Discussion:
fork-like functions and async-signal-safety
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Alan Somers
2023-12-01 16:26:51 UTC
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fork() is not async-signal-safe, and therefore can't be used in a
signal handler. Nor can it be used from a multithreaded program
(unless the child only executes aysnc-signal-safe functions).

_Fork(), OTOH, is async-signal-safe. According to fork(2), "It can be
used safely from signal handlers, but then no userspace services
(malloc(3) or rtld(1)) are available in the child if forked from
multi-threaded parent."

But can you use malloc and rtld in the child if _Fork was invoked from
a multi-threaded process, but not from a signal handler? That's not
clear to me.

And what about pdfork? Neither sigaction(2) nor pdfork(2) specify
whether pdfork is async-signal-safe. Is it?

FYI the reason I'm asking is that I'm trying to determine whether it
would be possible to make cap_init() async-signal-safe.

-Alan


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Konstantin Belousov
2023-12-01 19:58:04 UTC
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Post by Alan Somers
fork() is not async-signal-safe, and therefore can't be used in a
signal handler.
... if the process is multithreaded.
Post by Alan Somers
Nor can it be used from a multithreaded program
(unless the child only executes aysnc-signal-safe functions).
More precisely, it is not guaranteed to work in the child of the fork of
mt process. I believe (with high confidence) that it works for the
FreeBSD implementation.
Post by Alan Somers
_Fork(), OTOH, is async-signal-safe. According to fork(2), "It can be
used safely from signal handlers, but then no userspace services
(malloc(3) or rtld(1)) are available in the child if forked from
multi-threaded parent."
But can you use malloc and rtld in the child if _Fork was invoked from
a multi-threaded process, but not from a signal handler? That's not
clear to me.
No.
Post by Alan Somers
And what about pdfork? Neither sigaction(2) nor pdfork(2) specify
whether pdfork is async-signal-safe. Is it?
pdfork() is same as fork().
Post by Alan Somers
FYI the reason I'm asking is that I'm trying to determine whether it
would be possible to make cap_init() async-signal-safe.
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