73 lines
2.9 KiB
Plaintext
73 lines
2.9 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 Malloc and free
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.NH
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Malloc and free
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.PP
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The job of malloc(3) is to turn the rather simple
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brk(2) facility into a service programs can
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actually use without getting hurt.
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.PP
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The archetypical malloc(3) implementation keeps track of the memory between
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the end of the bss section, as defined by the
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.B _end
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symbol, and the current brk(2) point using a linked list of chunks of memory.
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Each item on the list has a status as either free or used, a pointer
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to the next entry and in most cases to the previous as well, to speed
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up inserts and deletes in the list.
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.PP
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When a malloc(3) request comes in, the list is traversed from the
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front and if a free chunk big enough to hold the request is found,
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it is returned, if the free chunk is bigger than the size requested,
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a new free chunk is made from the excess and put back on the list.
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.PP
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When a chunk is
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.B free(3) 'ed,
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the chunk is found in the list, its status
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is changed to free and if one or both of the surrounding chunks
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are free, they are collapsed to one.
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.PP
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A third kind of request,
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.B realloc(3) ,
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will resize
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a chunk, trying to avoid copying the contents if possible.
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It is seldom used, and has only had a significant impact on performance
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in a few special situations.
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The typical pattern of use is to malloc(3) a chunk of the maximum size
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needed, read in the data and adjust the size of the chunk to match the
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size of the data read using realloc(3).
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.PP
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For reasons of efficiency, the original implementation of malloc(3)
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put the small structure used to contain the next and previous pointers
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plus the state of the chunk right before the chunk itself.
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.PP
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As a matter of fact, the canonical malloc(3) implementation can be
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studied in the ``Old testament'', chapter 8 verse 7 [Kernighan & Ritchie]
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.PP
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Various optimisations can be applied to the above basic algorithm:
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.IP
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If in freeing a chunk, we end up with the last chunk on the list being
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free, we can return that to the kernel by calling brk(2) with the first
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address of that chunk and then make the previous chunk the last on the
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chain by terminating its ``next'' pointer.
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.IP
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A best-fit algorithm can be used instead of first-fit at an expense
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of memory, because statistically fewer chances to brk(2) backwards will
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present themselves.
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.IP
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Splitting the list in two, one for used and one for free chunks, to
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speed the searching.
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.IP
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Putting free chunks on one of several free lists, depending on their size,
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to speed allocation.
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.IP
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\&...
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