POOL(2) POOL(2)
NAME
poolalloc, poolfree, poolmsize, poolrealloc, poolcompact,
poolcheck, poolblockcheck, pooldump - general memory
management routines
SYNOPSIS
#include <u.h>
#include <libc.h>
#include <pool.h>
void* poolalloc(Pool* pool, ulong size)
void poolfree(Pool* pool, void* ptr)
ulong poolmsize(Pool* pool, void* ptr)
void* poolrealloc(Pool* pool, void* ptr, ulong size)
void poolcompact(Pool* pool)
void poolcheck(Pool *pool)
void poolblockcheck(Pool *pool, void *ptr)
void pooldump(Pool *pool);
DESCRIPTION
These routines provide a general memory management facility.
Memory is retrieved from a coarser allocator (e.g. sbrk or
the kernel's xalloc) and then allocated to callers. The
routines are locked and thus may safely be used in multipro-
cess programs.
Poolalloc attempts to allocate a block of size size; it
returns a pointer to the block when successful and nil oth-
erwise. The call poolalloc(0) returns a non-nil pointer.
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 poolalloc. The call poolfree(nil) is legal
and is a no-op. Poolrealloc attempts to resize to nsize
bytes the block associated with ptr, which must have been
previously returned by poolalloc or 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 poolrealloc, the return value should be used rather
than ptr to access the block. If the request cannot be sat-
isfied, poolrealloc returns nil, and the old pointer remains
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valid.
When blocks are allocated, there is often some extra space
left at the end that would usually go unused. Poolmsize
grows the block to encompass this extra space and returns
the new size.
The poolblockcheck and poolcheck routines validate a single
allocated block or the entire pool, respectively. They call
panic (see below) if corruption is detected. Pooldump
prints a summary line for every block in the pool, using the
print function (see below).
The Pool structure itself provides much of the setup inter-
face.
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;
};
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,
};
The pool obtains arenas of memory to manage by calling the
the given alloc routine. The total number of requested
bytes will not exceed maxsize. Each allocation request will
be for at least minarena bytes.
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When a new arena is allocated, the pool routines try to
merge it with the surrounding arenas, in an attempt to com-
bat fragmentation. If merge is non-nil, it is called with
the addresses of two blocks from alloc that the pool rou-
tines suspect might be adjacent. If they are not mergeable,
merge must return zero. If they are mergeable, merge should
merge them into one block in its own bookkeeping and return
non-zero.
To ease fragmentation and make block reuse easier, the sizes
requested of the pool routines are rounded up to a multiple
of quantum before the carrying out requests. If, after
rounding, the block size is still less than minblock bytes,
minblock will be used as the block size.
Poolcompact defragments the pool, moving blocks in order to
aggregate the free space. Each time it moves a block, it
notifies the move routine that the contents have moved. At
the time that move is called, the contents have already
moved, so from should never be dereferenced. If no move
routine is supplied (i.e. it is nil), then calling
poolcompact is a no-op.
When the pool routines need to allocate a new arena but can-
not, either because alloc has returned nil or because doing
so would use more than maxsize bytes, poolcompact is called
once to defragment the memory and the request is retried.
Pools are protected by the pool routines calling lock (when
non-nil) before modifying the pool, and calling unlock when
finished.
When internal corruption is detected, panic is called with a
print(2) style argument that specifies what happened. It is
assumed that panic never returns. When the pool routines
wish to convey a message to the caller (usually because log-
ging is turned on; see below), print is called, also with a
print(2) style argument.
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 POOL_ANTAGONISM is set, poolalloc
fills blocks with non-zero garbage before releasing them to
the user, and 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 allo-
cated block and the constant varies in order to distinguish
different markings. Freed blocks use the constant
0xF7000000, newly allocated blocks 0xF9000000, and newly
created unallocated blocks 0xF1000000. For example, if
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POOL(2) POOL(2)
POOL_ANTAGONISM is set and poolalloc returns a block start-
ing at 0x00012345, each word of the block will contain the
value 0xF90012345. Recognizing these numbers in memory-
related crashes can help diagnose things like double-frees
or dangling pointers.
Setting 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 mag-
nitude when many blocks are in use. If POOL_VERBOSITY is
set, the entire pool structure is printed (via print) each
time the pool is locked or unlocked. POOL_DEBUGGING enables
internal debugging output, whose format is unspecified and
volatile. It should not be used by most programs. When
POOL_LOGGING is set, a single line is printed via print at
the beginning and end of each pool call. If logstack is not
nil, it will be called as well. This provides a mechanism
for external programs to search for leaks. (See
debugmalloc(2) for one such mechanism.)
The pool routines are strict about the amount of space call-
ers 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 poolrealloc or free (or at
the next entry into the allocator, when POOL_PARANOIA is
set), and panic will be called. Since forgetting to allo-
cate space for the terminating NUL on strings is such a com-
mon error, if POOL_TOLERANCE is set and a single NUL is
found written past the end of a block, print will be called
with a notification, but panic will not be.
EXAMPLE
A complete example follows.
static void* sbrkalloc(ulong);
static int sbrkmerge(void*, void*);
static void plock(Pool*);
static void punlock(Pool*);
static void pprint(Pool*, char*, ...);
static void ppanic(Pool*, char*, ...);
typedef struct Private Private;
struct Private {
Lock;
int fd;
};
static Private sbrkpriv;
Pool sbrkmem = {
.name= "sbrkmem",
.maxsize= 2*1024*1024*1024,
.minarena= 4*1024,
.quantum= 32,
.alloc= sbrkalloc,
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.merge= sbrkmerge,
.flags= 0,
.lock= plock,
.unlock= punlock,
.print= pprint,
.panic= ppanic,
.private= &sbrkpriv,
};
/* sbrk with minimal bookkeeping so we can handle merge calls */
static void*
sbrkalloc(ulong n)
{
long *x;
n += 8; /* two longs for us */
x = sbrk(n);
if((int)x == -1)
x = nil;
x[0] = (n+7)&~7; /* sbrk rounds size up to mult. of 8 */
x[1] = 0xDeadBeef;
return x+2;
}
static int
sbrkmerge(void *x, void *y)
{
long *lx, *ly;
lx = x;
if(lx[-1] != 0xDeadBeef)
abort();
if((uchar*)lx+lx[-2] == (uchar*)y) {
ly = y;
lx[-2] += ly[-2];
return 1;
}
return 0;
}
static void
plock(Pool *p)
{
Private *priv = p->private;
lock(priv);
}
static void
punlock(Pool *p)
{
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POOL(2) POOL(2)
Private *priv = p->private;
unlock(priv);
}
static void
pprint(Pool *p, char *fmt, ...)
{
Private *priv = p->private;
int n;
va_list v;
char buf[128];
va_start(v, fmt);
n = doprint(buf, buf+sizeof buf, fmt, v)-buf;
va_end(v);
write(priv->fd, buf, n);
}
static void
ppanic(Pool*, char *fmt, ...)
{
va_list v;
int n;
char buf[128];
va_start(v, fmt);
n = doprint(buf, buf+sizeof buf, fmt, v)-buf;
va_end(v);
write(2, "panic: ", 7);
write(2, buf, n);
write(2, "0, 1);
abort();
}
void*
malloc(ulong size)
{
return poolalloc(&sbrkmem, size);
}
SOURCE
/sys/src/libc/port/pool.c
SEE ALSO
debugmalloc(2), malloc(2), brk(2)
BUGS
The implementation of POOL_TOLERANCE is slightly incomplete
- there exists one case in which the pool is still not tol-
erant of NUL bytes.
Errstr is not always set.
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POOL(2) POOL(2)
The example is too long.
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