46 lines
2.1 KiB
Plaintext
46 lines
2.1 KiB
Plaintext
.\"
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.\" ----------------------------------------------------------------------------
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.\" "THE BEER-WARE LICENSE" (Revision 42):
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.\" <phk@FreeBSD.org> wrote this file. As long as you retain this notice you
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.\" can do whatever you want with this stuff. If we meet some day, and you think
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.\" this stuff is worth it, you can buy me a beer in return. Poul-Henning Kamp
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.\" ----------------------------------------------------------------------------
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.\"
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.\" $FreeBSD$
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.\"
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.ds RH Alternative implementations
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.NH
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Alternative implementations
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.PP
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These problems were actually the inspiration for the first alternative
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malloc implementations.
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Since their main aim was debugging, they would often use techniques
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like allocating a guard zone before and after the chunk,
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and possibly filling these guard zones
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with some pattern, so accesses outside the allocated chunk could be detected
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with some decent probability.
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Another widely used technique is to use tables to keep track of which
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chunks are actually in which state and so on.
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.PP
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This class of debugging has been taken to its practical extreme by
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the product "Purify" which does the entire memory-coloring exercise
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and not only keeps track of what is in use and what isn't, but also
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detects if the first reference is a read (which would return undefined
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values) and other such violations.
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.PP
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Later actual complete implementations of malloc arrived, but many of
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these still based their workings on the basic schema mentioned previously,
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disregarding that in the meantime virtual memory and paging have
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become the standard environment.
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.PP
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The most widely used "alternative" malloc is undoubtedly ``gnumalloc''
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which has received wide acclaim and certainly runs faster than
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most stock mallocs. It does, however, tend to fare badly in
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cases where paging is the norm rather than the exception.
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.PP
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The particular malloc that prompted this work basically didn't bother
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reusing storage until the kernel forced it to do so by refusing
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further allocations with sbrk(2).
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That may make sense if you work alone on your own personal mainframe,
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but as a general policy it is less than optimal.
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