Say I have a big block of mapped memory I finished using. It came from mmaping anonymous memory or using MAP_PRIVATE. I could munmap it, then have malloc mmap again the next time I make a big enough allocation.
Could I instead give the memory to malloc directly? Could I say "Hey malloc, here's an address range I mapped. Go use it for heap space. Feel free to mprotect, mremap, or even munmap it as you wish."?
I'm using glibc on linux.
glibc malloc calls __morecore (a function pointer) to obtain more memory. See <malloc.h>. However, this will not work in general because the implementation assumes that the function behaves like sbrk and returns memory from a single, larger memory region. In practice, with glibc malloc, the only realistic way to make memory available for reuse by malloc is calling munmap.
Other malloc implementations allow donating memory (in some cases as internal interfaces). For example, musl's malloc has a function called __malloc_donate which should do what you are asking for.