Using unistd functions vs calling syscall straight in the code

[Murali Vijayaraghavan <vmurali@csail.mit.edu>]

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Hi

I am trying to run C programs on a barebones (MIPS-like) processor
simulator without any OS. The simulator mainly implements the userspace
ISA, with no syscall instruction support in hardware. I was hoping to
instead support some of the system calls (like open, read, write, etc,
mainly for debugging purposes) by using custom instructions, one for each
(or a few similar) system call(s). For that, the implementation of
functions like read and write should be calling these custom instructions
in assembly, in other words, I have to port the system call layer to my
simulator. I looked at musl among other libc implementations, and this was
the only one whose structure I could understand well, making it easy to
port. I did successfully and easily port it for my purposes, which brings
to my question/comment.

You guys do have a unistd implementation which supposedly implements each
of the system calls. But you are not consistent with the use of these
functions to perform the unistd-implemented tasks. Wouldn't it be a lot
cleaner to call these functions instead of calling syscall / syscall_cp
directly from the other (top-level) functions? Was there some rationale or
is it just code evolution?

Thanks
Murali

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