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Date: Fri, 3 Apr 2020 11:28:54 +0200
From: Szabolcs Nagy <>
Cc: "zhangwentao (M)" <>
Subject: Re: doubt about fork

* guolongqiang <> [2020-04-03 06:37:44 +0000]:
> Hello,
> I have a problem about multi threads fork. The implement of fork not lock such as stdio file,
> __thread_list_lock, or other global mutexs in musl libc before syscall of SYS_clone, this will
> cause dead lock in child. Is this a bug?

the standard is pretty clear that the child after fork
in a multi-threaded process can only do async-signal-safe
operations, anything that may lock is not as-safe.

incidentally glibc tries to make certain operations work
in the child by taking libc internal locks before fork and
releasing them after, however that does not give strong
guarantees of stdio use in the child because glibc supports
malloc interposition and the interposed malloc can't
reliably handle fork (i.e locks may be held in the child)
so stdio (and all libc apis that may internally allocate)
can still deadlock i.e. you don't get more guarantees on
other implementations either than the standard.

> Hope your response!
> --Guo Longqiang

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