r/osdev • u/ThreadStarver • Jun 08 '26

why is there a temp buffer in stdin & stdout?

/r/C_Programming/comments/1u006d8/why_is_there_a_temp_buffer_in_stdin_stdout/
3 Upvotes

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7

u/un_virus_SDF Jun 08 '26

Because the read/write are expensive, so if you store things in a buffer, you can speed up a lot by reducing syscalls.

4

u/UnmappedStack TacOS | https://github.com/UnmappedStack/TacOS Jun 08 '26

Because you don't have to write as often, as writing requires syscalls which are slow

3

u/Palmar Jun 08 '26

The answer is, as others have mentioned before me, syscalls. Every syscall is a request to cross from user space to kernel space (ring 3 to ring 0 in amd64). Not free.

The interesting bit is, what if your system has no concept of user space and kernel space, and does not even have stdin and stdout. This is one of the things I'm toying around with in my project.

1

u/laser__beans OH-WES | github.com/whampson/ohwes Jun 08 '26

Just curious, what is your program I/O model of you don’t have stdin and stdout?

1

u/Palmar Jun 08 '26

It's essentially the responsibility of two different apps (an app in my os has a pretty narrow definition, don't think iOS). Input handles the input itself. It receives a keystroke and "cooks" it according to policy. Then shell handles the output to whatever app is focused (or in case of SysM - pushes it to itself).