Eryk Sun added the comment:
Unix waitpid() packs the process exit status and terminating signal number into
a single status value. As specified by POSIX [1], the WEXITSTATUS function
returns only the lower 8 bits of the process exit status. In theory, waitid()
and wait6() can return the full 32-bit status value in the siginfo_t si_status.
Apparently NetBSD does this [2], but evidently Linux does not:
>>> p = subprocess.Popen(['python3', '-c','import os; os._exit(257)'])
>>> os.waitid(os.P_PID, p.pid, os.WEXITED).si_status
1
For Windows, programs sometimes return 16-bit WinAPI error codes or 32-bit
HRESULT error codes. A critical error or unhandled exception may even result in
returning a 32-bit NTSTATUS code (e.g. if you cancel the Windows error
reporting dialog).
Currently the Python 3 implementation of sys.exit and os._exit converts to a
signed long, but the Windows exit code is an unsigned long. To use integer
return codes in the range 0x80000000-0xFFFFFFFF requires manually converting to
the corresponding negative signed value.
Technically the two error types that require 32-bit values are signed (i.e.
HRESULT and NTSTATUS), so this shouldn't be a problem. However, in practice
sometimes a function -- and definitely subprocess.Popen -- returns an HRESULT
error code as an unsigned value.
It would be convenient, at least on Windows, if handle_system_exit in
Python/pythonrun.c converted the value using PyLong_AsUnsignedLongMask instead
of PyLong_AsLong. Similarly os._exit could use Argument Clinic's "unsigned int"
format, which also calls PyLong_AsUnsignedLongMask.
The problem with using PyLong_AsLong on Windows (32-bit or 64-bit) is that it
overflows with an error value of -1 for values greater than 0x7FFFFFFF. In this
case, os._exit raises OverflowError. handle_system_exit, on the other hand,
ignores the exception. It just uses the -1 error value as the exit code, i.e.
0xFFFFFFFF.
[1]: http://pubs.opengroup.org/onlinepubs/9699919799/functions/wait.html
[2]: https://www.daemon-systems.org/man/wait.2.html
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nosy: +eryksun
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