Re: XCAR/XCDR as lvalues

Hrvoje Niksic wrote: > > > (In fact, on *real* Unixes, we could simply use sigaction() to trap > > > SIGSEGV and have the handler check whether the SEGV was caused by > > > modifying a pure cons, and signal an error in that case. Of course, > > > this would not work on Linux.) > > > > Why? > > Because I am unable to make this trivial program (or a variation > thereof) to compile and run on Linux: [snip] Try the following instead. #include <stdio.h> #include <stdlib.h> #include <signal.h> #include <sigcontext.h> void segv_handler(int sig, struct sigcontext ctx) { printf ("Fault address: %p\n", ctx.cr2); exit (0); } int main (void) { struct sigaction act; sigaction (SIGSEGV, NULL, &act); act.sa_handler = segv_handler; sigemptyset (&act.sa_mask); act.sa_flags = 0; sigaction (SIGSEGV, &act, NULL); *(char *)0xdeadbeef = 'x'; return 0; /* unreached */ } Clearly it isn't portable (the sigcontext structure is processor-specific), but then I'm not sure that there is any portable method. Stevens[1] indicates that SA_SIGINFO and struct siginfo are specific to SVR4, and that BSD 4.3+ signal handlers have the prototype: