plan9fox/sys/man/2/pool

.TH POOL 2
.SH NAME
poolalloc, poolallocalign, poolfree, poolmsize, poolrealloc, poolcompact, poolcheck, poolblockcheck,
pooldump \- general memory management routines
.SH SYNOPSIS
.B #include <u.h>
.br
.B #include <libc.h>
.br
.B #include <pool.h>
.PP
.B
void*	poolalloc(Pool* pool, ulong size)
.PP
.B
void*	poolallocalign(Pool *pool, ulong size, 
.br
.B
                ulong align, long offset, ulong span)
.PP
.B
void	poolfree(Pool* pool, void* ptr)
.PP
.B
ulong	poolmsize(Pool* pool, void* ptr)
.PP
.B
void*	poolrealloc(Pool* pool, void* ptr, ulong size)
.PP
.B
void	poolcompact(Pool* pool)
.PP
.B
void	poolcheck(Pool *pool)
.PP
.B
void	poolblockcheck(Pool *pool, void *ptr)
.PP
.B
void	pooldump(Pool *pool);
.SH DESCRIPTION
These routines provide a general memory management facility.
Memory is retrieved from a coarser allocator (e.g. 
.I sbrk
or the kernel's 
.IR xalloc )
and then allocated to callers.
The routines are locked and thus may safely be used in
multiprocess programs.
.PP
.I Poolalloc
attempts to allocate a block of size
.BR size ;
it returns a pointer to the block when successful and nil otherwise.
The call
.B "poolalloc(0)
returns a non-nil pointer.
.I Poolfree
returns an allocated block to the pool.
It is an error to free a block more than once or to free a
pointer not returned by
.IR poolalloc .
The call
.B "poolfree(nil)
is legal and is a no-op.
.PP
.I Poolallocalign
attempts to allocate a block of size
.B size
with the given alignment constraints.
If
.I align
is non-zero,
the returned pointer is aligned to be equal to
.I offset
modulo
.IR align .
If
.I span
is non-zero,
the
.I n
byte block allocated will not span a
.IR span -byte
boundary.
.PP
.I Poolrealloc
attempts to resize to
.B nsize
bytes the block associated with
.BR ptr ,
which must have been previously returned by
.I poolalloc
or 
.IR poolrealloc .
If the block's size can be adjusted, a (possibly different) pointer to the new block is returned.
The contents up to the lesser of the old and new sizes are unchanged.
After a successful call to
.IR poolrealloc ,
the return value should be used rather than
.B ptr
to access the block.
If the request cannot be satisfied, 
.I poolrealloc
returns nil, and the old pointer remains valid.
.PP
When blocks are allocated, there is often some extra space left at the end
that would usually go unused.
.IR Poolmsize
grows the block to encompass this extra space and returns the new size.
.PP
The 
.I poolblockcheck
and
.I poolcheck
routines validate a single allocated block or the entire pool, respectively.
They call 
.B panic
(see below)
if corruption is detected.
.I Pooldump
prints a summary line for every block in the
pool, using the
.B print
function (see below).
.PP
The
.B Pool
structure itself provides much of the setup interface.
.IP
.EX
.ta \w'\fL    'u +\w'\fLulong 'u +\w'\fLlastcompact;  'u
typedef struct Pool Pool;
struct Pool {
	char*	name;
	ulong	maxsize;	/* of entire Pool */
	ulong	cursize;	/* of Pool */
	ulong	curfree;	/* total free bytes in Pool */
	ulong	curalloc;	/* total allocated bytes in Pool */
	ulong	minarena;	/* smallest size of new arena */
	ulong	quantum;	/* allocated blocks should be multiple of */
	ulong	minblock;	/* smallest newly allocated block */
	int	flags;
	int	nfree;	/* number of calls to free */
	int	lastcompact;	/* nfree at time of last poolcompact */
	void*	(*alloc)(ulong);
	int	(*merge)(void*, void*);
	void	(*move)(void* from, void* to);
	void	(*lock)(Pool*);
	void	(*unlock)(Pool*);
	void	(*print)(Pool*, char*, ...);
	void	(*panic)(Pool*, char*, ...);
	void	(*logstack)(Pool*);
	void*	private;
};
.ta \w'\fL    'u +\w'POOL_ANTAGONISM 'u
enum {  /* flags */
	POOL_ANTAGONISM	= 1<<0,
	POOL_PARANOIA	= 1<<1,
	POOL_VERBOSITY	= 1<<2,
	POOL_DEBUGGING	= 1<<3,
	POOL_LOGGING	= 1<<4,
	POOL_TOLERANCE	= 1<<5,
	POOL_NOREUSE	= 1<<6,
};
.EE
.PP
The pool obtains arenas of memory to manage by calling the the given
.B alloc
routine.
The total number of requested bytes will not exceed 
.BR maxsize .
Each allocation request will be for at least
.B minarena
bytes.
.PP
When a new arena is allocated, the pool routines try to
merge it with the surrounding arenas, in an attempt to combat fragmentation.
If 
.B merge
is non-nil, it is called with the addresses of two blocks from
.B alloc
that the pool routines suspect might be adjacent.
If they are not mergeable, 
.B merge
must return zero.
If they are mergeable, 
.B merge
should merge them into one block in its own bookkeeping
and return non-zero.
.PP
To ease fragmentation and make 
block reuse easier, the sizes requested of the pool routines are rounded up to a multiple of
.B quantum
before 
the carrying out requests.
If, after rounding, the block size is still less than
.B minblock
bytes, 
.B minblock
will be used as the block size.
.PP
.I Poolcompact
defragments the pool, moving blocks in order to aggregate
the free space.
Each time it moves a block, it notifies the
.B move
routine that the contents have moved.
At the time that
.B move
is called, the contents have already moved,
so 
.B from
should never be dereferenced.
If no
.B move
routine is supplied (i.e. it is nil), then calling
.I poolcompact
is a no-op.
.PP
When the pool routines need to allocate a new arena but cannot,
either because
.B alloc
has returned nil or because doing so would use more than
.B maxsize
bytes,
.I poolcompact
is called once to defragment the memory
and the request is retried.
.PP
.I Pools
are protected by the pool routines calling
.B lock
(when non-nil)
before modifying the pool, and
calling
.B unlock
when finished.
.PP
When internal corruption is detected,
.B panic
is called with a 
.IR print (2)
style argument that specifies what happened.
It is assumed that 
.B panic
never returns.
When the pool routines wish to convey a message
to the caller (usually because logging is turned on; see below),
.B print
is called, also with a 
.IR print (2)
style argument.
.PP
.B Flags
is a bit vector that tweaks the behavior of the pool routines
in various ways.
Most are useful for debugging in one way or another.
When
.B POOL_ANTAGONISM
is set,
.I poolalloc
fills blocks with non-zero garbage before releasing them to the user,
and
.I poolfree
fills the blocks on receipt.
This tickles both user programs and the innards of the allocator.
Specifically, each 32-bit word of the memory is marked with a pointer value exclusive-or'ed
with a constant.
The pointer value is the pointer to the beginning of the allocated block
and the constant varies in order to distinguish different markings.
Freed blocks use the constant 
.BR 0xF7000000 ,
newly allocated blocks
.BR 0xF9000000 ,
and newly created unallocated blocks
.BR 0xF1000000 .
For example, if 
.B POOL_ANTAGONISM 
is set and
.I poolalloc
returns a block starting at 
.BR 0x00012345 ,
each word of the block will contain the value
.BR 0xF90012345 .
Recognizing these numbers in memory-related crashes can
help diagnose things like double-frees or dangling pointers.
.PP
Setting
.B POOL_PARANOIA
causes the allocator to walk the entire pool whenever locking or unlocking itself,
looking for corruption.
This slows runtime by a few orders of magnitude
when many blocks are in use.
If 
.B POOL_VERBOSITY
is set, 
the entire pool structure is printed
(via 
.BR print )
each time the pool is locked or unlocked.
.B POOL_DEBUGGING
enables internal
debugging output,
whose format is unspecified and volatile.
It should not be used by most programs.
When
.B POOL_LOGGING
is set, a single line is printed via 
.B print
at the beginning and end of each pool call.
If 
.B logstack
is not nil, 
it will be called as well.
This provides a mechanism for external programs to search for leaks.
(See 
.IR leak (1)
for one such program.)
.PP
The pool routines are strict about the amount of space callers use.
If even a single byte is written past the end of the allotted space of a block, they
will notice when that block is next used in a call to
.I poolrealloc
or 
.I free
(or at the next entry into the allocator, when
.B POOL_PARANOIA
is set),
and
.B panic
will be called.
Since forgetting to allocate space for the
terminating NUL on strings is such a common error,
if
.B POOL_TOLERANCE 
is set and a single NUL is found written past the end of a block,
.B print
will be called with a notification, but
.B panic
will not be.
.PP
When
.B POOL_NOREUSE
is set,
.B poolfree
fills the passed block with garbage rather than
return it to the free pool.
.SH SOURCE
.B /sys/src/libc/port/pool.c
.SH SEE ALSO
.IR malloc (2),
.IR brk (2)
.PP
.B /sys/src/libc/port/malloc.c
is a complete example.