[tmerr]

This has me thinking, it could be a fun project to prototype a convenient file interface based on pointers as a C library. I imagine it's possible to get something close to what you want in terms of interface (not sure about performance).

I suspect in some cases it will be more convenient and in other cases it will be less convenient to use. The write interface isn't so bad for some use cases like appending to logs. It's also not bad for its ability to treat different types of objects (files, pipes, network) generically.

But if you want to do all manipulation through pointers it should be doable. To support appending to files you'd probably need some way to explicitly grow the file size and hand back a new pointer. Some relevant syscalls would probably be open, ftruncate, and mmap.

[eru]

You can do something like that, but you still need to be able to tell your OS when you want to sync what writes to disk.

[tmerr]

Yes the situation sounds similar to regular files to me. Iiuc mmap can implicitly sync to files, but it is only guaranteed after unmapped (akin to closing a file) or an explicit call to msync (akin to fflush).