10.7 Dynamic Memory Allocation

Dynamic memory allocation is the runtime allocation of uninitialized memory for arbitrary-sized data objects by various standard C library dynamic memory management functions that are explicitly called by the programmer (such as calloc, malloc and realloc) along with the C++ new operator. The memory for the objects is allocated from a region of reserved memory, usually called the heap. Memory allocated in this way can be freed for reuse by calling additional library routines (such as free) or the C++ delete operator, although indiscriminate use of this feature can lead to memory fragmentation.

There are two libc implementations of dynamic memory allocation available, allowing the programmer to select between a reduced code footprint or better memory management. These implementations can be selected using the -mmalloc-variant=scheme option.

Lite allocation uses a simple algorithm for allocating and freeing chunks of memory from the heap, resulting in a smaller code footprint and almost no runtime overhead. However, memory can easily fragment when allocating and subsequently freeing memory chunks in this way. This dynamic memory allocation implementation is selected when using the -mmalloc-variant=lite option or when this option has not been specified.

Binned allocation maintains free (unallocated) chunks of memory in bins, grouped by size. Algorithms are used to search for suitable memory chunks within the bins when requested. This algorithm adds some runtime overhead but overcomes many of the memory fragmentation problems associated with other schemes. The binned form of dynamic memory allocation will be utilized when using the -mmalloc-variant=binned option has been specified.

By default, no heap is used. If your program uses dynamic memory allocation, you must set aside memory to use as the heap. Most C++ applications will require a heap. In MPLAB X, you can specify a heap size in the xc32-ld (32-Bit Linker) category in the Project Properties dialog. MPLAB X will automatically pass the option to the linker when building your project. Alternatively, a heap can be created by specifying its size using the --defsym=_min_heap_size linker command line option. An example of allocating a heap of 512 bytes using the command line is:

xc32-gcc -mprocessor=32MZ2048ECH100foo.c -Wl,--defsym=_min_heap_size=512

An example of allocating a heap of 0xF000 bytes using the xc32-g++ driver is:

xc32-g++ -mprocessor=32MZ2048ECH100vector.cpp -Wl,--defsym=_min_heap_size=0xF000

The linker allocates the heap immediately before the stack.