On Wednesday, November 20, 2024 at 1:06:39 AM UTC-6 tobia...@gmx.de wrote:
The new version of cysignals, released just a couple of hours ago, now builds using Meson and works fine on Windows. @Marc, Nathan & collaborators: I've seen that CyPari has some Windows-specific signal handling. Would be awesome if these changes could be added to cysignals (if necessary and not done already) and then you could perhaps use cysignals in Cypari. Hi Tobias, I don't understand what you are saying. When you say "works on Windows" do you mean that the build works? I am certain that sig_on and sig_off do not work on Windows if you have not added any Windows-specific signal handling. Also we *do* use cysignals in CyPari -- that is, we use our modified version of cysignals, which works on Windows. The Windows-specific signal handling is essential if you want to make sig_on and sig_off work in Windows, and cysignals is what provides those functions. The core problem on Windows that must be solved to make sig_on and sig_off work is this (1) sig_on and sig_off use longjmp inside of a signal handler (2) on Windows a signal handler runs in a separate thread and therefore has its own stack. Consequently setjmp and longjmp cannot be used in a signal handler on Windows. The way setjmp and longjmp work is that setjmp takes a snapshot of the main execution stack which is saved in a global variable and longjmp restores the state of the main execution stack to what it was at the time the snapshot was taken. The way that sig_on and sig_off work is that sig_on installs some special signal handlers and then calls setjmp. That should happen immediately before calling some external library function, say from libpari. If that library function produces a killer signal, say SIG_SEGV, then the special SIG_SEGV signal handler calls longjmp to restore the state of the stack before the library function was called. It then executes its own exit procedure which prints a message about the segfault and exits cleanly. If the library function executes with no signals then sig_off should be called immediately to restore normal signal handling. Obviously if longjmp is called from a thread which has a different main execution stack all hell will break loose. The trick that we use in the CyPari version of cysignals to avoid this pitfall is that we exploit the fact that, in truth, there is one and only one signal handler on Windows which *does* run on the main execution stack of the process receiving the signal. That is the handler for SIG_FPE - floating point exception (which usually does not terminate a process). To make this work our special handler for, say, SIG_SEGV sets some global state and then raises SIG_FPE. Our handler for SIG_FPE checks the global state to decide whether the SIG_FPE is really a floating point exception or was raised by some other signal handler. - Marc -- You received this message because you are subscribed to the Google Groups "sage-devel" group. To unsubscribe from this group and stop receiving emails from it, send an email to sage-devel+unsubscr...@googlegroups.com. To view this discussion visit https://groups.google.com/d/msgid/sage-devel/843570bf-56a3-4223-a446-a43a615f5a83n%40googlegroups.com.
