Post by @mcc@mastodon.social

I have been called on to write a "Partial Free" function for previously allocated virtual memory. EG, someone allocated a large block of memory with mmap(), and now they want to free a section *out of the middle or end* of it.

My reading of the POSIX munmap doc https://pubs.opengroup.org/onlinepubs/7908799/xsh/munmap.html is that munmap does this already, because it deallocates "pages" not previous allocations, and I only need to call munmap() with the requested-deallocated range (assuming the range to be page-aligned)

Correct?

munmap

0 Boosts

0 Likes

yuubi

@yuubi@furry.engineer 11 min. ago

@mcc i've seen glibc on linux turn malloc into "mmap a large block and munmap the last about half of it", so yes that's a common usage pattern. the munmap took most of the time.

David Smith

@Catfish_Man@mastodon.social 12 min. ago

@mcc correct. In case it matters, some kernels do not handle this especially efficiently.

John Ripley

@jripley@mastodon.social 16 min. ago

@mcc That is also my understanding, and what I recall is also the implementation in various kernel sources (including Linux). mmap is not malloc, operates on independent pages, and munmap is not required to use only values returned by a previous mmap. You can even munmap pages which were present at process launch, and never explicitly mmap'd.

mcc

@mcc@mastodon.social 20 min. ago

For clarity, it is more important my function run correctly on BSD derived systems than Linux derived systems

Federation Bot

@Federation_Bot 14 min. ago

@mcc Lol I replied to this as if it was the other thread, sorry

Federation Bot

@Federation_Bot 16 min. ago

@mcc Several that I know of have proprietary languages, but a lot are simply HPGL and the interfaces are serial, and you can literally pipe the HPGL text saved from Inkscape onto the USB TTY file at least on Linux. The Silhouette has a plugin for Inkscape, but say, the Cricut you'd have to gut the electronics entirely and figure that out