MAP_POPULATE is intentionally disallowed currently by the Cosmo Libc pledge() implementation, because it's Linux-specific, I've rarely seen it used, and I was erring on the side of caution since I wasn't sure how much prefaulting reads could be considered a borderline privileged operation. If you remove that flag, then mmap() should work fine.

MAP_STACK can be used correctly with Cosmo by saying MAP_STACK | MAP_ANONYMOUS. That's all you need. You can also call Cosmo's _mapstack() function. Cosmo is somewhat unique in that it polyfills the platform-specific underlying flags automatically when you do that. So you shouldn't include MAP_GROWSDOWN because we chose to adopt the FreeBSD behavior which we polyfill on Linux using MAP_GROWSDOWN. See our docs for further details. https://github.com/jart/cosmopolitan/blob/0a589add4167c1b587...

I see. I should check again because things can change but my app is performance sensitive and when I checked last it seemed to give myself a boost since I was writing to the memory so quickly.

One of the reasons why I use MAP_GROWSDOWN is because it gives me a page guard. Your link mentions/implements a page guard. Well done

I tried my complicated app for fun removing the MAP_POPULATE flag. I get an assert because it seems like mmap didn't align the pointer to 4K. I don't need this to work I was just playing around with it

As you wish. I've just pushed a change removing the restriction on MAP_POPULATE. https://github.com/jart/cosmopolitan/commit/ccd057a85daf0d2d... Now that we have -n (maximum niceness flag) I doubt anyone will object. If your app is public and you're building it with Cosmo, then I look forward to the opportunity to see it someday. Please stay in touch. You're also welcome to join our Discord community! https://discord.gg/mw3j3sa2