1.12 Why do processes never decrease in size?

When you free memory back to the heap with free(), on almost all systems that doesn't reduce the memory usage of your program. The memory free()d is still part of the process' address space, and will be used to satisfy future malloc() requests.

If you really need to free memory back to the system, look at using mmap() to allocate private anonymous mappings. When these are unmapped, the memory really is released back to the system. Certain implementations of malloc() (e.g. in the GNU C Library) automatically use mmap() where available to perform large allocations; these blocks are then returned to the system on free().

Of course, if your program increases in size when you think it shouldn't, you may have a `memory leak' – a bug in your program that results in unused memory not being freed.

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