malloc, calloc, realloc, free, reallocf -- general purpose memory allocation
Standard C Library (libc, -lc)
calloc(size_t number, size_t size);
realloc(void *ptr, size_t size);
reallocf(void *ptr, size_t size);
const char * _malloc_options;
(*_malloc_message)(char *p1, char *p2, char *p3, char *p4);
The malloc() function allocates size bytes of memory. The allocated
space is suitably aligned (after possible pointer coercion) for storage
of any type of object. If the space is at least pagesize bytes in length
(see getpagesize(3)), the returned memory will be page boundary aligned
as well. If malloc() fails, a NULL pointer is returned.
Note that malloc() does NOT normally initialize the returned memory to
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 value of the newly allocated portion of the memory is undefined.
If the requested memory cannot be allocated, NULL is returned and
the memory referenced by ptr is valid and unchanged. If memory can be
allocated, the memory referenced by ptr is freed and a pointer to the
newly allocated memory is returned. Note that this may be different from
the value passed as 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
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 routines,
various flags will be set or reset, which affect the workings of
this allocation 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 behavior
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
J Each byte of new memory allocated by malloc(), realloc() or
reallocf() as well as all memory returned by free(), realloc() or
reallocf() will be initialized to 0xd0. This options also sets
the ``R'' option. This is intended for debugging and will impact
H Pass a hint to the kernel about pages unused by the allocation
functions. This will help performance if the system is paging
excessively. This option is off by default.
R Causes the realloc() and reallocf() functions to always reallocate
memory even if the initial allocation was sufficiently
large. This can substantially aid in compacting memory.
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 This option implicitly sets the ``J'' and ``R'' options, and then
zeros out the bytes that were requested. This is intended for
debugging and will impact performance negatively.
< Reduce the size of the cache by a factor of two. The default
cache size is 16 pages. This option can be specified multiple
> Double the size of the cache by a factor of two. The default
cache size is 16 pages. This option can be specified multiple
The ``J'' and ``Z'' options are intended for testing and debugging. An
application which changes its behavior when these options are used is
To set a systemwide reduction of cache size, and to dump core whenever a
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";
The following environment variables affect the execution of the allocation
MALLOC_OPTIONS If the environment variable MALLOC_OPTIONS is set, the
characters it contains will be interpreted as flags to
the allocation functions.
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.
DEBUGGING MALLOC PROBLEMS [Toc] [Back]
The major difference between this implementation and other allocation
implementations is that the free pages are not accessed unless allocated,
and are aggressively returned to the kernel for reuse.
Most allocation implementations will store a data structure containing
a linked list in the free chunks of memory, used to tie all
the free memory together. That can be suboptimal, as every time
the free-list is traversed, the otherwise unused, and likely paged
out, pages are faulted into primary memory. On systems which are
paging, this can result in a factor of five increase in the number
of page-faults done by a process.
A side effect of this architecture is that many minor transgressions on
the interface which would traditionally not be detected are in fact
detected. As a result, programs that have been running happily for years
may suddenly start to complain loudly, when linked with this allocation
The first and most important 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 listed in the
next section, it is likely because it depends on the storage being filled
with zero bytes. Try running it with ``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, more likely after than before 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 allocation implementations
available on the 'Net which focus on detecting and pinpointing problems
by trading performance for extra sanity checks and detailed diagnostics.
If malloc(), calloc(), realloc() or free() 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
will assure death of the process.
The following is a brief description of possible error messages and their
(ES): mumble mumble mumble The allocation functions were compiled with
``EXTRA_SANITY'' defined, and an error was found during the additional
error checking. Consult the source code for further information.
mmap(2) failed, check limits This most likely means that the system is
dangerously overloaded or that the process' limits are incorrectly specified.
freelist is destroyed The internal free-list has been corrupted.
out of memory The ``X'' option was specified and an allocation of memory
The following is a brief description of possible warning messages and
chunk/page is already free The process attempted to free() memory which
had already been freed.
junk pointer, ... A pointer specified to one of the allocation functions
points outside the bounds of the memory of which they are aware.
malloc() has never been called No memory has been allocated, yet something
is being freed or realloc'ed.
modified (chunk-/page-) pointer The pointer passed to free() or
realloc() has been modified.
pointer to wrong page The pointer that free(), realloc(), or reallocf()
is trying to free does not reference a possible page.
recursive call A process has attempted to call an allocation function
recursively. This is not permitted. In particular, signal handlers
should not attempt to allocate memory.
unknown char in MALLOC_OPTIONS An unknown option was specified. Even
with the ``A'' option set, this warning is still only a warning.
brk(2), mmap(2), alloca(3), getpagesize(3), memory(3)
The malloc(), calloc(), realloc() and free() functions conform to ISO/IEC
9899:1990 (``ISO C89'').
The present allocation implementation started out as a file system for a
drum attached to a 20bit binary challenged computer which was built with
discrete germanium transistors. It has since graduated to handle primary
storage rather than secondary. It first appeared in its new shape and
ability in FreeBSD 2.2.
The reallocf() function first appeared in FreeBSD 3.0.
Poul-Henning Kamp <phk@FreeBSD.org>
The messages printed in case of problems provide no detail about the
It can be argued that returning a NULL pointer when asked to allocate
zero bytes is a silly response to a silly question.
FreeBSD 5.2.1 August 27, 1996 FreeBSD 5.2.1 [ Back ]