Problem usage memory in C

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For good reasons, virtually no memory allocator returns blocks to the OS

Memory can only be removed from your program in units of pages, and even that is unlikely to be observed.

calloc(3) and malloc(3) do interact with the kernel to get memory, if necessary. But very, very few implementations of free(3) ever return memory to the kernel1, they just add it to a free list that calloc() and malloc() will consult later in order to reuse the released blocks. There are good reasons for this design approach.

Even if a free() wanted to return memory to the system, it would need at least one contiguous memory page in order to get the kernel to actually protect the region, so releasing a small block would only lead to a protection change if it was the last small block in a page.

Theory of Operation

So malloc(3) gets memory from the kernel when it needs it, ultimately in units of discrete page multiples. These pages are divided or consolidated as the program requires. Malloc and free cooperate to maintain a directory. They coalesce adjacent free blocks when possible in order to be able to provide large blocks. The directory may or may not involve using the memory in freed blocks to form a linked list. (The alternative is a bit more shared-memory and paging-friendly, and it involves allocating memory specifically for the directory.) Malloc and free have little if any ability to enforce access to individual blocks even when special and optional debugging code is compiled into the program.

1. The fact that very few implementations of free() attempt to return memory to the system is not at all due to the implementors slacking off.

Interacting with the kernel is much slower than simply executing library code, and the benefit would be small. Most programs have a steady-state or increasing memory footprint, so the time spent analyzing the heap looking for returnable memory would be completely wasted. Other reasons include the fact that internal fragmentation makes page-aligned blocks unlikely to exist, and it’s likely that returning a block would fragment blocks to either side. Finally, the few programs that do return large amounts of memory are likely to bypass malloc() and simply allocate and free pages anyway.

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