MALLOC(3) BSD Programmer's Manual MALLOC(3)NAME
malloc, calloc, realloc, free - general purpose memory allocation func-
tions
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
free(void *ptr);
char * malloc_options;
DESCRIPTION
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.
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
initialized to nul bytes.
The realloc() function changes the size of the previously allocated memo-
ry 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 value of the newly allocated portion of the memory is unde-
fined. If the requested memory cannot be allocated, NULL is returned and
the memory referenced by ptr is valid and unchanged. 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 allocation implementation.
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), and failure to allocate memory become fatal. The process
will call abort(3) in these cases.
J Each byte of new memory allocated by malloc() or realloc() as
well as all memory returned by free() or realloc() will be ini-
tialized to 0xd0. This options also sets the ``R'' option. This
is intended for debugging and will impact performance negatively.
H Pass a hint to the kernel about pages unused by the allocation
functions. This may help performance if the system is paging ex-
cessively.
R Cause the realloc() function to always reallocate memory even if
the initial allocation was sufficiently large. This can substan-
tially aid in compacting memory.
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 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:
extern char *malloc_options;
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
times.
> Double the size of the cache by a factor of two. The default
cache size is 16 pages. This option can be specified multiple
times.
The ``J'' and ``Z'' options are intended for testing and debugging. An
application which changes its behavior when these options are used is
flawed.
EXAMPLES
To specify in the source that a program does no return value checking on
calls to these functions:
extern char *malloc_options;
malloc_options = "X";
ENVIRONMENT
The following environment variables affect the execution of the alloca-
tion functions:
MALLOC_OPTIONS
If the environmental variable MALLOC_OPTIONS is set, the characters
it contains will be interpreted as flags to the allocation func-
tions.
RETURN VALUES
The malloc() and calloc() functions return a pointer to the allocated
memory if successful; otherwise a NULL pointer is returned.
The realloc() function returns a pointer, possibly identical to ptr, to
the allocated memory if successful; otherwise a NULL pointer is returned,
in which case the memory referenced by ptr is still available and intact.
The free() function returns no value.
DEBUGGING MALLOC PROBLEMS
The major difference between this implementation and other allocation im-
plementations 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 con-
taining 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 de-
tected. As a result, programs that have been running happily for years
may suddenly start to complain loudly, when linked with this allocation
implementation.
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 be-
have differently without emitting any of the messages listed in the next
section, it is likely because it depends on the storage being filled with
nul bytes. Try running it with ``Z'' option set; if that improves the
situation, this diagnosis has been confirmed. If the program still mis-
behaves, 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.
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.
DIAGNOSTIC MESSAGES
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 following is a brief description of possible error messages and their
meanings:
(ES): mumble mumble mumble
The allocation functions were compiled with ``EXTRA_SANITY'' de-
fined, and an error was found during the additional error check-
ing. Consult the source code for further information.
allocation failed
If the ``A'' option is specified it is a fatal error for an allo-
cation function to fail.
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.
The following is a brief description of possible warning messages and
their meanings:
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 re-
alloc'ed.
modified (chunk-/page-) pointer
The pointer passed to free() or realloc() has been modified.
pointer to wrong page
The pointer that malloc() or calloc() is trying to free does not
reference a possible page.
recursive call
A process has attempted to call an allocation function recursive-
ly. This is not permitted. In particular, signal handlers
should not attempt to allocate memory.
out of memory
The ``X'' option was specified and an allocation of memory
failed.
unknown char in MALLOC_OPTIONS
An unknown option was specified. Even with the ``A'' option set,
this warning is still only a warning.
SEE ALSObrk(2), alloca(3), getpagesize(3), memory(3)STANDARDS
The malloc(), calloc(), realloc() and free() functions conform to ANSI C
X3.159-1989 (``ANSI C '').
BUGS
The messages printed in case of problems provide no detail about the ac-
tual values.
It can be argued that returning a null pointer when asked to allocate ze-
ro bytes is a silly response to a silly question.
This implementation was authored by Poul-Henning Kamp. Please report any
problems to him at <phk@FreeBSD.org>.
HISTORY
The present allocation implementation started out as a filesystem 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 release 2.2.
BSDI BSD/OS August 27, 1996 4
