Hello Dave,
On 2/20/21 10:30 AM, Alejandro Colomar (man-pages) wrote:
I'll forward these 2 mails to the mailing list (and Michael (mtk) too;
maybe he can correct me).
I'll answer in a reply to this email.
-------- Forwarded Message --------
Subject: Bug reporting
Date: Sat, 20 Feb 2021 08:09:50 +0300
From: Dave Chupreev <cdn.dev@xxxxxxxxx>
To: alx.manpages@xxxxxxxxx <alx.manpages@xxxxxxxxx>
Hello, Alejandro.
Pages this <https://man7.org/linux/man-pages/dir_section_2.html> and
this <https://man7.org/linux/man-pages/man2/syscalls.2.html> have
different count of 'exit' calls, 4 and 2 respectively.
According to these pages
(<https://github.com/torvalds/linux/blob/v5.11/arch/x86/entry/syscalls/syscall_32.tbl>,
<https://github.com/torvalds/linux/blob/v5.11/arch/x86/entry/syscalls/syscall_64.tbl>),
there are only 2 system calls containing 'exit' - *exit* and
*exit_group*. So it seems '_Exit' and '_exit' are redundant here
<https://man7.org/linux/man-pages/dir_section_2.html>. Please correct me
if I am wrong.
Thanks in advance!
-------- Forwarded Message --------
Subject: Bug reporting
Date: Sat, 20 Feb 2021 08:18:56 +0300
From: Dave Chupreev <cdn.dev@xxxxxxxxx>
To: alx.manpages@xxxxxxxxx <alx.manpages@xxxxxxxxx>
Hello, Alejandro.
Entry exit(2) on this
<https://man7.org/linux/man-pages/dir_section_2.html> page directs to
page with _exit(2) in the header.
According to these pages
(<https://github.com/torvalds/linux/blob/v5.11/arch/x86/entry/syscalls/syscall_32.tbl>,
<https://github.com/torvalds/linux/blob/v5.11/arch/x86/entry/syscalls/syscall_64.tbl>),
system call itself is called *exit* so it would be nice to fix the
header to avoid the confusion. Please correct me if I am wrong.
Thanks in advance!
Okay I admit there's a bit of inconsistency in these functions.
exit_group(2) apart (which I think is quite clear).
___
So, the C standard specifies two exit() vairants:
exit() and _Exit()
and POSIX specifies 3 exit() variants:
_exit(), _Exit(), and exit()
_exit() and _Exit are identical, and the only reason for _Exit() to
exist is history, and the fact that the C standard couldn't use _exit()
as it only reserved names that started with an underscore and an
uppercase letter, but not a lowercase one, for future revisions. So
when it adopted the posix _exit(), it had to rename it with an available
name.
exit() does something more than _exit().
That's what the standards say.
___
Now let's discuss the implementation, which is where the confusion starts.
In the Linux kernel, there's only one system call, and it's called exit
(its entry point is sys_exit(), and can be called using
syscall(SYS_exit, ...) *but don't*), BUT it implements the functionality
of _exit() (as the standards call it; see above).
That's the reason that the manual page _exit(2) has a link page called
exit(2).
But syscalls can't be called directly (you have to use syscall(2)), so
the C library (glibc) provides a wrapper function for the system call.
And it does it by following the standards, so the wrapper function for
the exit system call is _exit(), as POSIX calls it. And lately, the
second wrapper _Exit() was added, being an alias to _exit().
These correspond to the manual pages _exit(2) and _Exit(2).
And then glibc has to provide the exit() function, as the standards
specify it (which remember, was different than the kernel exit system
call), and the kernel doesn't provide a system call with such
functionality, so glibc has to implement it, probably using _exit(), but
with much more functionality. This is not a system call, but a library
function.
And that's why there's an exit(3) manual page.
___
To put some more context, FreeBSD (and I guess other BSDs as well)
provides _exit() as a system call,
and then _Exit() and exit() are functions which I guess wrap around
_exit(), _Exit() probably just calling _exit().
It provides the following manual pages:
_exit(2), _Exit(3), and exit(3)
___
BTW, even though you can use syscall(2), to call the system calls, don't
take my words that you can do it as a recommendation to do it. If
possible, always use the C library (glibc) wrappers, which hide the
complexity of calling some system calls (and sometimes even bugs) in
their wrappers.
___
I hope I was clear and precise. Michael, please correct me if I was
wrong in some point.
Cheers,
Alex
--
Alejandro Colomar
Linux man-pages comaintainer; https://www.kernel.org/doc/man-pages/
http://www.alejandro-colomar.es/
