From mboxrd@z Thu Jan  1 00:00:00 1970
From: Florian Weimer <fweimer@redhat.com>
Subject: Re: Official Linux system wrapper library?
Date: Wed, 14 Nov 2018 13:10:16 +0100
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Cc: Willy Tarreau <w@1wt.eu>, "Michael Kerrisk (man-pages)" <mtk.manpages@gmail.com>, Daniel Colascione <dancol@google.com>, linux-kernel <linux-kernel@vger.kernel.org>, Joel Fernandes <joelaf@google.com>, Linux API <linux-api@vger.kernel.org>, Vlastimil Babka <vbabka@suse.cz>, Carlos O'Donell <carlos@redhat.com>, "libc-alpha@sourceware.org" <libc-alpha@sourceware.org>
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* Adam Borowski:

> On Sun, Nov 11, 2018 at 12:46:35PM +0100, Florian Weimer wrote:
>> A lot of multi-threaded applications assume that most high-level
>> functionality remains usable even after fork in a multi-threaded
>> process.
>
> How would this be even possible?  Currently fork kills all threads
> (save for the caller).

glibc's fork acquires several locks around fork.  Other mallocs install
fork handlers, too.

> Glibc's manpage also warns:
>
> # After a fork() in a multithreaded program, the child can safely call only
> # async-signal-safe functions (see signal-safety(7)) until such time as it
> # calls execve(2).
>
> Which makes sense as its malloc uses a mutex, and you can't take a breath
> without a library call using malloc somewhere (or in C++, the language
> itself).

Right, but applications require a working malloc after fork,
unfortunately.  opendir is often used to enumerate file descriptors
which need closing, for example.

Thanks,
Florian