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From:   Florian Weimer <fweimer@redhat.com>
To:     Adam Borowski <kilobyte@angband.pl>
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>
Subject: Re: Official Linux system wrapper library?
References: <CAKOZuesB4R=dCz4merWQN0FSCGrXmOgUUr4ienSbStBJguNv8g@mail.gmail.com>
        <bbc12da5-830e-99a7-95e3-d9da42947dc9@gmail.com>
        <20181111081725.GA30248@1wt.eu>
        <3664a508-ca74-4ff0-39a6-34543194a24e@gmail.com>
        <20181111111143.GB4189@1wt.eu>
        <87zhufvntw.fsf@oldenburg.str.redhat.com>
        <20181114120348.or5id3hzrmltkyvb@angband.pl>
Date:   Wed, 14 Nov 2018 13:10:16 +0100
In-Reply-To: <20181114120348.or5id3hzrmltkyvb@angband.pl> (Adam Borowski's
        message of "Wed, 14 Nov 2018 13:03:48 +0100")
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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