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MALLOC(3)	       FreeBSD Library Functions Manual		     MALLOC(3)

NAME
     malloc, calloc, realloc, free, reallocf, malloc_usable_size -- general
     purpose memory allocation functions

LIBRARY
     Standard C	Library	(libc, -lc)

SYNOPSIS
     #include <stdlib.h>

     void *
     malloc(size_t size);

     void *
     calloc(size_t number, size_t size);

     void *
     realloc(void *ptr,	size_t size);

     void *
     reallocf(void *ptr, size_t	size);

     void
     free(void *ptr);

     const char	* _malloc_options;

     void
     (*_malloc_message)(const char *p1,	const char *p2,	const char *p3,
	 const char *p4);

     #include <malloc_np.h>

     size_t
     malloc_usable_size(const void *ptr);

DESCRIPTION
     The malloc() function allocates size bytes	of uninitialized memory.  The
     allocated space is	suitably aligned (after	possible pointer coercion) for
     storage of	any type of object.

     The calloc() function allocates space for number objects, each size bytes
     in	length.	 The result is identical to calling malloc() with an argument
     of	``number * size'', with	the exception that the allocated memory	is
     explicitly	initialized to zero bytes.

     The realloc() function changes the	size of	the previously allocated mem-
     ory referenced by ptr to size bytes.  The contents	of the memory are
     unchanged up to the lesser	of the new and old sizes.  If the new size is
     larger, the contents of the newly allocated portion of the	memory are
     undefined.	 Upon success, the memory referenced by	ptr is freed and a
     pointer to	the newly allocated memory is returned.	 Note that realloc()
     and reallocf() may	move the memory	allocation, resulting in a different
     return value than ptr.  If	ptr is NULL, the realloc() function behaves
     identically to malloc() for the specified size.

     The reallocf() function is	identical to the realloc() function, except
     that it will free the passed pointer when the requested memory cannot be
     allocated.	 This is a FreeBSD specific API	designed to ease the problems
     with traditional coding styles for	realloc	causing	memory leaks in
     libraries.

     The free()	function causes	the allocated memory referenced	by ptr to be
     made available for	future allocations.  If	ptr is NULL, no	action occurs.

     The malloc_usable_size() function returns the usable size of the alloca-
     tion pointed to by	ptr.  The return value may be larger than the size
     that was requested	during allocation.  The	malloc_usable_size() function
     is	not a mechanism	for in-place realloc();	rather it is provided solely
     as	a tool for introspection purposes.  Any	discrepancy between the
     requested allocation size and the size reported by	malloc_usable_size()
     should not	be depended on,	since such behavior is entirely	implementa-
     tion-dependent.

TUNING
     Once, when	the first call is made to one of these memory allocation rou-
     tines, various flags will be set or reset,	which affects the workings of
     this allocator implementation.

     The ``name'' of the file referenced by the	symbolic link named
     /etc/malloc.conf, the value of the	environment variable MALLOC_OPTIONS,
     and the string pointed to by the global variable _malloc_options will be
     interpreted, in that order, from left to right as flags.

     Each flag is a single letter, optionally prefixed by a non-negative base
     10	integer	repetition count.  For example,	``3N'' is equivalent to
     ``NNN''.  Some flags control parameter magnitudes,	where uppercase
     increases the magnitude, and lowercase decreases the magnitude.  Other
     flags control boolean parameters, where uppercase indicates that a	behav-
     ior is set, or on,	and lowercase means that a behavior is not set,	or
     off.

     A	     All warnings (except for the warning about	unknown	flags being
	     set) become fatal.	 The process will call abort(3)	in these
	     cases.

     B	     Double/halve the per-arena	lock contention	threshold at which a
	     thread is randomly	re-assigned to an arena.  This dynamic load
	     balancing tends to	push threads away from highly contended	are-
	     nas, which	avoids worst case contention scenarios in which
	     threads disproportionately	utilize	arenas.	 However, due to the
	     highly dynamic load that applications may place on	the allocator,
	     it	is impossible for the allocator	to know	in advance how sensi-
	     tive it should be to contention over arenas.  Therefore, some
	     applications may benefit from increasing or decreasing this
	     threshold parameter.  This	option is not available	for some con-
	     figurations (non-PIC).

     C	     Double/halve the size of the maximum size class that is a multi-
	     ple of the	cacheline size (64).  Above this size, subpage spacing
	     (256 bytes) is used for size classes.  The	default	value is 512
	     bytes.

     D	     Use sbrk(2) to acquire memory in the data storage segment (DSS).
	     This option is enabled by default.	 See the ``M'' option for
	     related information and interactions.

     F	     Double/halve the per-arena	maximum	number of dirty	unused pages
	     that are allowed to accumulate before informing the kernel	about
	     at	least half of those pages via madvise(2).  This	provides the
	     kernel with sufficient information	to recycle dirty pages if
	     physical memory becomes scarce and	the pages remain unused.  The
	     default is	512 pages per arena; MALLOC_OPTIONS=10f	will prevent
	     any dirty unused pages from accumulating.

     G	     When there	are multiple threads, use thread-specific caching for
	     objects that are smaller than one page.  This option is enabled
	     by	default.  Thread-specific caching allows many allocations to
	     be	satisfied without performing any thread	synchronization, at
	     the cost of increased memory use.	See the	``R'' option for
	     related tuning information.  This option is not available for
	     some configurations (non-PIC).

     J	     Each byte of new memory allocated by malloc(), realloc() or
	     reallocf()	will be	initialized to 0xa5.  All memory returned by
	     free(), realloc() or reallocf() will be initialized to 0x5a.
	     This is intended for debugging and	will impact performance	nega-
	     tively.

     K	     Double/halve the virtual memory chunk size.  The default chunk
	     size is the maximum of 1 MB and the largest page size that	is
	     less than or equal	to 4 MB.

     M	     Use mmap(2) to acquire anonymously	mapped memory.	This option is
	     enabled by	default.  If both the ``D'' and	``M'' options are
	     enabled, the allocator prefers anonymous mappings over the	DSS,
	     but allocation only fails if memory cannot	be acquired via	either
	     method.  If neither option	is enabled, then the ``M'' option is
	     implicitly	enabled	in order to assure that	there is a method for
	     acquiring memory.

     N	     Double/halve the number of	arenas.	 The default number of arenas
	     is	two times the number of	CPUs, or one if	there is a single CPU.

     P	     Various statistics	are printed at program exit via	an atexit(3)
	     function.	This has the potential to cause	deadlock for a multi-
	     threaded process that exits while one or more threads are execut-
	     ing in the	memory allocation functions.  Therefore, this option
	     should only be used with care; it is primarily intended as	a per-
	     formance tuning aid during	application development.

     Q	     Double/halve the size of the maximum size class that is a multi-
	     ple of the	quantum	(8 or 16 bytes,	depending on architecture).
	     Above this	size, cacheline	spacing	is used	for size classes.  The
	     default value is 128 bytes.

     R	     Double/halve magazine size, which approximately doubles/halves
	     the number	of rounds in each magazine.  Magazines are used	by the
	     thread-specific caching machinery to acquire and release objects
	     in	bulk.  Increasing the magazine size decreases locking over-
	     head, at the expense of increased memory usage.  This option is
	     not available for some configurations (non-PIC).

     U	     Generate ``utrace'' entries for ktrace(1),	for all	operations.
	     Consult the source	for details on this option.

     V	     Attempting	to allocate zero bytes will return a NULL pointer
	     instead of	a valid	pointer.  (The default behavior	is to make a
	     minimal allocation	and return a pointer to	it.)  This option is
	     provided for System V compatibility.  This	option is incompatible
	     with the ``X'' option.

     X	     Rather than return	failure	for any	allocation function, display a
	     diagnostic	message	on stderr and cause the	program	to drop	core
	     (using abort(3)).	This option should be set at compile time by
	     including the following in	the source code:

		   _malloc_options = "X";

     Z	     Each byte of new memory allocated by malloc(), realloc() or
	     reallocf()	will be	initialized to 0.  Note	that this initializa-
	     tion only happens once for	each byte, so realloc()	and reallocf()
	     calls do not zero memory that was previously allocated.  This is
	     intended for debugging and	will impact performance	negatively.

     The ``J'' and ``Z'' options are intended for testing and debugging.  An
     application which changes its behavior when these options are used	is
     flawed.

IMPLEMENTATION NOTES
     Traditionally, allocators have used sbrk(2) to obtain memory, which is
     suboptimal	for several reasons, including race conditions,	increased
     fragmentation, and	artificial limitations on maximum usable memory.  This
     allocator uses both sbrk(2) and mmap(2) by	default, but it	can be config-
     ured at run time to use only one or the other.  If	resource limits	are
     not a primary concern, the	preferred configuration	is MALLOC_OPTIONS=dM
     or	MALLOC_OPTIONS=DM.  When so configured,	the datasize resource limit
     has little	practical effect for typical applications; use
     MALLOC_OPTIONS=Dm if that is a concern.  Regardless of allocator configu-
     ration, the vmemoryuse resource limit can be used to bound	the total vir-
     tual memory used by a process, as described in limits(1).

     This allocator uses multiple arenas in order to reduce lock contention
     for threaded programs on multi-processor systems.	This works well	with
     regard to threading scalability, but incurs some costs.  There is a small
     fixed per-arena overhead, and additionally, arenas	manage memory com-
     pletely independently of each other, which	means a	small fixed increase
     in	overall	memory fragmentation.  These overheads are not generally an
     issue, given the number of	arenas normally	used.  Note that using sub-
     stantially	more arenas than the default is	not likely to improve perfor-
     mance, mainly due to reduced cache	performance.  However, it may make
     sense to reduce the number	of arenas if an	application does not make much
     use of the	allocation functions.

     In	addition to multiple arenas, this allocator supports thread-specific
     caching for small objects (smaller	than one page),	in order to make it
     possible to completely avoid synchronization for most small allocation
     requests.	Such caching allows very fast allocation in the	common case,
     but it increases memory usage and fragmentation, since a bounded number
     of	objects	can remain allocated in	each thread cache.

     Memory is conceptually broken into	equal-sized chunks, where the chunk
     size is a power of	two that is greater than the page size.	 Chunks	are
     always aligned to multiples of the	chunk size.  This alignment makes it
     possible to find metadata for user	objects	very quickly.

     User objects are broken into three	categories according to	size: small,
     large, and	huge.  Small objects are smaller than one page.	 Large objects
     are smaller than the chunk	size.  Huge objects are	a multiple of the
     chunk size.  Small	and large objects are managed by arenas; huge objects
     are managed separately in a single	data structure that is shared by all
     threads.  Huge objects are	used by	applications infrequently enough that
     this single data structure	is not a scalability issue.

     Each chunk	that is	managed	by an arena tracks its contents	as runs	of
     contiguous	pages (unused, backing a set of	small objects, or backing one
     large object).  The combination of	chunk alignment	and chunk page maps
     makes it possible to determine all	metadata regarding small and large
     allocations in constant time.

     Small objects are managed in groups by page runs.	Each run maintains a
     bitmap that tracks	which regions are in use.  Allocation requests that
     are no more than half the quantum (8 or 16, depending on architecture)
     are rounded up to the nearest power of two.  Allocation requests that are
     more than half the	quantum, but no	more than the minimum cacheline-multi-
     ple size class (see the ``Q'' option) are rounded up to the nearest mul-
     tiple of the quantum.  Allocation requests	that are more than the minumum
     cacheline-multiple	size class, but	no more	than the minimum subpage-mul-
     tiple size	class (see the ``C'' option) are rounded up to the nearest
     multiple of the cacheline size (64).  Allocation requests that are	more
     than the minimum subpage-multiple size class are rounded up to the	near-
     est multiple of the subpage size (256).  Allocation requests that are
     more than one page, but small enough to fit in an arena-managed chunk
     (see the ``K'' option), are rounded up to the nearest run size.  Alloca-
     tion requests that	are too	large to fit in	an arena-managed chunk are
     rounded up	to the nearest multiple	of the chunk size.

     Allocations are packed tightly together, which can	be an issue for	multi-
     threaded applications.  If	you need to assure that	allocations do not
     suffer from cacheline sharing, round your allocation requests up to the
     nearest multiple of the cacheline size.

DEBUGGING MALLOC PROBLEMS
     The first thing to	do is to set the ``A'' option.	This option forces a
     coredump (if possible) at the first sign of trouble, rather than the nor-
     mal policy	of trying to continue if at all	possible.

     It	is probably also a good	idea to	recompile the program with suitable
     options and symbols for debugger support.

     If	the program starts to give unusual results, coredump or	generally
     behave differently	without	emitting any of	the messages mentioned in the
     next section, it is likely	because	it depends on the storage being	filled
     with zero bytes.  Try running it with the ``Z'' option set; if that
     improves the situation, this diagnosis has	been confirmed.	 If the	pro-
     gram still	misbehaves, the	likely problem is accessing memory outside the
     allocated area.

     Alternatively, if the symptoms are	not easy to reproduce, setting the
     ``J'' option may help provoke the problem.

     In	truly difficult	cases, the ``U'' option, if supported by the kernel,
     can provide a detailed trace of all calls made to these functions.

     Unfortunately this	implementation does not	provide	much detail about the
     problems it detects; the performance impact for storing such information
     would be prohibitive.  There are a	number of allocator implementations
     available on the Internet which focus on detecting	and pinpointing	prob-
     lems by trading performance for extra sanity checks and detailed diagnos-
     tics.

DIAGNOSTIC MESSAGES
     If	any of the memory allocation/deallocation functions detect an error or
     warning condition,	a message will be printed to file descriptor
     STDERR_FILENO.  Errors will result	in the process dumping core.  If the
     ``A'' option is set, all warnings are treated as errors.

     The _malloc_message variable allows the programmer	to override the	func-
     tion which	emits the text strings forming the errors and warnings if for
     some reason the stderr file descriptor is not suitable for	this.  Please
     note that doing anything which tries to allocate memory in	this function
     is	likely to result in a crash or deadlock.

     All messages are prefixed by ``<progname>:	(malloc)''.

RETURN VALUES
     The malloc() and calloc() functions return	a pointer to the allocated
     memory if successful; otherwise a NULL pointer is returned	and errno is
     set to ENOMEM.

     The realloc() and reallocf() functions return a pointer, possibly identi-
     cal to ptr, to the	allocated memory if successful;	otherwise a NULL
     pointer is	returned, and errno is set to ENOMEM if	the error was the
     result of an allocation failure.  The realloc() function always leaves
     the original buffer intact	when an	error occurs, whereas reallocf() deal-
     locates it	in this	case.

     The free()	function returns no value.

     The malloc_usable_size() function returns the usable size of the alloca-
     tion pointed to by	ptr.

ENVIRONMENT
     The following environment variables affect	the execution of the alloca-
     tion functions:

     MALLOC_OPTIONS  If	the environment	variable MALLOC_OPTIONS	is set,	the
		     characters	it contains will be interpreted	as flags to
		     the allocation functions.

EXAMPLES
     To	dump core whenever a problem occurs:

	   ln -s 'A' /etc/malloc.conf

     To	specify	in the source that a program does no return value checking on
     calls to these functions:

	   _malloc_options = "X";

SEE ALSO
     limits(1),	madvise(2), mmap(2), sbrk(2), alloca(3), atexit(3),
     getpagesize(3), getpagesizes(3), memory(3), posix_memalign(3)

STANDARDS
     The malloc(), calloc(), realloc() and free() functions conform to ISO/IEC
     9899:1990 (``ISO C90'').

HISTORY
     The reallocf() function first appeared in FreeBSD 3.0.

     The malloc_usable_size() function first appeared in FreeBSD 7.0.

FreeBSD	9.3		      September	26, 2009		   FreeBSD 9.3

NAME | LIBRARY | SYNOPSIS | DESCRIPTION | TUNING | IMPLEMENTATION NOTES | DEBUGGING MALLOC PROBLEMS | DIAGNOSTIC MESSAGES | RETURN VALUES | ENVIRONMENT | EXAMPLES | SEE ALSO | STANDARDS | HISTORY

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