Hello all,

After having a discussion on IRC with some friends of mine about system
call conventions, we couldn't exactly determine why pipe(2)'s calling
convention has to be different from the rest. Unlike most system calls,
pipe(2) has two return values. Instead of just copying out an array of
two elements, it uses two registers to store the file descriptor
numbers.

It seems a lot of BSD-style system calls used to work that way, but
pipe(2) seems to be the only system call on FreeBSD that uses this
today. Some system calls only seem to set td_retval[1] to zero, which
makes little sense to me. Maybe those assignments can be removed.

In my opinion there are a couple of disadvantages of having multiple
return values:

- As documented in syscall(2), there is no way to obtain the second
return value if you use this functions.

- Each of those system calls needs to have its own implementation
written in assembly for each architecture we support. Why can hundreds
of system calls be handled in a generic fashion, while interfaces like
pipe(2) can't?

As a small experiment I've written a patch to allocate a new system call
(506) which uses a generic calling convention to implement pipe(2). It
seems Linux also uses this method, so I've removed linux_pipe() from the
Linuxolator as well, which seems to work.

I could commit this if people think it makes sense. Any comments?

--
Ed Schouten
WWW: http://80386.nl/

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