The heap memory manager provides simple dynamic memory allocation mechanisms for embedded systems. It was introduced in HALFRED to fight the chaos of heap management.
A heap is simply a dedicated pool of memory, on which the program can allocate (reserve) some space for further use. The allocation is done dynamically at runtime, thus it's often called dynamic memory. When the allocated memory on the heap is not needed anymore, the program should free it (cancel reservation, deallocate). This is true for most PC applications. In embedded systems freeing heap memory is often not needed and even depreciated, because of the memory fragmentation effect and allocation time uncertainty.
The default and most portable dynamic memory management is provided by standard C functions: malloc() and free(). These are often used in more complex embedded systems, but for low-complex or highly resource/time-constrained applications, the default implementations are often too time and resource consuming. Fortunately, in most embedded systems the C library implementation allow the user to alter the default behavior of malloc() and free(), so that the dynamic memory management is done the right, application-specific way. Unfortunately this approach is not so portable and depends on the particular C library implementation used.
Because of these drawbacks, some embedded real-time operating systems have their own memory managers. A good example is the FreeRTOS system, were heap management is done via pvPortMalloc and vPortFree portable functions. The only drawback, is that when there are is more than one heap memory manager working in a single application, there is also more than one heap space. And most often the heap space is oversized due to the fact that it is hard to define the exact maximum amount of heap needed. So in case of more stack space, the memory waste increases and it is more complex to track several heaps (for example checking overflows) than just one.
To make HALFRED-based applications less vulnerable to heap problems, a dedicated heap engine has been introduced.
The heap memory is allocated by a call to HEAP_Alloc() and freed by a call to HEAP_Free(). This is simple and straightforward. The external behavior of these functions should be identical to malloc() and free().
The heap engine is controlled by the HAL_HEAP_MODE definition in hal_config.h. These modes control the internal behavior of HEAP_Alloc() and HEAP_Free(). There are 4 modes of operation defined:
This is the default mode, the most passive and "uninvasive". In this mode HALFRED's internal heap memory manager is disabled (does not compile). The heap will be serviced by standard malloc() and free() implementation, unless there is a default OS implementation (like pvPortMalloc and vPortFree for FreeRTOS). Use this mode if you don't know what else to do.
In this mode the heap will be serviced by a simple HALFRED's internal heap manager. This also means, that whenever possible, OS heap will also be serviced by this HALFRED's internal heap manager. Remember that the simple internal memory manager does NOT provide methods for freeing allocated heap space! In this mode additional functions: HEAP_GetSpaceUsed and HEAP_GetSpaceLeft are available to check how much space is used and how much space is left on the heap. This is the most recommended mode for HALFRED-based applications.
In this mode the heap will be serviced by more advanced HALFRED's internal heap manager. This also means, that whenever possible, OS heap will also be serviced by this HALFRED's internal heap manager. This internal memory manager provide methods for both allocating and freeing heap space. In this mode additional functions: HEAP_GetSpaceUsed and HEAP_GetSpaceLeft are available to check how much space is used and how much space is left on the heap.
This is the user-defined mode. In this mode, the user is responsible for providing HEAP_Alloc() and HEAP_Free() implementations.
Size of heap.
In modes 1 and 2 the heap size is defined by HAL_HEAP_SIZE definition in hal_config.h. In mode 0 the size of the heap is defined in other way, specific to the C library used.
Memory blocks alignment.
It is possible to align every block allocated on the heap to a number of bytes, although it only works in MODE 1. To do that, the HAL_HEAP_ALIGNMENT must be defined. HAL_HEAP_ALIGNMENT equal to zero or one will have no effect on the alignment. By default the blocks on the heap are aligned to four bytes (HAL_HEAP_ALIGNMENT equal to 4).
Tracing memory allocation.
In MODE 1, the heap manager will throw diagnostic messages through the DIAG module on every allocation, when HAL_HEAP_DEBUG definition is present and equal to one.
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