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MALLOC(3) NetBSD Library Functions Manual MALLOC(3)
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malloc, calloc, realloc, free -- general purpose memory allocation func-
Standard C Library (libc, -lc)
calloc(size_t number, size_t size);
realloc(void *ptr, size_t size);
const char * _malloc_options;
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.
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
A All warnings (except for the warning about unknown flags being
set) become fatal. The process will call abort(3) in these
H Use madvise(2) 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
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 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
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 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() 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
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
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
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
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 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() 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.
The following environment variables affect the execution of the alloca-
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), memory(3), posix_memalign(3)
The malloc(), calloc(), realloc() and free() functions conform to ISO/IEC
9899:1990 (``ISO C90'').
NetBSD 5.1 October 15, 2007 NetBSD 5.1