
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than a few thousand bytes of stack space are risking their
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portability. All large data structures should be placed in
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<P><BR><FONT SIZE="4"><STRONG>Static Variables</STRONG></FONT><BR>
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<P>Static variables also proved a problem on the
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Macintosh. It turned out that the Macintosh
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THINK C compiler did not allow
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more than 32K of statics <I>in the entire program</I> . For
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a while this restriction was a serious threat to the program
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as it was discovered that constant strings were included in
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this total! However, some searching revealed a compiler
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option that removed the strings from the static category.
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<P>Nevertheless, the 32K limit is rather severe. Again, it
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seems that for portability reasons, C programs that use a
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lot of static variables are risking their portability. As a
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result, the FunnelWeb code avoids static variables where
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possible in favour of the heap.
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