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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
     memory 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
     allocation 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
     implementation-dependent.

TUNING
     Once, when the first call is made to one of these memory allocation
     routines, various flags will be set or reset, which affect 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
             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
             arenas, 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
             sensitive 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
             configurations (non-PIC).

     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.

     H       Obsoleted by the ``F'' option.  MALLOC_OPTIONS=H sets the per-
             arena maximum number of dirty unused pages to 0, and
             MALLOC_OPTIONS=h resets the per-arena maximum number of dirty
             unused pages to the default.  This option will be removed in
             FreeBSD 8.0.

     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
             negatively.

     K       Double/halve the virtual memory chunk size.  The default chunk
             size is 1 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 four 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
             executing in the memory allocation functions.  Therefore, this
             option should only be used with care; it is primarily intended as
             a performance tuning aid during application development.

     Q       Double/halve the size of the allocation quantum.  The default
             quantum is the minimum allowed by the architecture (typically 8
             or 16 bytes).

     S       Double/halve the size of the maximum size class that is a
             multiple of the quantum.  Above this size, power-of-two spacing
             is used for size classes.  The default value is 512 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 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
             initialization 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
     configured 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
     configuration, the vmemoryuse resource limit can be used to bound the
     total virtual 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
     completely 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 substantially more arenas than the default is not likely to improve
     performance, 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.

     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 no larger than one half of a 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 and logarithmic time, respectively.

     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 (see the ``Q'' option) are rounded up
     to the nearest power of two (typically 2, 4, or 8).  Allocation requests
     that are more than half the quantum, but no more than the maximum
     quantum-multiple size class (see the ``S'' option) are rounded up to the
     nearest multiple of the quantum.  Allocation requests that are larger
     than the maximum quantum-multiple size class, but no larger than one half
     of a page, are rounded up to the nearest power of two.  Allocation
     requests that are larger than half of a page, 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
     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 cache line sharing, round your allocation requests up to the
     nearest multiple of the cache line 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
     normal 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
     program 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
     problems by trading performance for extra sanity checks and detailed
     diagnostics.

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
     function 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
     identical 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()
     deallocates it in this case.

     The free() function returns no value.

     The malloc_usable_size() function returns the usable size of the
     allocation pointed to by ptr.

ENVIRONMENT
     The following environment variables affect the execution of the
     allocation 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), 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 11.0-PRERELEASE        February 17, 2008       FreeBSD 11.0-PRERELEASE

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

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