If you replace your call segfault.segfault() with os.kill(os.getpid(),
signal.SIGSEGV) everything works as expected. The problem is that the
signal is just caught in the c handler (and a flag is set) and then the
program continues with the offending *c = 'a'; statement, which again
ends with a SIGSEGV and the handler being called.
The documentation explicitly states "Because the C signal handler always
returns, it makes little sense to catch synchronous errors like SIGFPE
or SIGSEGV."
I think raising that InvalidArgument exception is most probably the
right thing to do (or at least printing a warning). Those that really
want to handle SIGSEGV should do it from C anyway.