Unlike programming languages such as C/C++, MicroPython hides memory management details from the developer by supporting automatic memory management of a ":ref:`Python heap<heap>`" that holds all Python objects. MicroPython uses garbage collection (GC) for automatic memory management. The garbage collector is responsible for freeing memory which is no longer in use.

Specifically, MicroPython uses a Mark and Sweep garbage collection algorithm. This algorithm has a mark phase that scans the heap marking all live objects, and then a sweep phase goes through the heap reclaiming all unmarked objects.

The MicroPython garbage collector is by default automatic, but manual control is available through the :mod:`gc` built-in module.

Awareness of the garbage collector is needed when writing C code that allocates memory from the "Python heap" (i.e. functions m_malloc(), m_malloc0(), m_free(), etc).

The mark phase of the garbage collector scans for live pointers to heap memory starting from the following roots:

• The stack of the main Python runtime (or REPL).
• The stacks of each "Python thread", for ports which implement Python threads on top of native operating system threads or tasks.
• The "root pointers" defined in C code using the macro MP_REGISTER_ROOT_POINTER. These are the recommended way to have statically scoped pointers to the Python heap.
• Tracked allocations made with the m_tracked_calloc(), m_tracked_realloc and m_tracked_free() functions. These special functions allow allocating a block of memory which is always considered live by the garbage collector. Similar to memory allocation in C, this memory is only freed by calling m_tracked_free() or by soft reset. There is a small memory usage and runtime overhead to each tracked allocation. This feature is not enabled by default on all ports.

The garbage collector then recursively scans and marks all the memory pointed to by the root pointers, until all addresses are exhausted. This is sufficient to find all Python objects that are still in use by the MicroPython runtime.

However, the following memory will not be scanned by the garbage collector and could be freed prematurely:

• Static or global C variables which contain pointers to heap memory.
• Pointers which don't point to the "head" of an allocated buffer (i.e. to the exact address returned by m_malloc()), but instead to an address inside the allocated buffer (for example, a pointer to a nested struct). For performance reasons, the garbage collector doesn't mark the enclosing buffer in these cases.
• The stack of any thread or RTOS task which isn't running Python code or manually registered as a "Python thread" (for ports which support native threads or tasks).

Ways to avoid use-after-free in these scenarios:

• Use the tracked allocation API m_tracked_calloc(), m_tracked_realloc() and m_tracked_free().
• Register a root pointer (see above), instead of storing a pointer in a static variable.
• Restructure the code, for example by having an API where Python code initialises a singleton Python object (implemented in C) which holds all of the relevant pointers instead of having them in static variables.

All MicroPython objects are referred to by the mp_obj_t data type. This is usually word-sized (i.e. the same size as a pointer on the target architecture), and can be typically 32-bit (STM32, nRF, ESP32, Unix x86) or 64-bit (Unix x64). It can also be greater than a word-size for certain object representations, for example OBJ_REPR_D has a 64-bit sized mp_obj_t on a 32-bit architecture.

An mp_obj_t represents a MicroPython object, for example an integer, float, type, dict or class instance. Some objects, like booleans and small integers, have their value stored directly in the mp_obj_t value and do not require additional memory. Other objects have their value store elsewhere in memory (for example on the garbage-collected heap) and their mp_obj_t contains a pointer to that memory. A portion of mp_obj_t is the tag which tells what type of object it is.

See py/mpconfig.h for the specific details of the available representations.

Pointer tagging

Because pointers are word-aligned, when they are stored in an mp_obj_t the lower bits of this object handle will be zero. For example on a 32-bit architecture the lower 2 bits will be zero:

********|********|********|******00

These bits are reserved for purposes of storing a tag. The tag stores extra information as opposed to introducing a new field to store that information in the object, which may be inefficient. In MicroPython the tag tells if we are dealing with a small integer, interned (small) string or a concrete object, and different semantics apply to each of these.

For small integers the mapping is this:

********|********|********|*******1

Where the asterisks hold the actual integer value. For an interned string or an immediate object (e.g. True) the layout of the mp_obj_t value is, respectively:

********|********|********|*****010

********|********|********|*****110

While a concrete object that is none of the above takes the form:

********|********|********|******00

The stars here correspond to the address of the concrete object in memory.

The value of a small integer is stored directly in the mp_obj_t and will be allocated in-place, not on the heap or elsewhere. As such, creation of small integers does not affect the heap. Similarly for interned strings that already have their textual data stored elsewhere, and immediate values like None, False and True.

Everything else which is a concrete object is allocated on the heap and its object structure is such that a field is reserved in the object header to store the type of the object.

+++++++++++
+         +
+ type    + object header
+         +
+++++++++++
+         + object items
+         +
+         +
+++++++++++

The heap's smallest unit of allocation is a block, which is four machine words in size (16 bytes on a 32-bit machine, 32 bytes on a 64-bit machine). Another structure also allocated on the heap tracks the allocation of objects in each block. This structure is called a bitmap.

The bitmap tracks whether a block is "free" or "in use" and use two bits to track this state for each block.

The mark-sweep garbage collector manages the objects allocated on the heap, and also utilises the bitmap to mark objects that are still in use. See py/gc.c for the full implementation of these details.

Allocation: heap layout

The heap is arranged such that it consists of blocks in pools. A block can have different properties:

• ATB(allocation table byte): If set, then the block is a normal block
• FREE: Free block
• HEAD: Head of a chain of blocks
• TAIL: In the tail of a chain of blocks
• MARK : Marked head block
• FTB(finaliser table byte): If set, then the block has a finaliser