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 <redacted>
To:     alx.manpages@gmail.com <redacted>



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 <redacted>
To:     alx.manpages@gmail.com <redacted>



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/