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Memory Heaps

Zephyr provides a collection of utilities that allow threads to dynamically allocate memory.

Synchronized Heap Allocator

Creating a Heap

The simplest way to define a heap is statically, with the K_HEAP_DEFINE macro. This creates a static struct k_heap variable with a given name that manages a memory region of the specified size.

Heaps can also be created to manage arbitrary regions of application-controlled memory using k_heap_init().

Allocating Memory

Memory can be allocated from a heap using k_heap_alloc(), passing it the address of the heap object and the number of bytes desired. This functions similarly to standard C malloc(), returning a NULL pointer on an allocation failure.

The heap supports blocking operation, allowing threads to go to sleep until memory is available. The final argument is a k_timeout_t timeout value indicating how long the thread may sleep before returning, or else one of the constant timeout values K_NO_WAIT or K_FOREVER.

Releasing Memory

Memory allocated with k_heap_alloc() must be released using k_heap_free(). Similar to stanard C free(), the pointer provided must be either a NULL value or a pointer previously returned by k_heap_alloc() for the same heap. Freeing a NULL value is defined to have no effect.

Low Level Heap Allocator

The underlying implementation of the struct k_heap abstraction is provided a data structure named struct sys_heap. This implements exactly the same allocation semantics, but provides no kernel synchronization tools. It is available for applications that want to manage their own blocks of memory in contexts (for example, userspace) where synchronization is unavailable or more complicated. Unlike k_heap, all calls to any sys_heap functions on a single heap must be serialized by the caller. Simultaneous use from separate threads is disallowed.


Internally, the sys_heap memory block is partitioned into “chunks” of 8 bytes. All allocations are made out of a contiguous region of chunks. The first chunk of every allocation or unused block is prefixed by a chunk header that stores the length of the chunk, the length of the next lower (“left”) chunk in physical memory, a bit indicating whether the chunk is in use, and chunk-indexed link pointers to the previous and next chunk in a “free list” to which unused chunks are added.

The heap code takes reasonable care to avoid fragmentation. Free block lists are stored in “buckets” by their size, each bucket storing blocks within one power of two (i.e. a bucket for blocks of 3-4 chunks, another for 5-8, 9-16, etc…) this allows new allocations to be made from the smallest/most-fragmented blocks available. Also, as allocations are freed and added to the heap, they are automatically combined with adjacent free blocks to prevent fragmentation.

All metadata is stored at the beginning of the contiguous block of heap memory, including the variable-length list of bucket list heads (which depend on heap size). The only external memory required is the struct sys_heap structure itself.

The sys_heap functions are unsynchronized. Care must be taken by any users to prevent concurrent access. Only one context may be inside one of the API functions at a time.

The heap code takes care to present high performance and reliable latency. All sys_heap API functions are guaranteed to complete within constant time. On typical architectures, they will all complete within 1-200 cycles. One complexity is that the search of the minimum bucket size for an allocation (the set of free blocks that “might fit”) has a compile-time upper bound of iterations to prevent unbounded list searches, at the expense of some fragmentation resistance. This CONFIG_SYS_HEAP_ALLOC_LOOPS value may be chosen by the user at build time, and defaults to a value of 3.

System Heap

The system heap is a predefined memory allocator that allows threads to dynamically allocate memory from a common memory region in a malloc()-like manner.

Only a single system heap is be defined. Unlike other heaps or memory pools, the system heap cannot be directly referenced using its memory address.

The size of the system heap is configurable to arbitrary sizes, subject to space availability.

A thread can dynamically allocate a chunk of heap memory by calling k_malloc(). The address of the allocated chunk is guaranteed to be aligned on a multiple of pointer sizes. If a suitable chunk of heap memory cannot be found NULL is returned.

When the thread is finished with a chunk of heap memory it can release the chunk back to the system heap by calling k_free().

Defining the Heap Memory Pool

The size of the heap memory pool is specified using the CONFIG_HEAP_MEM_POOL_SIZE configuration option.

By default, the heap memory pool size is zero bytes. This value instructs the kernel not to define the heap memory pool object.

Allocating Memory

A chunk of heap memory is allocated by calling k_malloc().

The following code allocates a 200 byte chunk of heap memory, then fills it with zeros. A warning is issued if a suitable chunk is not obtained.

Note that the application will actually allocate a 256 byte memory block, since that is the closest matching size supported by the heap memory pool.

char *mem_ptr;

mem_ptr = k_malloc(200);
if (mem_ptr != NULL)) {
    memset(mem_ptr, 0, 200);
} else {
    printf("Memory not allocated");

Releasing Memory

A chunk of heap memory is released by calling k_free().

The following code allocates a 75 byte chunk of memory, then releases it once it is no longer needed. (A 256 byte memory block from the heap memory pool is actually used to satisfy the request.)

char *mem_ptr;

mem_ptr = k_malloc(75);
... /* use memory block */

Suggested Uses

Use the heap memory pool to dynamically allocate memory in a malloc()-like manner.

Configuration Options

Related configuration options:

API Reference

group heap_apis


void *k_malloc(size_t size)

Allocate memory from heap.

This routine provides traditional malloc() semantics. Memory is allocated from the heap memory pool.


Address of the allocated memory if successful; otherwise NULL.

  • size: Amount of memory requested (in bytes).

void k_free(void *ptr)

Free memory allocated from heap.

This routine provides traditional free() semantics. The memory being returned must have been allocated from the heap memory pool or k_mem_pool_malloc().

If ptr is NULL, no operation is performed.



  • ptr: Pointer to previously allocated memory.

void *k_calloc(size_t nmemb, size_t size)

Allocate memory from heap, array style.

This routine provides traditional calloc() semantics. Memory is allocated from the heap memory pool and zeroed.


Address of the allocated memory if successful; otherwise NULL.

  • nmemb: Number of elements in the requested array

  • size: Size of each array element (in bytes).