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Comment by wtracy | Hacker News Reader

Malloc doesn't interact with the kernel at all. The kernel sees pages, not the data structures that malloc manages. The kernel doesn't even know whether you free()'d the memory by the time the process exits.

There is exactly zero difference from the operating system's perspective between freeing and not freeing the memory before program termination (except that one might have a higher peak memory usage).

The classic implementations of several standard Unix utilities deliberately never deallocated memory for performance reasons. If there was a problem with this practice, I would expect Bell Labs--of all places--to know not to do it.

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>There is exactly zero difference from the operating system's perspective (...)

Close, but not quite: in case of larger allocations, malloc() tends to use mmap( , , , | MAP_ANON) rather than brk() to request memory from the OS. For example, the glibc's malloc() uses mmap() when requested size exceeds MMAP_THRESHOLD, which is 128kB by default.

The mmap() approach gives large, continuous memory blocks that can also be easily free()'d via munmap() with little to no bookkeeping needed[1] - not being subject to the same fragmentation woes as memory allocated via brk() - as long as your address space is significantly larger than allocated memory.

That aside, I fully agree with the author of 9cc.

[1] in fact a simplistic libc memory allocator could directly wrap malloc() around mmap(), free() around munmap() and realloc() around mrealloc(), leveraging the in-kernel allocator at the cost of one syscall with at least two context switches at each call - i.e., slowww

Using mmap/munmap isn't quite that trivial: You need to remember how large each allocation is in order to know how many pages to unmap.

An implementation certainly could return memory to the OS on calls to free(), but to my knowledge none of the widely used implementations do so. (I would be interested to learn of counterexamples!)

jemalloc can use madvise(MADV_FREE) to return pages to the OS.

Did I mention kernels anywhere? I simply shared my experience.

If that doesn't fit neatly into your delusions it's really not my problem.

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