Heap Memory Manager


Version 1.1

Author:  Walt Karas


1  Introduction


This document describes a portable heap memory manager, also known as a dynamic memory allocator.  The manager consists of several functions implemented in ANSI C.  These functions have worst-case time complexity of O(log N), where N is the number of distinct free memory block sizes.  Allocation is done on a strict best-fit basis.  Adjacent free memory blocks are always coalesced into a single, larger free block.


This document, as well as the source code it describes, is in the public domain.


The heap facility provided by this manager is similar to the heap facility provided by the malloc and free functions in the C standard library.  A successful allocation returns a contiguous range of bytes in memory of at least the requested size.  Allocated memory has to be explicitly freed/deallocated (no garbage collection).


The manager code (optionally) performs limited self-consistency checks in order to detect heap corruption.


This manager could be used to implement the malloc/free functions in the C standard library.  It could also be used to implement a heap memory manager for shared memory, which in turn could be used to implement allocators of shared memory for C++ Standard Template Library containers.


2  Source Files


The files containing the source code for the manager are:





The header file that defines the interface to the heap memory manager.


The header file containing definitions that may have to be changed for a particular port of the heap manager.  Included by heapmm.h.


The internal header file for the manager.  Files external to the manager should not include this file.


The implementation file for basic functions that are always needed when using the heap manager.


The implementation of the hmm_alloc function.


The implementation of the hmm_grow_chunk function.


The implementation of the hmm_true_size function.


The implementation of the hmm_largest_available function.


The implementation of the hmm_resize function.


The implementation of the hmm_shrink_chunk function.


The implementation of the function that aborts execution if the manager’s (optional) self-auditing finds corruption of the heap.


Test suite for the manager.


Header file for test suite (only).


Program that (very crudely) measures the speed of manager functions versus the malloc and free functions of the standard library.


These files, along with this document (heapmm.html) are in the zip archive heapmm.zip.


The source code for the manager includes the header files cavl_if.h and cavl_impl.h from the C AVL Tree Generic Package (version 1.4 or later).


3  Assumptions and Definitions


The first assumption is that sizeof(char) is 1.  This is required by the ISO C++ standard.  I’m not sure it’s required by ANSI C, but I’ve never seen a C implementation where this wasn’t the case.


If a pointer p of type char * contains an address such that (* (T *) p) is a valid lvalue for any type T, then p contains an aligned address.


The address align unit (AAU) is some positive number of bytes N such that, if p, a pointer of type char *, contains an aligned address, then the expressions p + N and p – N also evaluate to aligned addresses.


In order for the manager to be ported to a processor architecture, a valid address align unit for the processor must exist.  The smaller the AAU, the fewer wasted “pad” bytes will be necessary.  Generally, to avoid skewed memory reads and writes, you should use the width in bytes of your processor’s data bus as the AAU.  For that reason, the default AAU is sizeof(int).  The preprocessor symbol HMM_ADDR_ALIGN_UNIT, defined in hmm_cnfg.h, translates to the address align unit.  If eliminating pad bytes concerns you more than avoid skewed memory operations, you can make the AAU even smaller.  An AAU of 1 can be used for the Intel x86, VAX and other architectures which do not generate bus errors.  An AAU of 2 can be used for the Motorola 68000 and the PDP-11 architectures.


A chunk of memory is a large contiguous range of addresses, which the manager subdivides to satisfy allocation requests.  The subdivided pieces of a chunk are called blocks.  The first byte of a chunk must be at an aligned address.  The number of bytes in a block or chunk must be a whole number of block align units (BAUs).  A block align unit is a whole number of AAUs.  The preprocessor symbol HMM_BLOCK_ALIGN_UNIT, defined in hmm_cnfg.h, translates to the block align unit.  The default BAU is simply one AAU.  Using a larger BAU causes the sizes of blocks to be rounded up with pad AAUs, thus wasting some space.  The advantage is that the number of distinct block sizes is reduced.  The time it takes to allocate and free memory increases as the number of distinct block sizes increases.


The implementation depends on the assumption that sizeof(T *) == sizeof(void *) for any type T.  It also assumes that, if p is a pointer variable of type void *, and p is null, then  * (T **) &p == (T *) 0  for any type T.


4  Interface


The prototypes for these functions and the typedefs are provided by heapmm.h.


4.1  Typedef hmm_size_aau


An unsigned integral type with enough precision to hold the size of a block in AAUs.  unsigned long by default.


4.2  Typedef hmm_size_bau


An unsigned integral type with enough precision to hold the size of a block or chunk in BAUs.  unsigned long by default.


Let N be the number of bits of precision in hmm_size_bau.  Due to details of the implementation, the maximum chunk size is the largest unsigned number of precision N – 1.  For example, if hmm_size_bau is a 32-bit unsigned integer, the maximum chunk size is 231 – 1. If you attempt to add or grow a chunk in excess of the maximum size, the results are undefined.


4.3  Typedef hmm_descriptor


The descriptor for a heap that is managed by this manager.  The fields num_baus_can_shrink and end_of_shrinkable_chunk should be treated as read-only by the user code.  See the descriptions of the hmm_free and hmm_shrink_chunk functions for the usage of these two fields.  The other fields of this structure should be treated as private (neither readable nor writable) by the user code.


4.4  Function hmm_init


void hmm_init(hmm_descriptor *desc);


Initializes the descriptor to an empty state.  A descriptor must be initialized before passing it to any other function.


4.5  Function hmm_new_chunk


void hmm_new_chunk(hmm_descriptor *desc, void *first_byte_of_chunk, hmm_size_bau num_block_align_units_in_chunk);


Assigns a chunk to the heap with the given descriptor.  The chunk can then be subdivided to satisfy allocation requests.  A chunk can only be assigned to a single heap, but a heap can have multiple chunks assigned to it.  The first byte of the chunk has to be at an aligned address.


Assigning very small chunks to a heap will result in corruption.  (“Very small” means smaller than MIN_BLOCK_BAUS + DUMMY_END_BLOCK_BAUS.  These values are defined in hmm_intrnl.h.)


4.6  Function hmm_alloc


void * hmm_alloc(hmm_descriptor *desc, hmm_size_aau num_addr_align_units);


Allocate a contiguous range of AAUs of size num_addr_align_units from a heap.  A pointer to the first byte of the allocated memory is returned if the function is successful.  A null pointer is returned if the function fails.


4.7  Function hmm_free


void hmm_free(hmm_descriptor *desc, void *mem);


Frees (deallocates) memory previously allocated from the given heap.  The mem parameter has to be same value returned by the hmm_alloc function.


If, after the free operation, it would be possible to shrink the size of a chunk, the descriptor field num_baus_can_shrink will be set to the maximum number of block align units the chunk could be shrunk by.  If it is not possible to shrink any chunk as a result of the free operation, num_baus_can_shrink will be set to zero.


If num_baus_can_shrink is set to a non-zero value, the (void) pointer field end_of_shrinkable_chunk (also in the descriptor) will point to one byte after the last byte in the chunk that can be shrunk.


4.8  Function hmm_grow_chunk


void hmm_grow_chunk(hmm_descriptor *desc, void *end_of_chunk, hmm_size_bau num_block_align_units_to_add);


Adds space to a chunk at the end.  The chunk must already have been assigned to the heap whose descriptor is given.  end_of_chunk should point to the first byte after the last byte in the chunk (prior to growing).


Growing a chunk by a very small amount may result in corruption.  (“Very small” means smaller than MIN_BLOCK_BAUS.  This value is defined in hmm_intrnl.h.)


This function is redundant, in the sense that you could add the additional heap space as a new chunk, rather than growing an existing chunk.  The reason to grow the existing chunk instead is that the manager is not smart enough to coalesce adjacent free blocks in different chunks, even when the chunks are adjacent.  There is also a small amount of per-chunk overhead.


4.9  Function hmm_true_size


hmm_size_aau hmm_true_size(void *mem);


Sometimes, when you allocate some memory, you will actually get a few more AAUs than you requested.  This function returns the number of AAUs that are actually usable in a currently allocated block.  mem has to be pointer value that was returned by hmm_alloc.


4.10  Function hmm_largest_available


hmm_size_aau hmm_largest_available(hmm_descriptor *desc);


Returns the largest number of AAUs that could successfully be allocated (by a single call to hmm_alloc) from the heap with the given descriptor.


4.11  Function hmm_shrink_chunk


void hmm_shrink_chunk(hmm_descriptor *desc, hmm_size_bau num_block_align_units_to_shrink);


Shrinks a chunk.  (The starting location of the chunk does not change, only the ending location.)  This function should only be called immediately after a call to hmm_free that set the num_baus_can_shrink descriptor field to a non-zero value.  No other manager functions should be called between the calls to hmm_free and hmm_shrink_chunk.  The value passed for num_block_align_units_to_shrink has to be less than or equal to num_baus_can_shrink.  The (void) pointer end_of_shrinkable_chunk in the descriptor, as set by hmm_free, points to the first byte past the last byte in the chunk that will be shrunk.  (User code must not change the value of end_of_shrinkable_chunk.)


If the descriptor field num_baus_can_shrink contains the number of  BAUs in the chunk, the chunk can be made to “disappear” by this function.  But, if an attempt is made to shrink the chunk to a very small size, corruption will result.  (“Very small” is smaller than MIN_BLOCK_BAUS + DUMMY_END_BLOCK_BAUS.  These values are defined in hmm_intrnl.h.)


4.12  Function hmm_resize


int hmm_resize(hmm_descriptor *desc, void *mem, hmm_size_aau new_num_addr_align_units);


Attempts to change the size of a previously-allocated block.  mem must be pointer that was returned by hmm_alloc.  The function returns 0 if it was able to resize the block to the desired size.  Otherwise, the function returns -1.


Note that this function will not move the block the way the C Standard Library function realloc will.


5  Overview of Implementation


Each block in a chunk starts on an aligned address, and consists of a whole number of block align units.  At the beginning of each block is the block head.  The block head consists of the following structure:


typedef struct head_struct
    hmm_size_bau previous_block_size, block_size;


hmm_size_bau is the (configurable) unsigned type for holding sizes of blocks and chunks.  The most significant bit of previous_block_size and the most significant bit of block_size are combined together to form a block status field.  If both bits in the block status field are zero, this indicates that the block has been allocated (by a call to hmm_alloc).  Any of the other 3 possible combinations of values of the bits in the block status field indicate that the block is free.  The remaining bits is block_size contain the number of block align units in the block.  The remaining bits in previous_block_size contain the block size of the preceding block, or zero if the block is the first block in a chunk.  The size of the block head is the minimum number of address align units required to hold a head_record.


Each chunk ends with a dummy end block.  In the dummy end block, both bits in the block status field are zero (although it cannot be allocated).  block_size contains zero, and previous_block_size contains the size of the last (non-dummy) block in the chunk.  The size of a dummy end block is the minimum number of block align units need to hold a head_record.


The remainder of a (non-dummy) block after the head is the block payload.  In an allocated block, the payload contains user data.  When you call hmm_alloc, the pointer returned is a pointer to the first byte of the payload of the block that was allocated for you.  In a free block, the payload contains a pointer record.  A pointer record has this structure:


typedef struct ptr_struct
    struct ptr_struct *self, *prev, *next;


Free blocks are held in a data structure called the free collection.  The pointers in the pointer record are used to link a free block into the free collection.  Each free block is located in a bin along with all other free blocks of exactly the same size.  The first block in each bin is a node in an AVL tree keyed by block size.  The self and prev pointers in the block’s pointer record are the node’s child pointers.  The node’s balance factor (-1, 0 or 1) is encoded using the 3 possible bit value combinations for a free block in the block status field.  The next pointer points to the pointer record of the next block in the bin (or is null if there is only one block in the bin).


For blocks in a bin after the first, prev and next point to the previous and the next block in the bin (making the bin a doubly-linked list).  self points to the pointer record it is inside of.  (This will make sense in a minute.)  The block status field contains any of the 3 possible “free” values (doesn’t matter which).


When allocating a block, you search the AVL tree for the smallest block whose payload is the same size or greater than the size requested.  If there are other blocks in the bin of the block you find, you grab the second block in the bin.  This avoids a (relatively) time-consuming change to the AVL tree structure.  But if there is only one block in the bin, you have to delete it from the AVL tree.  If the “extra” space in the block is big enough, you split the extra space off as a new block.  You then put this new block into the free collection.


When freeing a block, you check to see if the adjacent blocks are free.  (Since the previous block’s size is in the head of the block you’re freeing, it’s easy to locate the previous block’s head.)  If (one or both) adjacent blocks are free, you coalesce them into a single free block.  Before coalescing an adjacent block, you have to take it out of the free collection. You look at the self pointer in the pointer record of the adjacent block.  If the self pointer is actually pointing to itself, the adjacent block can’t be the first block in a bin, because a node in an AVL tree cannot be its own child.  In this case, taking the adjacent block out of the free collection is simply a matter of deleting it from the doubly-linked list for the bin.  If you’re not lucky, and the adjacent block is an AVL tree node, it’s better to substitute another block from the bin into the adjacent block’s position in the tree.  (Deleting a node from an AVL tree can result in a lot of complex rebalancing.)  But if the adjacent free block is alone in its bin, deleting it from the AVL tree is unavoidable.  After any coalescing is complete, you insert the newly-freed block into the free collection.


It’s a common pattern in programs that a bunch of blocks are allocated at about the same time, then freed later at about the same time.  A group of successive allocations are often satisfied by subdividing a single large block, causing the allocated blocks to be located adjacently in the order of allocation.  If these blocks are freed together, they coalesce back into the original big block.  To optimize for this case, when a block is freed by a call to hmm_free, the block is not immediately put into the free collection.  This give the block a chance to coalesce with succeeding blocks that are freed before taking the time to do an insertion into the free collection.  (There is a pointer in the heap descriptor to keep track of the last freed block.)


I chose to implement this in C rather than C++ because of the widespread (but unjustified) resistance to using C++ in low-level programming.  There are a few macros in the code that translate to expressions with multiple occurrences of macro parameters.  I tried to not pass expressions as parameters to these macros, but it wasn’t always easy to avoid it.  So, if you’re using C++, and your faith in your compiler’s repeated subexpression detection is weak, you might want to convert these macros to inline functions.


6  Other Heap Memory Managers


This manager was designed to have reasonable performance regardless of the pattern of allocations and frees, and to be easily portable to arbitrary environments.  Doug Lea's memory allocator might be a better choice if you are more interested in average-case performance where the typical allocation size is relatively small.


Links to other heap managers can be found here.  You may also want to read an interesting (PDF) paper that compares the performance of different heap managers.


In an ideal world, developers of heap-intensive and performance-sensitive applications would have multiple heap managers available to them.  This would allow the selection of the best heap manager for an application based on comparative performance testing.  Having a variety of heap managers that are packaged so they are operating-system-independent would be helpful in reaching this goal.


7  Version History


Version 1.0



Version 1.1