How to use mmap to allocate a memory in heap?

Question

Just the question stated, how can I use mmap() to allocate a memory in heap? This is my only option because malloc() is not a reentrant function.

Answer 1

Why do you need reentrancy? The only time it's needed is for calling a function from a signal handler; otherwise, thread-safety is just as good. Both malloc and mmap are thread-safe. Neither is async-signal-safe per POSIX. In practice, mmap probably works fine from a signal handler, but the whole idea of allocating memory from a signal handler is a very bad idea.

If you want to use mmap to allocate anonymous memory, you can use (not 100% portable but definitely best):

p = mmap(0, size, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0);

The portable but ugly version is:

int fd = open("/dev/zero", O_RDWR);
p = mmap(0, size, PROT_READ|PROT_WRITE, MAP_PRIVATE, fd, 0);
close(fd);

Note that MAP_FAILED, not NULL, is the code for failure.

Answer 2

Make a simple slab allocator

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Although allocating memory in a signal handler¹ does seem like something best avoided, it certainly can be done.

No, you can't directly use malloc(). If you want it to be in the heap then mmap won't work either.

My suggestion is that you make a special-purpose slab allocator based on malloc.

Decide exactly what size of object you want and preallocate some number of them. Allocate them initially with malloc() and save them for concurrent use later. There are intrinsically reentrant queue-and-un-queue functions that you can use to obtain and release these blocks. If they only need to be managed from the signal handler then even that isn't necessary.

Problem solved!

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^{1. And if you are not doing that then it seems like you have an embedded system or could just use malloc().}