Re: Official Linux system wrapper library?

Florian Weimer
2018-11-14T12:10:28Z

* 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