Is munmap_chunk() a function? If so, what does it do?

Question

Sometimes code that use heap allocation functions throw this error: munmap_chunk(): invalid pointer It appears by context munmap_chunk() is a function, and these errors are this function throwing them. What is this function and what does this function do?

My question is rather simple, so there is no reason to provide a lot of extra information: What is munmap_chunk()?

Answer 1

Two simple functions malloc and free must solve contradicting problems:

• Memory must be returned to the kernel as fast as possible.
• Memory allocations must be fast, so memory cannot not be returned to the kernel immediately and should be reused. Coupled with memory fragmentation, this makes it impossible to return memory to the kernel (glibc doesn't do this at all).

To solve both, allocators (at least glibc, musl, windows) have a threshold for a size of an allocated object: allocations greater than that size are done by direct call to mmap/VirtualAlloc while freeing such chunks goes directly to munmap/VirtualFree.

This makes it possible to keep malloc fast for small allocations, while keeping it memory-efficient for large ones (as it can free large chunks of memory immediately).

The threshold value is usually about 128 KB and larger (see DEFAULT_MMAP_THRESHOLD_MIN in glibc, MMAP_THRESHOLD in musl).

Function munmap_chunk in glibc is responsible for that and is usually called like this:

if (chunk_is_mmapped(p)) {
    munmap_chunk(p);
    return;
}

You will see munmap_chunk(): invalid pointer message when passing a pointer to a chunk of memory that looks like mmap-allocated chunk, but upon investigation has invalid size, which means there is a memory corruption or free misusage.

Aside from this, it is also responsible of keeping track of a number of allocated through mmap chunks to workaround VMA merging issue on Linux kernel issues.