libc/stdlib/malloc.3

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.\"     @(#)malloc.3	8.1 (Berkeley) 6/4/93
.\" $FreeBSD: src/lib/libc/stdlib/malloc.3,v 1.73 2007/06/15 22:32:33 jasone Exp $
.\"
.Dd June 20, 2009
.Dt MALLOC 3
.Os
.Sh NAME
.Nm malloc , calloc , realloc , free
.Nd general purpose memory allocation functions
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In stdlib.h
.Ft void *
.Fn malloc "size_t size"
.Ft void *
.Fn calloc "size_t number" "size_t size"
.Ft void *
.Fn realloc "void *ptr" "size_t size"
.Ft void
.Fn free "void *ptr"
.Ft const char *
.Va _malloc_options ;
.Sh DESCRIPTION
The
.Fn malloc
function allocates
.Fa size
bytes of uninitialized memory.
The allocated space is suitably aligned (after possible pointer coercion)
for storage of any type of object.
.Pp
The
.Fn calloc
function allocates space for
.Fa number
objects,
each
.Fa size
bytes in length.
The result is identical to calling
.Fn malloc
with an argument of
.Dq "number * size" ,
with the exception that the allocated memory is explicitly initialized
to zero bytes.
.Pp
The
.Fn realloc
function changes the size of the previously allocated memory referenced by
.Fa ptr
to
.Fa 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.
Upon success, the memory referenced by
.Fa ptr
is freed and a pointer to the newly allocated memory is returned.
Note that
.Fn realloc
may move the memory allocation, resulting in a different return value than
.Fa ptr .
If
.Fa ptr
is
.Dv NULL ,
the
.Fn realloc
function behaves identically to
.Fn malloc
for the specified size.
.Pp
When using
.Fn realloc
one must be careful to avoid the following idiom:
.Pp
.Bd -literal -offset indent
nsize += 50;
if ((p = realloc(p, nsize)) == NULL)
	return (NULL);
.Ed
.Pp
Do not adjust the variable describing how much memory has been allocated
until one knows the allocation has been successful.
This can cause aberrant program behavior if the incorrect size value is used.
In most cases, the above sample will also result in a leak of memory.
As stated earlier, a return value of
.Dv NULL
indicates that the old object still remains allocated.
Better code looks like this:
.Bd -literal -offset indent
newsize = size + 50;
if ((p2 = realloc(p, newsize)) == NULL) {
	if (p)
		free(p);
	p = NULL;
	return (NULL);
}
p = p2;
size = newsize;
.Ed
.Pp
The
.Fn free
function causes the allocated memory referenced by
.Fa ptr
to be made available for future allocations.
If
.Fa ptr
is
.Dv NULL ,
no action occurs.
.Sh 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.
.Pp
The
.Dq name
of the file referenced by the symbolic link named
.Pa /etc/malloc.conf ,
the value of the environment variable
.Ev MALLOC_OPTIONS ,
and the string pointed to by the global variable
.Va _malloc_options
will be interpreted, in that order, character by character as flags.
.Pp
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 off.
.Bl -tag -width indent
.It A
All warnings (except for the warning about unknown
flags being set) become fatal.
The process will call
.Xr abort 3
in these cases.
.It H
Use
.Xr madvise 2
when pages within a chunk are no longer in use, but the chunk as a whole cannot
yet be deallocated.
This is primarily of use when swapping is a real possibility, due to the high
overhead of the
.Fn madvise
system call.
.It J
Each byte of new memory allocated by
.Fn malloc ,
.Fn realloc
will be initialized to 0xa5.
All memory returned by
.Fn free ,
.Fn realloc
will be initialized to 0x5a.
This is intended for debugging and will impact performance negatively.
.It K
Increase/decrease the virtual memory chunk size by a factor of two.
The default chunk size is 1 MB.
This option can be specified multiple times.
.It 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.
.It P
Various statistics are printed at program exit via an
.Xr 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.
.It Q
Increase/decrease the size of the allocation quantum by a factor of two.
The default quantum is the minimum allowed by the architecture (typically 8 or
16 bytes).
This option can be specified multiple times.
.It 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.
.It U
Generate
.Dq utrace
entries for
.Xr ktrace 1 ,
for all operations.
Consult the source for details on this option.
.It V
Attempting to allocate zero bytes will return a
.Dv 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
.Dq X
option.
.It X
Rather than return failure for any allocation function,
display a diagnostic message on
.Dv stderr
and cause the program to drop
core (using
.Xr abort 3 ) .
This option should be set at compile time by including the following in
the source code:
.Bd -literal -offset indent
_malloc_options = "X";
.Ed
.It Z
Each byte of new memory allocated by
.Fn malloc ,
.Fn realloc
will be initialized to 0.
Note that this initialization only happens once for each byte, so
.Fn realloc
does not zero memory that was previously allocated.
This is intended for debugging and will impact performance negatively.
.El
.Pp
The
.Dq J
and
.Dq Z
options are intended for testing and debugging.
An application which changes its behavior when these options are used
is flawed.
.Sh 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 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.
.Pp
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.
.Pp
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.
.Pp
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.
.Pp
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
.Dq 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
.Dq 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
.Dq 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.
.Pp
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.
.Sh DEBUGGING MALLOC PROBLEMS
The first thing to do is to set the
.Dq 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.
.Pp
It is probably also a good idea to recompile the program with suitable
options and symbols for debugger support.
.Pp
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
.Dq 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.
.Pp
Alternatively, if the symptoms are not easy to reproduce, setting the
.Dq J
option may help provoke the problem.
.Pp
In truly difficult cases, the
.Dq U
option, if supported by the kernel, can provide a detailed trace of
all calls made to these functions.
.Pp
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.
.Sh DIAGNOSTIC MESSAGES
If any of the memory allocation/deallocation functions detect an error or
warning condition, a message will be printed to file descriptor
.Dv STDERR_FILENO .
Errors will result in the process dumping core.
If the
.Dq A
option is set, all warnings are treated as errors.
.Pp
The
.Va _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
.Dv 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.
.Pp
All messages are prefixed by
.Dq Ao Ar progname Ac Ns Li \&: Pq malloc .
.Sh RETURN VALUES
The
.Fn malloc
and
.Fn calloc
functions return a pointer to the allocated memory if successful; otherwise
a
.Dv NULL
pointer is returned and
.Va errno
is set to
.Er ENOMEM .
.Pp
The
.Fn realloc
function returns a pointer, possibly identical to
.Fa ptr ,
to the allocated memory
if successful; otherwise a
.Dv NULL
pointer is returned, and
.Va errno
is set to
.Er ENOMEM
if the error was the result of an allocation failure.
The
.Fn realloc
function always leaves the original buffer intact
when an error occurs.
.Pp
The
.Fn free
function returns no value.
.Sh ENVIRONMENT
The following environment variables affect the execution of the allocation
functions:
.Bl -tag -width ".Ev MALLOC_OPTIONS"
.It Ev MALLOC_OPTIONS
If the environment variable
.Ev MALLOC_OPTIONS
is set, the characters it contains will be interpreted as flags to the
allocation functions.
.El
.Sh EXAMPLES
To dump core whenever a problem occurs:
.Pp
.Bd -literal -offset indent
ln -s 'A' /etc/malloc.conf
.Ed
.Pp
To specify in the source that a program does no return value checking
on calls to these functions:
.Bd -literal -offset indent
_malloc_options = "X";
.Ed
.Sh SEE ALSO
.\" .Xr limits 1 ,
.Xr madvise 2 ,
.Xr mmap 2 ,
.Xr sbrk 2 ,
.Xr alloca 3 ,
.Xr atexit 3 ,
.Xr getpagesize 3 ,
.Xr memory 3 ,
.Xr posix_memalign 3
.Sh STANDARDS
The
.Fn malloc ,
.Fn calloc ,
.Fn realloc
and
.Fn free
functions conform to
.St -isoC .