Skip to content

Heap Memory

lbuild module: modm:platform:heap

Your applicaton is linked against the newlib-nano libc, which only requires the implementation of the void* sbrk(ptrdiff_t size) hook to use the heap.

However, the sbrk mechanism can only grow and shrink forward and backwards in memory and in particular it does not support discontinuous jumps from one memory section to another. The limitation stems from being designed for use with a MMU which would simply map another physical page into the linear virtual address space so that the heap section appears continuous to sbrk.

Since we do not have a MMU on Cortex-M, this strategy limits the use of the default newlib allocator to one continuous memory region. Therefore this module implements alternative allocators for devices with multiple memory regions with different traits and extends the C++ operator new to access them.

See the modm:architecture:memory module for what kind of memory traits exist.

Heap is not Implemented Error

This module is not included by default, and any attempt to use the heap fails with one or multiple linker error messages similiar to this:

`_sbrk_r' referenced in section `.text._malloc_r'
    of libc_nano.a(lib_a-nano-mallocr.o): defined in discarded section
    of libmodm.a(no_heap.o)

This is to prevent you from accidentally using the heap, which may not be desirable for your application. If this occurs you have three choices. You can:

  1. find and remove calls to malloc/new in your application, or
  2. include this module with its predefined allocators, or
  3. implement your own allocator.

Predefined Allocators

There are several trade-offs to each allocator, however, as a rule of thumb, choose:

  • newlib for devices with one large continuous RAM region.
  • block for devices with one very small RAM region.
  • tlsf for devices with multiple, different discontinuous RAM regions.

Multi-SRAM regions

For devices which contain separate memories laid out in a continuous way (often called SRAM1, SRAM2, SRAM3, etc.) the newlib and block strategies choose the largest continuous memory region, even though unaligned accesses across memory regions may not be supported in hardware and lead to a bus fault! Consider using the TLSF implementation, which does not suffer from this issue.

Allocators are not interrupt- or thread-safe

No locking is implemented by default, if you need this feature, consider implementing your own custom allocator algorithm!


The newlib-nano allocator is a simple linked list, its overhead is therefore low, but the access time may not be good. Due to the limitations of the sbrk mechanism only the largest memory region is used as heap! Depending on the device memory architecture this can leave large memory regions unused.

Consider using the TLSF allocator for devices with multiple discontinuous memories.


For devices with very small memories, we recommend using the block allocator strategy, which uses a very light-weight and simple algorithm. This also only operates on one continuous memory region as heap.

The Block allocator does not implement realloc!

This is a bug in modm:driver:block.allocator and currently a modm_assert will fail.


To use all non-statically allocated memory for heap, use the TLSF strategy, which natively supports multiple memory regions. This implementation treats all internal memories as separate regions, so unaligned access across memory boundaries is not an issue. To request heap memory of different traits, see the modm:architecture:memory module.

TLSF has static overhead

The TLSF implementation has a static overhead of about 1kB per memory trait group, however, these can then contain multiple discontinuous regions. The upside of this large static allocation is very fast allocation times of O(1), but we recommend using TLSF only for devices with multiple large memory regions.

Custom Allocator

To implement your own allocator do not include this module. Instead initialize your heap in the function __modm_initialize_memory(), which gets called by the startup script after hardware init, but before static constructors are called (see modm:platform:cortex-m for details).

The simplest way to do so is to allocate a huge array into one of the heap sections and use this as your heap. Consult modm:platform:core for what heap sections your target provides!

modm_section(".heap1") // always the main heap section
uint8_t heap_begin[10*1024]; // 10 kB heap
const uint8_t *const heap_end{heap_begin + sizeof(heap_begin)};

extern "C" void __modm_initialize_memory()
    // Initialize your specific allocator algorithm here

Static constructors are only called afterwards!

Since constructors may call the heap, it must be initialized before static constructors are called. Only trivially constructed (POD) objects are already initialized!

Using the HeapTable

If you prefer a little more control, include the modm:architecture:memory module to get access to the internal modm::platform::HeapTable API, which lists memory regions by traits and sizes.

For example to find the largest continuous memory section with default traits you can use this code:

const uint8_t *heap_begin{nullptr};
const uint8_t *heap_end{nullptr};
extern "C" void __modm_initialize_memory()
    bool success = HeapTable::find_largest(&heap_begin, &heap_end,
    modm_assert(success, "heap.init", "No default memory section found!");

If you want to know more about the available memory regions, you can iterate over the heap table directly. This gives you full control over where you want to place you heap. You can print this table at runtime to get a feel for it:

for (const auto [traits, start, end, size] : modm::platform::HeapTable())
    MODM_LOG_INFO.printf("Memory section %#x @[0x%p,0x%p](%u)\n",
                         traits.value, start, end, size);

Providing sbrk

To use the builtin allocator from newlib, all you need to provide is an implementation of the sbrk function. A simple implementation for a [heap_begin, heap_end] memory region looks like this:

const uint8_t *heap_top{heap_begin};
extern "C" void* _sbrk_r(struct _reent *,  ptrdiff_t size)
    const uint8_t *const heap = heap_top;
    heap_top += size;
    modm_assert(heap_top < heap_end, "heap.sbrk", "Heap overflowed!");
    return (void*) heap;

Wrapping malloc

To use a completely custom allocator, you need to replace the newlib allocator by wrapping the malloc, calloc, realloc and free functions via the linker by adding this to your project configuration:

  <!-- repos, modules, options, etc... -->
    <collect name=":build:linkflags">-Wl,-wrap,_malloc_r</collect>
    <collect name=":build:linkflags">-Wl,-wrap,_calloc_r</collect>
    <collect name=":build:linkflags">-Wl,-wrap,_realloc_r</collect>
    <collect name=":build:linkflags">-Wl,-wrap,_free_r</collect>

And then implement the following functions with your custom allocator:

extern "C" void *
__wrap__malloc_r(struct _reent *, size_t size)
    return allocator.malloc(size);
extern "C" void *
__wrap__calloc_r(struct _reent *, size_t size)
    return allocator.calloc(size);
extern "C" void *
__wrap__realloc_r(struct _reent *, void *ptr, size_t size)
    return allocator.realloc(ptr, size);
extern "C" void 
__wrap__free_r(struct _reent *, void *p)

This is particularly recommended if you need a thread-safe malloc, which you implement here via the _reent struct. Consult newlib docs for details.

sbrk is not called anymore

When wrapping these malloc functions, _sbrk_r is not called anymore, and therefore is thrown away by the linker, thus the linker error disappears. You therefore do not need to implement it, not even as a stub.

To also support memory traits, you need to overwrite the default implementation of malloc_traits(size, traits) which would otherwise just ignore the traits:

extern "C" void *
malloc_traits(size_t size, uint32_t ctraits)
    // Convert back from C land to C++ land:
    const modm::MemoryTraits traits{ctraits};
    if (traits & modm::MemoryTrait::AccessDMA) {
        // check for space in DMA-able heap regions
    } else {
        // check other regions
    return ptr;

This module is only available for sam, stm32.



Heap allocator algorithms

Default: tlsf
Inputs: [block, newlib, tlsf]
Input Dependency: block -> modm:driver:block.allocator
Input Dependency: tlsf -> modm:tlsf



modm_platform_heap modm: platform: heap

modm_architecture_assert modm: architecture: assert


modm_architecture_memory modm: architecture: memory


modm_driver_block_allocator modm: driver: block.allocator


modm_tlsf modm: tlsf


Limited availability: Check with 'lbuild discover' if this module is available for your target!