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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.

     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.

     E	     Double/halve the size of the maximum medium size class.  The
	     valid range is from one page to one half chunk.  The default
	     value is 32 KiB.

     F	     Halve/double the per-arena	minimum	ratio of active	to dirty
	     pages.  Some dirty	unused pages may be allowed to accumulate,
	     within the	limit set by the ratio,	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 minimum ratio is 32:1; MALLOC_OPTIONS=6F will disable
	     dirty page	purging.

     G	     Double/halve the approximate interval (counted in terms of
	     thread-specific cache allocation/deallocation events) between
	     full thread-specific cache	garbage	collection sweeps.  Garbage
	     collection	is actually performed incrementally, one size class at
	     a time, in	order to avoid large collection	pauses.	 The default
	     sweep interval is 8192; MALLOC_OPTIONS=14g	will disable garbage
	     collection.

     H	     Double/halve the number of	thread-specific	cache slots per	size
	     class.  When there	are multiple threads, each thread uses a
	     thread-specific cache for small and medium	objects.  Thread-spe-
	     cific caching allows many allocations to be satisfied without
	     performing	any thread synchronization, at the cost	of increased
	     memory use.  See the ``G''	option for related tuning information.
	     The default number	of cache slots is 128; MALLOC_OPTIONS=7h will
	     disable thread-specific caching.  Note that one cache slot	per
	     size class	is not a valid configuration due to implementation
	     details.

     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 4 MiB.

     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.

     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_FILENO 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 and medium objects, in order to make it possible	to
     completely	avoid synchronization for most small and medium	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 four categories according to size: small,
     medium, large, and	huge.  Small objects are smaller than one page.
     Medium objects range from one page	to an upper limit determined at	run
     time (see the ``E'' option).  Large objects are smaller than the chunk
     size.  Huge objects are a multiple	of the chunk size.  Small, medium, 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	struc-
     ture 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 or medium	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 and medium 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	mini-
     mum cacheline-multiple size class (see the	``Q'' option) are rounded up
     to	the nearest multiple of	the quantum.  Allocation requests that are
     more than the minimum cacheline-multiple size class, but no more than the
     minimum subpage-multiple 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, but no	more
     than the maximum subpage-multiple size class are rounded up to the	near-
     est multiple of the subpage size (256).  Allocation requests that are
     more than the maximum subpage-multiple size class,	but no more than the
     maximum medium size class (see the	``M'' option) are rounded up to	the
     nearest medium size class;	spacing	is an automatically determined power
     of	two and	ranges from the	subpage	size to	the page size.	Allocation
     requests that are more than the maximum medium size class,	but small
     enough to fit in an arena-managed chunk (see the ``K'' option), are
     rounded up	to the nearest run size.  Allocation requests that are too
     large to fit in an	arena-managed chunk are	rounded	up to the nearest mul-
     tiple 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_FILENO 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		       January 31, 2010			   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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