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docs: clarify PSRAM default allocation behavior

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Clarify that MALLOC_CAP_DEFAULT describes a memory capability
rather than the malloc() placement policy, so PSRAM-backed
capability allocations are documented correctly when malloc()
remains internal-only by default.

Closes https://github.com/espressif/esp-idf/issues/18391

Made-with: Cursor
This commit is contained in:
Marius Vikhammer
2026-03-27 10:05:28 +08:00
parent c9dd3aca0b
commit d09e551417
5 changed files with 32 additions and 7 deletions
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docs/en/api-guides/external-ram.rst
+5 -3
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@@ -87,11 +87,13 @@ It is recommended to access the PSRAM by ESP-IDF heap memory allocator (see next
Add External RAM to the Capability Allocator
--------------------------------------------
Select this option by choosing ``Make RAM allocatable using heap_caps_malloc(..., MALLOC_CAP_SPIRAM)`` from :ref:`CONFIG_SPIRAM_USE`.
Select this option by choosing ``Add RAM to heap_caps allocator (malloc() stays internal by default)`` from :ref:`CONFIG_SPIRAM_USE`.
When enabled, memory is mapped to data virtual address space and also added to the :doc:`capabilities-based heap memory allocator </api-reference/system/mem_alloc>` using ``MALLOC_CAP_SPIRAM``.
When enabled, memory is mapped to data virtual address space and also added to the :doc:`capabilities-based heap memory allocator </api-reference/system/mem_alloc>` using ``MALLOC_CAP_SPIRAM``. Since this memory is also tagged with ``MALLOC_CAP_DEFAULT``, calls such as ``heap_caps_malloc(size, MALLOC_CAP_DEFAULT)`` can still return PSRAM pointers.
To allocate memory from external RAM, a program should call ``heap_caps_malloc(size, MALLOC_CAP_SPIRAM)``. After use, this memory can be freed by calling the normal ``free()`` function.
To explicitly allocate memory from external RAM, a program should call ``heap_caps_malloc(size, MALLOC_CAP_SPIRAM)``. After use, this memory can be freed by calling the normal ``free()`` function.
In this mode, standard ``malloc()`` does not allocate from external RAM by default because it uses a separate default allocation policy.
.. _external_ram_config_malloc: