367 lines
8.6 KiB
Plaintext
367 lines
8.6 KiB
Plaintext
.TH POOL 2
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.SH NAME
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poolalloc, poolallocalign, poolfree, poolmsize, poolisoverlap, poolrealloc, poolcompact, poolcheck, poolblockcheck,
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pooldump \- general memory management routines
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.SH SYNOPSIS
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.B #include <u.h>
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.br
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.B #include <libc.h>
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.br
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.B #include <pool.h>
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.PP
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.B
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void* poolalloc(Pool* pool, ulong size)
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.PP
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.B
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void* poolallocalign(Pool *pool, ulong size,
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.br
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.B
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ulong align, long offset, ulong span)
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.PP
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.B
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void poolfree(Pool* pool, void* ptr)
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.PP
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.B
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ulong poolmsize(Pool* pool, void* ptr)
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.PP
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.B
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int poolisoverlap(Pool* pool, void* ptr, ulong len)
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.PP
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.B
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void* poolrealloc(Pool* pool, void* ptr, ulong size)
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.PP
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.B
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int poolcompact(Pool* pool)
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.PP
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.B
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void poolcheck(Pool *pool)
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.PP
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.B
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void poolblockcheck(Pool *pool, void *ptr)
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.PP
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.B
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void pooldump(Pool *pool);
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.SH DESCRIPTION
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These routines provide a general memory management facility.
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Memory is retrieved from a coarser allocator (e.g.
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.I sbrk
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or the kernel's
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.IR xalloc )
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and then allocated to callers.
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The routines are locked and thus may safely be used in
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multiprocess programs.
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.PP
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.I Poolalloc
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attempts to allocate a block of size
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.BR size ;
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it returns a pointer to the block when successful and nil otherwise.
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The call
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.B "poolalloc(0)
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returns a non-nil pointer.
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.I Poolfree
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returns an allocated block to the pool.
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It is an error to free a block more than once or to free a
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pointer not returned by
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.IR poolalloc .
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The call
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.B "poolfree(nil)
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is legal and is a no-op.
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.PP
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.I Poolallocalign
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attempts to allocate a block of size
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.B size
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with the given alignment constraints.
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If
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.I align
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is non-zero,
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the returned pointer is aligned to be equal to
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.I offset
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modulo
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.IR align .
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If
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.I span
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is non-zero,
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the
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.I n
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byte block allocated will not span a
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.IR span -byte
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boundary.
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.PP
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.I Poolrealloc
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attempts to resize to
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.B nsize
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bytes the block associated with
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.BR ptr ,
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which must have been previously returned by
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.I poolalloc
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or
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.IR poolrealloc .
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If the block's size can be adjusted, a (possibly different) pointer to the new block is returned.
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The contents up to the lesser of the old and new sizes are unchanged.
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After a successful call to
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.IR poolrealloc ,
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the return value should be used rather than
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.B ptr
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to access the block.
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If the request cannot be satisfied,
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.I poolrealloc
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returns nil, and the old pointer remains valid.
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.PP
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When blocks are allocated, there is often some extra space left at the end
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that would usually go unused.
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.IR Poolmsize
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grows the block to encompass this extra space and returns the new size.
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.PP
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.I Poolisoverlap
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checks if the byte span
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.BR [ptr , ptr + len)
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overlaps the arenas of the specified
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.BR pool ,
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returning non-zero when there is overlap or zero if none.
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.PP
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The
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.I poolblockcheck
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and
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.I poolcheck
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routines validate a single allocated block or the entire pool, respectively.
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They call
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.B panic
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(see below)
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if corruption is detected.
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.I Pooldump
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prints a summary line for every block in the
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pool, using the
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.B print
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function (see below).
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.PP
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The
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.B Pool
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structure itself provides much of the setup interface.
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.IP
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.EX
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.ta \w'\fL 'u +\w'\fLulong 'u +\w'\fLlastcompact; 'u
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typedef struct Pool Pool;
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struct Pool {
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char* name;
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uintptr maxsize; /* of entire Pool */
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uintptr cursize; /* of Pool */
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uintptr curfree; /* total free bytes in Pool */
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uintptr curalloc; /* total allocated bytes in Pool */
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ulong minarena; /* smallest size of new arena */
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ulong quantum; /* allocated blocks should be multiple of */
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ulong minblock; /* smallest newly allocated block */
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int flags;
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int nfree; /* number of calls to free */
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int lastcompact; /* nfree at time of last poolcompact */
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void* (*alloc)(ulong);
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int (*merge)(void*, void*);
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void (*move)(void* from, void* to);
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void (*lock)(Pool*);
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void (*unlock)(Pool*);
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void (*print)(Pool*, char*, ...);
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void (*panic)(Pool*, char*, ...);
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void (*logstack)(Pool*);
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void* private;
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};
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.ta \w'\fL 'u +\w'POOL_ANTAGONISM 'u
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enum { /* flags */
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POOL_ANTAGONISM = 1<<0,
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POOL_PARANOIA = 1<<1,
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POOL_VERBOSITY = 1<<2,
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POOL_DEBUGGING = 1<<3,
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POOL_LOGGING = 1<<4,
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POOL_TOLERANCE = 1<<5,
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POOL_NOREUSE = 1<<6,
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};
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.EE
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.PP
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The pool obtains arenas of memory to manage by calling the given
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.B alloc
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routine.
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The total number of requested bytes will not exceed
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.BR maxsize .
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Each allocation request will be for at least
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.B minarena
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bytes.
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.PP
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When a new arena is allocated, the pool routines try to
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merge it with the surrounding arenas, in an attempt to combat fragmentation.
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If
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.B merge
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is non-nil, it is called with the addresses of two blocks from
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.B alloc
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that the pool routines suspect might be adjacent.
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If they are not mergeable,
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.B merge
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must return zero.
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If they are mergeable,
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.B merge
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should merge them into one block in its own bookkeeping
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and return non-zero.
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.PP
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To ease fragmentation and make
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block reuse easier, the sizes requested of the pool routines are rounded up to a multiple of
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.B quantum
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before
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the carrying out requests.
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If, after rounding, the block size is still less than
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.B minblock
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bytes,
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.B minblock
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will be used as the block size.
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.PP
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.I Poolcompact
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defragments the pool, moving blocks in order to aggregate
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the free space.
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Each time it moves a block, it notifies the
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.B move
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routine that the contents have moved.
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At the time that
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.B move
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is called, the contents have already moved,
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so
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.B from
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should never be dereferenced.
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If no
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.B move
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routine is supplied (i.e. it is nil), then calling
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.I poolcompact
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is a no-op.
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.PP
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When the pool routines need to allocate a new arena but cannot,
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either because
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.B alloc
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has returned nil or because doing so would use more than
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.B maxsize
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bytes,
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.I poolcompact
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is called once to defragment the memory
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and the request is retried.
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.PP
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.I Pools
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are protected by the pool routines calling
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.B lock
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(when non-nil)
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before modifying the pool, and
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calling
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.B unlock
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when finished.
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.PP
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When internal corruption is detected,
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.B panic
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is called with a
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.IR print (2)
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style argument that specifies what happened.
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It is assumed that
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.B panic
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never returns.
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When the pool routines wish to convey a message
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to the caller (usually because logging is turned on; see below),
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.B print
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is called, also with a
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.IR print (2)
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style argument.
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.PP
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.B Flags
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is a bit vector that tweaks the behavior of the pool routines
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in various ways.
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Most are useful for debugging in one way or another.
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When
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.B POOL_ANTAGONISM
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is set,
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.I poolalloc
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fills blocks with non-zero garbage before releasing them to the user,
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and
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.I poolfree
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fills the blocks on receipt.
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This tickles both user programs and the innards of the allocator.
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Specifically, each 32-bit word of the memory is marked with a pointer value exclusive-or'ed
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with a constant.
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The pointer value is the pointer to the beginning of the allocated block
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and the constant varies in order to distinguish different markings.
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Freed blocks use the constant
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.BR 0xF7000000 ,
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newly allocated blocks
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.BR 0xF9000000 ,
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and newly created unallocated blocks
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.BR 0xF1000000 .
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For example, if
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.B POOL_ANTAGONISM
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is set and
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.I poolalloc
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returns a block starting at
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.BR 0x00012345 ,
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each word of the block will contain the value
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.BR 0xF90012345 .
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Recognizing these numbers in memory-related crashes can
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help diagnose things like double-frees or dangling pointers.
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.PP
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Setting
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.B POOL_PARANOIA
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causes the allocator to walk the entire pool whenever locking or unlocking itself,
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looking for corruption.
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This slows runtime by a few orders of magnitude
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when many blocks are in use.
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If
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.B POOL_VERBOSITY
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is set,
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the entire pool structure is printed
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(via
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.BR print )
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each time the pool is locked or unlocked.
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.B POOL_DEBUGGING
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enables internal
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debugging output,
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whose format is unspecified and volatile.
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It should not be used by most programs.
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When
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.B POOL_LOGGING
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is set, a single line is printed via
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.B print
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at the beginning and end of each pool call.
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If
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.B logstack
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is not nil,
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it will be called as well.
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This provides a mechanism for external programs to search for leaks.
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(See
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.IR leak (1)
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for one such program.)
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.PP
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The pool routines are strict about the amount of space callers use.
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If even a single byte is written past the end of the allotted space of a block, they
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will notice when that block is next used in a call to
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.I poolrealloc
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or
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.I free
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(or at the next entry into the allocator, when
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.B POOL_PARANOIA
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is set),
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and
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.B panic
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will be called.
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Since forgetting to allocate space for the
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terminating NUL on strings is such a common error,
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if
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.B POOL_TOLERANCE
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is set and a single NUL is found written past the end of a block,
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.B print
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will be called with a notification, but
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.B panic
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will not be.
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.PP
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When
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.B POOL_NOREUSE
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is set,
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.B poolfree
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fills the passed block with garbage rather than
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return it to the free pool.
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.SH SOURCE
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.B /sys/src/libc/port/pool.c
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.SH SEE ALSO
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.IR malloc (2),
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.IR brk (2)
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.PP
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.B /sys/src/libc/port/malloc.c
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is a complete example.
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