General purpose memory allocation functions

MALLOC(3)               NetBSD Library Functions Manual              MALLOC(3)

NAME
     malloc, calloc, realloc, free -- general purpose memory allocation func-
     tions

LIBRARY
     Standard C Library (libc, -lc)

SYNOPSIS
     #include 

     void *
     malloc(size_t size);

     void *
     calloc(size_t number, size_t size);

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

     void
     free(void *ptr);

     const char * _malloc_options;

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 value of the newly allocated portion of the memory is unde-
     fined.  Upon success, the memory referenced by ptr is freed and a pointer
     to the newly allocated memory is returned.  Note that realloc() 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 free() function causes the allocated memory referenced by ptr to be
     made available for future allocations.  If ptr is NULL, no action occurs.

TUNING
     Once, when the first call is made to one of these memory allocation rou-
     tines, 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, character by character as flags.

     Most flags are single letters, where uppercase indicates that the behav-
     ior is set, or on, and lowercase means that the behavior is not set, or
     off.

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

     H       Use  when pages within a chunk are no longer in use,
             but the chunk as a whole cannot yet be deallocated.  This is pri-
             marily of use when swapping is a real possibility, due to the
             high overhead of the madvise() system call.

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

     K       Increase/decrease the virtual memory chunk size by a factor of
             two.  The default chunk size is 1 MB.  This option can be speci-
             fied multiple times.

     N       Increase/decrease the number of arenas by a factor of two.  The
             default number of arenas is four times the number of CPUs, or one
             if there is a single CPU.  This option can be specified multiple
             times.

     P       Various statistics are printed at program exit via an 
             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       Increase/decrease the size of the allocation quantum by a factor
             of two.  The default quantum is the minimum allowed by the archi-
             tecture (typically 8 or 16 bytes).  This option can be specified
             multiple times.

     S       Increase/decrease the size of the maximum size class that is a
             multiple of the quantum by a factor of two.  Above this size,
             power-of-two spacing is used for size classes.  The default value
             is 512 bytes.  This option can be specified multiple times.

     U       Generate ``utrace'' entries for , 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 ).  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() will be
             initialized to 0.  Note that this initialization only happens
             once for each byte, so realloc() call 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
     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.

     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 multi-
     ple 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 infre-
     quently enough that this single data structure is not a scalability
     issue.

     Each chunk that is managed by an arena tracks its contents in a page map
     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 (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 quan-
     tum-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 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() function returns 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.

     The free() function returns no value.

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), , , , , ,
     , , 

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

NetBSD 4.0                     October 15, 2007                     NetBSD 4.0

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