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MALLOC(3)		 BSD Library Functions Manual		     MALLOC(3)

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

     Standard C	Library	(libc, -lc)

     #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);

     free(void *ptr);

     const char	* _malloc_options;

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

     #include <malloc_np.h>

     malloc_usable_size(const void *ptr);

     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 ex-
     plicitly 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 un-
     changed 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 un-
     defined.  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 li-

     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	re-
     quested allocation	size and the size reported by malloc_usable_size()
     should not	be depended on,	since such behavior is entirely	implementa-

     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 behavior 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

     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

     D	     Use sbrk(2) to acquire memory in the data storage segment (DSS).
	     This option is enabled by default.	 See the "M" option for	re-
	     lated 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

     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 de-

     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-

     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	allo-
	     cation 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

     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 in-
	     stead 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 ap-
     plication which changes its behavior when these options are used is

     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 re-
     quests.  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 re-
     quests that are no	more than half the quantum (8 or 16, depending on ar-
     chitecture) are rounded up	to the nearest power of	two.  Allocation re-
     quests that are more than half the	quantum, but no	more than the minimum
     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 nearest
     multiple of the subpage size (256).  Allocation requests that are more
     than the maximum subpage-multiple size class, but no more than the	maxi-
     mum 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 multiple	of the chunk

     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.

     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 be-
     have 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 im-
     proves the	situation, this	diagnosis has been confirmed.  If the program
     still misbehaves, the likely problem is accessing memory outside the al-
     located 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-

     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)".

     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 re-
     sult of an	allocation failure.  The realloc() function always leaves the
     original buffer intact when an error occurs, whereas reallocf() deallo-
     cates 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.

     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.

     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";

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

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

     The reallocf() function first appeared in FreeBSD 3.0.

     The malloc_usable_size() function first appeared in FreeBSD 7.0.

BSD			       January 31, 2010				   BSD


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