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Sage Developer’s Guide, Release 6.1.1# (some long computation)sig_off()except KeyboardInterrupt:# (handle interrupt)Certain C libraries in Sage are written in a way that they will raise Python exceptions: libGAP and NTLcan raise RuntimeError and PARI can raise PariError. These exceptions behave exactly like theKeyboardInterrupt in the example above and can be caught by putting the sig_on() inside a try/exceptblock. See Error Handling in C Libraries to see how this is implmented.It is possible to stack sig_on() and sig_off(). If you do this, the effect is exactly the same as if only the outersig_on()/sig_off() was there. The inner ones will just change a reference counter and otherwise do nothing.Make sure that the number of sig_on() calls equal the number of sig_off() calls:def stack_sig_on():sig_on()sig_on()sig_on()# (some code)sig_off()sig_off()sig_off()Extra care must be taken with exceptions raised inside sig_on(). The problem is that, if you do not do anythingspecial, the sig_off() will never be called if there is an exception. If you need to raise an exception yourself, calla sig_off() before it:def raising_an_exception():sig_on()# (some long computation)if (something_failed):sig_off()raise RuntimeError("something failed")# (some more computation)sig_off()return somethingAlternatively, you can use try/finally which will also catch exceptions raised by subroutines inside the try:def try_finally_example():sig_on()try:# (some long computation, potentially raising exceptions)finally:sig_off()return somethingUsing sig_check()The function sig_check() behaves exactly as sig_on(); sig_off() (except that sig_check() is fastersince it does not involve a setjmp() call).sig_check() can be used to check for pending interrupts. If an interrupt happens outside of asig_on()/sig_off() block, it will be caught by the next sig_check() or sig_on().The typical use case for sig_check() is within tight loops doing complicated stuff (mixed Python and Cythoncode, potentially raising exceptions). It is safer to use and gives more control, because a KeyboardInterrupt canonly be raised during sig_check():44 Chapter 4. Writing Code for Sage
Sage Developer’s Guide, Release 6.1.1def sig_check_example():for x in foo:# (one loop iteration which does not take a long time)sig_check()Other SignalsApart from handling interrupts, sig_on() provides more general signal handling. It handles alarm() time-outsby raising an AlarmInterrupt (inherited from KeyboardInterrupt) exception.If the code inside sig_on() would generate a segmentation fault or call the C function abort() (or more generally,raise any of SIGSEGV, SIGILL, SIGABRT, SIGFPE, SIGBUS), this is caught by the interrupt framework and an exceptionis raised (RuntimeError for SIGABRT, FloatingPointError for SIGFPE and the custom exceptionSignalError, based on BaseException, otherwise):cdef extern from ’stdlib.h’:void abort()def abort_example():sig_on()abort()sig_off()sage: abort_example()Traceback (most recent call last):...RuntimeError: AbortedThis exception can then be caught as explained above. A segmentation fault or abort() unguarded by sig_on()would simply terminate Sage.Instead of sig_on(), there is also a function sig_str(s), which takes a C string s as argument. It behaves thesame as sig_on(), except that the string s will be used as a string for the exception. sig_str(s) should still beclosed by sig_off(). Example Cython code:cdef extern from ’stdlib.h’:void abort()def abort_example_with_sig_str():sig_str("custom error message")abort()sig_off()Executing this gives:sage: abort_example_with_sig_str()Traceback (most recent call last):...RuntimeError: custom error messageWith regard to ordinary interrupts (i.e. SIGINT), sig_str(s) behaves the same as sig_on(): a simpleKeyboardInterrupt is raised.Error Handling in C LibrariesSome C libraries can produce errors and use some sort of callback mechanism to report errors: an external errorhandling function needs to be set up which will be called by the C library if an error occurs.4.3. Coding in Cython 45
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