pipe(2) calling convention: why?

[Kostik Belousov]

On Sun, Nov 09, 2008 at 08:27:46PM +0100, Ed Schouten wrote:
> 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?
> 

The convention of returning pipe descriptors in the registers comes
back at least to the Six Edition. Check the Lion' book for the reference.
Amusingly, Solaris uses the same calling convention for pipe(2).

I do not see what we gain by the change. Now, we have one syscall and
some arch-dependend wrappers in the libc. After the patch, we get rid
of the wrappers, but grow two syscalls.

The only reason of doing this I can imagine is to allow syscall(2) to
work for SYS_pipe from C code. Since we did not heard complaints about
this for ~15 years, we can live with it.
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