MALLOC(3)

NAME
malloc, calloc, realloc, free, reallocf, malloc_usable_size - general purpose memory allocation functions

LIBRARY
libc

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 *
reallocf(void *ptr, size_t size);
void
free(void *ptr);
const char *
_malloc_options;
void
*_malloc_message(const char *p1, const char *p2, const char *p3, const char *p4);
#include <malloc_np.h>
size_t
malloc_usable_size(const void *ptr);

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 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 contents of the newly allocated portion of the memory are undefined. Upon success, the memory referenced by ptr is freed and a pointer to the newly allocated memory is returned. Note that realloc() and reallocf() 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 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 libraries.

The free() function causes the allocated memory referenced by ptr to be made available for future allocations. If ptr is NULL, no action occurs.

The malloc_usable_size() function returns the usable size of the allocation pointed to by ptr. The return value may be larger than the size that was requested during allocation. The malloc_usable_size() function is not a mechanism for in-place realloc(); rather it is provided solely as a tool for introspection purposes. Any discrepancy between the requested allocation size and the size reported by malloc_usable_size() should not be depended on, since such behavior is entirely implementation-dependent.

TUNING
Once, when the first call is made to one of these memory allocation routines, various flags will be set or reset, which affects 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, from left to right as flags.

Each flag is a single letter, optionally prefixed by a non-negative base 10 integer repetition count. For example, “3N” is equivalent to “NNN”. Some flags control parameter magnitudes, where uppercase increases the magnitude, and lowercase decreases the magnitude. Other flags control boolean parameters, where uppercase indicates that a behavior is set, or on, and lowercase means that a behavior is not set, or off.

A   All warnings (except for the warning about unknown flags being set) become fatal. The process will call abort(3) in these cases.

C   Double/halve the size of the maximum size class that is a multiple of the cacheline size (64). Above this size, subpage spacing (256 bytes) is used for size classes. The default value is 512 bytes.

D   Use sbrk(2) to acquire memory in the data storage segment (DSS). This option is enabled by default. See the “M” option for related information and interactions.

E   Double/halve the size of the maximum medium size class. The valid range is from one page to one half chunk. The default value is 32 KiB.

F   Halve/double the per-arena minimum ratio of active to dirty pages. Some dirty unused pages may be allowed to accumulate, within the limit set by the ratio, before informing the kernel about at least half of those pages via madvise(2). This provides the kernel with sufficient information to recycle dirty pages if physical memory becomes scarce and the pages remain unused. The default minimum ratio is 32:1; MALLOC_OPTIONS=6F will disable dirty page purging.

G   Double/halve the approximate interval (counted in terms of thread-specific cache allocation/deallocation events) between full thread-specific cache garbage collection sweeps. Garbage collection is actually performed incrementally, one size class at a time, in order to avoid large collection pauses. The default sweep interval is 8192; MALLOC_OPTIONS=14g will disable garbage collection.

H   Double/halve the number of thread-specific cache slots per size class. When there are multiple threads, each thread uses a thread-specific cache for small and medium objects. Thread-specific caching allows many allocations to be satisfied without performing any thread synchronization, at the cost of increased memory use. See the “G” option for related tuning information. The default number of cache slots is 128; MALLOC_OPTIONS=7h will disable thread-specific caching. Note that one cache slot per size class is not a valid configuration due to implementation details.

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

K   Double/halve the virtual memory chunk size. The default chunk size is 4 MiB.

M   Use mmap(2) to acquire anonymously mapped memory. This option is enabled by default. If both the “D” and “M” options are enabled, the allocator prefers anonymous mappings over the DSS, but allocation only fails if memory cannot be acquired via either method. If neither option is enabled, then the “M” option is implicitly enabled in order to assure that there is a method for acquiring memory.

N   Double/halve the number of arenas. The default number of arenas is two times the number of CPUs, or one if there is a single CPU.

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 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.

Q   Double/halve the size of the maximum size class that is a multiple of the quantum (8 or 16 bytes, depending on architecture). Above this size, cacheline spacing is used for size classes. The default value is 128 bytes.

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_FILENO 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(), or reallocf() will be initialized to 0. Note that this initialization only happens once for each byte, so realloc() and reallocf() calls 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
Traditionally, allocators have used sbrk(2) to obtain memory, which is suboptimal for several reasons, including race conditions, increased fragmentation, and artificial limitations on maximum usable memory. This allocator uses both sbrk(2) and mmap(2) by default, but it can be configured at run time to use only one or the other. If resource limits are not a primary concern, the preferred configuration is MALLOC_OPTIONS=dM or MALLOC_OPTIONS=DM. When so configured, the datasize resource limit has little practical effect for typical applications; use MALLOC_OPTIONS=Dm if that is a concern. Regardless of allocator configuration, the vmemoryuse resource limit can be used to bound the total virtual memory used by a process, as described in limits(1).

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.

In addition to multiple arenas, this allocator supports thread-specific caching for small and medium objects, in order to make it possible to completely avoid synchronization for most small and medium allocation requests. Such caching allows very fast allocation in the common case, but it increases memory usage and fragmentation, since a bounded number of objects can remain allocated in each thread cache.

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 four categories according to size: small, medium, large, and huge. Small objects are smaller than one page. Medium objects range from one page to an upper limit determined at run time (see the “E” option). Large objects are smaller than the chunk size. Huge objects are a multiple of the chunk size. Small, medium, 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.

Each chunk that is managed by an arena tracks its contents as runs of contiguous pages (unused, backing a set of small or medium 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 and medium 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 (8 or 16, depending on architecture) are rounded up to the nearest power of two. Allocation requests that are more than half the quantum, but no more than the minimum cacheline-multiple size class (see the “Q” option) are rounded up to the nearest multiple of the quantum. Allocation requests that are more than the minimum cacheline-multiple size class, but no more than the minimum subpage-multiple size class (see the “C” option) are rounded up to the nearest multiple of the cacheline size (64). Allocation requests that are more than the minimum subpage-multiple size class, but no more than the maximum subpage-multiple size class are rounded up to the nearest multiple of the subpage size (256). Allocation requests that are more than the maximum subpage-multiple size class, but no more than the maximum medium size class (see the “M” option) are rounded up to the nearest medium size class; spacing is an automatically determined power of two and ranges from the subpage size to the page size. Allocation requests that are more than the maximum medium size class, 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 cacheline sharing, round your allocation requests up to the nearest multiple of the cacheline 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 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 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 program 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 problems by trading performance for extra sanity checks and detailed diagnostics.

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 function which emits the text strings forming the errors and warnings if for some reason the STDERR_FILENO 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() 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.

The malloc_usable_size() function returns the usable size of the allocation pointed to by ptr.

ENVIRONMENT
The following environment variables affect the execution of the allocation 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), madvise(2), mmap(2), sbrk(2), alloca(3), atexit(3), getpagesize(3), getpagesizes(3), memory(3), posix_memalign(3)

STANDARDS
The malloc(), calloc(), realloc() and free() functions conform to isoC.

HISTORY
The reallocf() function first appeared in FreeBSD 3.0.

The malloc_usable_size() function first appeared in FreeBSD 7.0.