Two simultaneous heaps combining: kernel heap and libc heap

RE: Two simultaneous heaps combining: kernel heap and libc heap

Egor Dolgalev — Tue, 28 Apr 2026 19:06:43 GMT

Hi Priyanka,

I tried it. To set any value to CONFIG_COMMON_LIBC_MALLOC_ARENA_SIZE I have to enable CONFIG_COMMON_LIBC_MALLOC. Enabling CONFIG_COMMON_LIBC_MALLOC leads to malloc function compilation and I have conflict with my malloc implementation

RE: Two simultaneous heaps combining: kernel heap and libc heap

Priyanka — Tue, 28 Apr 2026 08:29:49 GMT

Hi Egor,

Could you try to allocate:

CONFIG_COMMON_LIBC_MALLOC_ARENA_SIZE=1

instead of leaving it 0 ? (It's just to satisfy the condition, but the actual allocation should go through your overriden method.)

Please try this and let me know how it goes.

-Priyanka

RE: Two simultaneous heaps combining: kernel heap and libc heap

Egor Dolgalev — Thu, 23 Apr 2026 08:18:41 GMT

Justin,

Thank you for this hints. I managed it with the next changes:

1. Override only C allocation functions malloc, calloc, realloc and free to kernel equivalents. Did several experiments and I see that C++ allocation is done via these C functions

2. Exclude Libc implementation of allocation functions and set heap arena size to 0 using next config:

CONFIG_COMMON_LIBC_MALLOC=n

CONFIG_COMMON_LIBC_MALLOC_ARENA_SIZE=0

CONFIG_COMMON_LIBC_CALLOC=n

CONFIG_COMMON_LIBC_REALLOCARRAY=n

It looks like these approach works in my case. May be you can add something to it.

But I faced with one issue for handling Settings subsytem over the MCU manager. I want to handle settings write and read using heap to not to allocate too much stack for McuMgr thread because we do settings read/write only during production stage and the size of the transferred data is significant. So, I tried to set MCUMGR_GRP_SETTINGS_BUFFER_TYPE_HEAP=y but got 'n' due to the dependency of the heap size which is 0

Here is the dependency from Kconfig:

config MCUMGR_GRP_SETTINGS_BUFFER_TYPE_HEAP
bool "Heap (dynamic size)"
depends on COMMON_LIBC_MALLOC_ARENA_SIZE > 0
help
Use dynamic heap memory allocation through malloc, if there is
insufficient heap memory for the allocation then the request will be
rejected.

What could be the solution for this issue?

RE: Two simultaneous heaps combining: kernel heap and libc heap

Justin012 — Wed, 22 Apr 2026 03:45:59 GMT

Hi Egor,

you’re right — having two heaps is not ideal, and unfortunately there’s no built-in way to truly merge them into one.

Simple approach that works:
Redirect everything to the Zephyr (kernel) heap. heardle game

That means:

Override malloc, free, calloc, realloc
Override C++ new / delete
Internally call k_malloc / k_free

This way you effectively use one heap only, and you can monitor it with Zephyr APIs.

Also, I’d avoid:

COMMON_LIBC_MALLOC_ARENA_SIZE = -1

Better set a fixed size or fully redirect allocations, otherwise memory usage is hard to control.

In short:
You can’t truly combine both heaps, but you can force everything to use the kernel heap, which is the cleanest solution.

Hope that helps