Ignoring return values from thread and locking functions is dangerous to say the least. The problem creates hard to find bugs in multithreaded programs.
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We were going to begin to harden Threads.h and Threads.m, but I decided to yank the library due to the concerns.
Attached are the partial hardening applied to Threads.h and Threads.m.
That code is not for Windows.
I suppose you should post patches to p7zip project tracker instead.