MALLOC(3)

NAME
malloc, calloc, realloc, free - 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
free(void *ptr);
const char *
_malloc_options;

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 value of the newly allocated portion of the memory is undefined. 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.

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.

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 off.

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

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 primarily 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 specified 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 times.

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   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.

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 flawed.

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.

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 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.

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 quantum-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 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 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 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 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() 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.

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), memory(3), posix_memalign(3)

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