0.8.3.1

[sbcl/lichteblau.git] / src / runtime / interrupt.c

/*
 * interrupt-handling magic
 */

/*
 * This software is part of the SBCL system. See the README file for
 * more information.
 *
 * This software is derived from the CMU CL system, which was
 * written at Carnegie Mellon University and released into the
 * public domain. The software is in the public domain and is
 * provided with absolutely no warranty. See the COPYING and CREDITS
 * files for more information.
 */


/* As far as I can tell, what's going on here is:
 *
 * In the case of most signals, when Lisp asks us to handle the
 * signal, the outermost handler (the one actually passed to UNIX) is
 * either interrupt_handle_now(..) or maybe_now_maybe_later(..).
 * In that case, the Lisp-level handler is stored in interrupt_handlers[..]
 * and interrupt_low_level_handlers[..] is cleared.
 *
 * However, some signals need special handling, e.g. 
 *
 * o the SIGSEGV (for e.g. Linux) or SIGBUS (for e.g. FreeBSD) used by the
 *   garbage collector to detect violations of write protection,
 *   because some cases of such signals (e.g. GC-related violations of
 *   write protection) are handled at C level and never passed on to
 *   Lisp. For such signals, we still store any Lisp-level handler
 *   in interrupt_handlers[..], but for the outermost handle we use
 *   the value from interrupt_low_level_handlers[..], instead of the
 *   ordinary interrupt_handle_now(..) or interrupt_handle_later(..).
 *
 * o the SIGTRAP (Linux/Alpha) which Lisp code uses to handle breakpoints,
 *   pseudo-atomic sections, and some classes of error (e.g. "function
 *   not defined").  This never goes anywhere near the Lisp handlers at all.
 *   See runtime/alpha-arch.c and code/signal.lisp 
 * 
 * - WHN 20000728, dan 20010128 */


#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <signal.h>

#include "runtime.h"
#include "arch.h"
#include "sbcl.h"
#include "os.h"
#include "interrupt.h"
#include "globals.h"
#include "lispregs.h"
#include "validate.h"
#include "monitor.h"
#include "gc.h"
#include "alloc.h"
#include "dynbind.h"
#include "interr.h"
#include "genesis/fdefn.h"
#include "genesis/simple-fun.h"

void run_deferred_handler(struct interrupt_data *data, void *v_context) ;
static void store_signal_data_for_later (struct interrupt_data *data, 
                                         void *handler, int signal,
                                         siginfo_t *info, 
                                         os_context_t *context);
boolean interrupt_maybe_gc_int(int signal, siginfo_t *info, void *v_context);

extern lispobj all_threads_lock;
extern int countdown_to_gc;

/*
 * This is a workaround for some slightly silly Linux/GNU Libc
 * behaviour: glibc defines sigset_t to support 1024 signals, which is
 * more than the kernel.  This is usually not a problem, but becomes
 * one when we want to save a signal mask from a ucontext, and restore
 * it later into another ucontext: the ucontext is allocated on the
 * stack by the kernel, so copying a libc-sized sigset_t into it will
 * overflow and cause other data on the stack to be corrupted */

#define REAL_SIGSET_SIZE_BYTES ((NSIG/8))

void sigaddset_blockable(sigset_t *s)
{
    sigaddset(s, SIGHUP);
    sigaddset(s, SIGINT);
    sigaddset(s, SIGQUIT);
    sigaddset(s, SIGPIPE);
    sigaddset(s, SIGALRM);
    sigaddset(s, SIGURG);
    sigaddset(s, SIGFPE);
    sigaddset(s, SIGTSTP);
    sigaddset(s, SIGCHLD);
    sigaddset(s, SIGIO);
    sigaddset(s, SIGXCPU);
    sigaddset(s, SIGXFSZ);
    sigaddset(s, SIGVTALRM);
    sigaddset(s, SIGPROF);
    sigaddset(s, SIGWINCH);
    sigaddset(s, SIGUSR1);
    sigaddset(s, SIGUSR2);
#ifdef LISP_FEATURE_SB_THREAD
    /* don't block STOP_FOR_GC, we need to be able to interrupt threads
     * for GC purposes even when they are blocked on queues etc */
    sigaddset(s, SIG_INTERRUPT_THREAD);
#endif
}

/* When we catch an internal error, should we pass it back to Lisp to
 * be handled in a high-level way? (Early in cold init, the answer is
 * 'no', because Lisp is still too brain-dead to handle anything.
 * After sufficient initialization has been completed, the answer
 * becomes 'yes'.) */
boolean internal_errors_enabled = 0;

struct interrupt_data * global_interrupt_data;

\f
/*
 * utility routines used by various signal handlers
 */

void 
build_fake_control_stack_frames(struct thread *th,os_context_t *context)
{
#ifndef LISP_FEATURE_X86
    
    lispobj oldcont;

    /* Build a fake stack frame or frames */

    current_control_frame_pointer =
        (lispobj *)(*os_context_register_addr(context, reg_CSP));
    if ((lispobj *)(*os_context_register_addr(context, reg_CFP))
        == current_control_frame_pointer) {
        /* There is a small window during call where the callee's
         * frame isn't built yet. */
        if (lowtag_of(*os_context_register_addr(context, reg_CODE))
            == FUN_POINTER_LOWTAG) {
            /* We have called, but not built the new frame, so
             * build it for them. */
            current_control_frame_pointer[0] =
                *os_context_register_addr(context, reg_OCFP);
            current_control_frame_pointer[1] =
                *os_context_register_addr(context, reg_LRA);
            current_control_frame_pointer += 8;
            /* Build our frame on top of it. */
            oldcont = (lispobj)(*os_context_register_addr(context, reg_CFP));
        }
        else {
            /* We haven't yet called, build our frame as if the
             * partial frame wasn't there. */
            oldcont = (lispobj)(*os_context_register_addr(context, reg_OCFP));
        }
    }
    /* We can't tell whether we are still in the caller if it had to
     * allocate a stack frame due to stack arguments. */
    /* This observation provoked some past CMUCL maintainer to ask
     * "Can anything strange happen during return?" */
    else {
        /* normal case */
        oldcont = (lispobj)(*os_context_register_addr(context, reg_CFP));
    }

    current_control_stack_pointer = current_control_frame_pointer + 8;

    current_control_frame_pointer[0] = oldcont;
    current_control_frame_pointer[1] = NIL;
    current_control_frame_pointer[2] =
        (lispobj)(*os_context_register_addr(context, reg_CODE));
#endif
}

void
fake_foreign_function_call(os_context_t *context)
{
    int context_index;
    struct thread *thread=arch_os_get_current_thread();

    /* Get current Lisp state from context. */
#ifdef reg_ALLOC
    dynamic_space_free_pointer =
        (lispobj *)(*os_context_register_addr(context, reg_ALLOC));
#ifdef alpha
    if ((long)dynamic_space_free_pointer & 1) {
        lose("dead in fake_foreign_function_call, context = %x", context);
    }
#endif
#endif
#ifdef reg_BSP
    current_binding_stack_pointer =
        (lispobj *)(*os_context_register_addr(context, reg_BSP));
#endif

    build_fake_control_stack_frames(thread,context);

    /* Do dynamic binding of the active interrupt context index
     * and save the context in the context array. */
    context_index =
        fixnum_value(SymbolValue(FREE_INTERRUPT_CONTEXT_INDEX,thread));
    
    if (context_index >= MAX_INTERRUPTS) {
        lose("maximum interrupt nesting depth (%d) exceeded", MAX_INTERRUPTS);
    }

    bind_variable(FREE_INTERRUPT_CONTEXT_INDEX,
                  make_fixnum(context_index + 1),thread);

    thread->interrupt_contexts[context_index] = context;

    /* no longer in Lisp now */
    foreign_function_call_active = 1;
}

/* blocks all blockable signals.  If you are calling from a signal handler,
 * the usual signal mask will be restored from the context when the handler 
 * finishes.  Otherwise, be careful */

void
undo_fake_foreign_function_call(os_context_t *context)
{
    struct thread *thread=arch_os_get_current_thread();
    /* Block all blockable signals. */
    sigset_t block;
    sigemptyset(&block);
    sigaddset_blockable(&block);
    sigprocmask(SIG_BLOCK, &block, 0);

    /* going back into Lisp */
    foreign_function_call_active = 0;

    /* Undo dynamic binding of FREE_INTERRUPT_CONTEXT_INDEX */
    unbind(thread);

#ifdef reg_ALLOC
    /* Put the dynamic space free pointer back into the context. */
    *os_context_register_addr(context, reg_ALLOC) =
        (unsigned long) dynamic_space_free_pointer;
#endif
}

/* a handler for the signal caused by execution of a trap opcode
 * signalling an internal error */
void
interrupt_internal_error(int signal, siginfo_t *info, os_context_t *context,
                         boolean continuable)
{
    lispobj context_sap = 0;

    fake_foreign_function_call(context);

    /* Allocate the SAP object while the interrupts are still
     * disabled. */
    if (internal_errors_enabled) {
        context_sap = alloc_sap(context);
    }

    sigprocmask(SIG_SETMASK, os_context_sigmask_addr(context), 0);

    if (internal_errors_enabled) {
        SHOW("in interrupt_internal_error");
#if QSHOW
        /* Display some rudimentary debugging information about the
         * error, so that even if the Lisp error handler gets badly
         * confused, we have a chance to determine what's going on. */
        describe_internal_error(context);
#endif
        funcall2(SymbolFunction(INTERNAL_ERROR), context_sap,
                 continuable ? T : NIL);
    } else {
        describe_internal_error(context);
        /* There's no good way to recover from an internal error
         * before the Lisp error handling mechanism is set up. */
        lose("internal error too early in init, can't recover");
    }
    undo_fake_foreign_function_call(context);
    if (continuable) {
        arch_skip_instruction(context);
    }
}

void
interrupt_handle_pending(os_context_t *context)
{
    struct thread *thread;
    struct interrupt_data *data;

    thread=arch_os_get_current_thread();
    data=thread->interrupt_data;
    SetSymbolValue(INTERRUPT_PENDING, NIL,thread);

    /* restore the saved signal mask from the original signal (the
     * one that interrupted us during the critical section) into the
     * os_context for the signal we're currently in the handler for.
     * This should ensure that when we return from the handler the
     * blocked signals are unblocked */

    memcpy(os_context_sigmask_addr(context), &data->pending_mask, 
           REAL_SIGSET_SIZE_BYTES);

    sigemptyset(&data->pending_mask);
    /* This will break on sparc linux: the deferred handler really wants
     * to be called with a void_context */
    run_deferred_handler(data,(void *)context); 
}
\f
/*
 * the two main signal handlers:
 *   interrupt_handle_now(..)
 *   maybe_now_maybe_later(..)
 *
 * to which we have added interrupt_handle_now_handler(..).  Why?
 * Well, mostly because the SPARC/Linux platform doesn't quite do
 * signals the way we want them done.  The third argument in the
 * handler isn't filled in by the kernel properly, so we fix it up
 * ourselves in the arch_os_get_context(..) function; however, we only
 * want to do this when we first hit the handler, and not when
 * interrupt_handle_now(..) is being called from some other handler
 * (when the fixup will already have been done). -- CSR, 2002-07-23
 */

void
interrupt_handle_now(int signal, siginfo_t *info, void *void_context)
{
    os_context_t *context = (os_context_t*)void_context;
    struct thread *thread=arch_os_get_current_thread();
#ifndef __i386__
    boolean were_in_lisp;
#endif
    union interrupt_handler handler;

#ifdef LISP_FEATURE_LINUX
    /* Under Linux on some architectures, we appear to have to restore
       the FPU control word from the context, as after the signal is
       delivered we appear to have a null FPU control word. */
    os_restore_fp_control(context);
#endif 
    handler = thread->interrupt_data->interrupt_handlers[signal];

    if (ARE_SAME_HANDLER(handler.c, SIG_IGN)) {
        return;
    }
    
#ifndef __i386__
    were_in_lisp = !foreign_function_call_active;
    if (were_in_lisp)
#endif
    {
        fake_foreign_function_call(context);
    }

#ifdef QSHOW_SIGNALS
    FSHOW((stderr,
           "/entering interrupt_handle_now(%d, info, context)\n",
           signal));
#endif

    if (ARE_SAME_HANDLER(handler.c, SIG_DFL)) {

        /* This can happen if someone tries to ignore or default one
         * of the signals we need for runtime support, and the runtime
         * support decides to pass on it. */
        lose("no handler for signal %d in interrupt_handle_now(..)", signal);

    } else if (lowtag_of(handler.lisp) == FUN_POINTER_LOWTAG) {
        /* Once we've decided what to do about contexts in a 
         * return-elsewhere world (the original context will no longer
         * be available; should we copy it or was nobody using it anyway?)
         * then we should convert this to return-elsewhere */

        /* CMUCL comment said "Allocate the SAPs while the interrupts
         * are still disabled.".  I (dan, 2003.08.21) assume this is 
         * because we're not in pseudoatomic and allocation shouldn't
         * be interrupted.  In which case it's no longer an issue as
         * all our allocation from C now goes through a PA wrapper,
         * but still, doesn't hurt */

        lispobj info_sap,context_sap = alloc_sap(context);
        info_sap = alloc_sap(info);
        /* Allow signals again. */
        sigprocmask(SIG_SETMASK, os_context_sigmask_addr(context), 0);

#ifdef QSHOW_SIGNALS
        SHOW("calling Lisp-level handler");
#endif

        funcall3(handler.lisp,
                 make_fixnum(signal),
                 info_sap,
                 context_sap);
    } else {

#ifdef QSHOW_SIGNALS
        SHOW("calling C-level handler");
#endif

        /* Allow signals again. */
        sigprocmask(SIG_SETMASK, os_context_sigmask_addr(context), 0);
        
        (*handler.c)(signal, info, void_context);
    }

#ifndef __i386__
    if (were_in_lisp)
#endif
    {
        undo_fake_foreign_function_call(context);
    }

#ifdef QSHOW_SIGNALS
    FSHOW((stderr,
           "/returning from interrupt_handle_now(%d, info, context)\n",
           signal));
#endif
}

/* This is called at the end of a critical section if the indications
 * are that some signal was deferred during the section.  Note that as
 * far as C or the kernel is concerned we dealt with the signal
 * already; we're just doing the Lisp-level processing now that we
 * put off then */

void
run_deferred_handler(struct interrupt_data *data, void *v_context) {
    (*(data->pending_handler))
        (data->pending_signal,&(data->pending_info), v_context);
}

boolean
maybe_defer_handler(void *handler, struct interrupt_data *data,
                    int signal, siginfo_t *info, os_context_t *context)
{
    struct thread *thread=arch_os_get_current_thread();
    if (SymbolValue(INTERRUPTS_ENABLED,thread) == NIL) {
        store_signal_data_for_later(data,handler,signal,info,context);
        SetSymbolValue(INTERRUPT_PENDING, T,thread);
        return 1;
    } 
    /* a slightly confusing test.  arch_pseudo_atomic_atomic() doesn't
     * actually use its argument for anything on x86, so this branch
     * may succeed even when context is null (gencgc alloc()) */
    if (
#ifndef __i386__
        (!foreign_function_call_active) &&
#endif
        arch_pseudo_atomic_atomic(context)) {
        store_signal_data_for_later(data,handler,signal,info,context);
        arch_set_pseudo_atomic_interrupted(context);
        return 1;
    }
    return 0;
}
static void
store_signal_data_for_later (struct interrupt_data *data, void *handler,
                             int signal, 
                             siginfo_t *info, os_context_t *context)
{
    data->pending_handler = handler;
    data->pending_signal = signal;
    if(info)
        memcpy(&(data->pending_info), info, sizeof(siginfo_t));
    if(context) {
        /* the signal mask in the context (from before we were
         * interrupted) is copied to be restored when
         * run_deferred_handler happens.  Then the usually-blocked
         * signals are added to the mask in the context so that we are
         * running with blocked signals when the handler returns */
        sigemptyset(&(data->pending_mask));
        memcpy(&(data->pending_mask),
               os_context_sigmask_addr(context),
               REAL_SIGSET_SIZE_BYTES);
        sigaddset_blockable(os_context_sigmask_addr(context));
    } else {
        /* this is also called from gencgc alloc(), in which case
         * there has been no signal and is therefore no context. */
        sigset_t new;
        sigemptyset(&new);
        sigaddset_blockable(&new);
        sigprocmask(SIG_BLOCK,&new,&(data->pending_mask));
    }
}


static void
maybe_now_maybe_later(int signal, siginfo_t *info, void *void_context)
{
    os_context_t *context = arch_os_get_context(&void_context);
    struct thread *thread=arch_os_get_current_thread();
    struct interrupt_data *data=thread->interrupt_data;
#ifdef LISP_FEATURE_LINUX
    os_restore_fp_control(context);
#endif 
    if(maybe_defer_handler(interrupt_handle_now,data,
                           signal,info,context))
        return;
    interrupt_handle_now(signal, info, context);
}

void
sig_stop_for_gc_handler(int signal, siginfo_t *info, void *void_context)
{
    os_context_t *context = arch_os_get_context(&void_context);
    struct thread *thread=arch_os_get_current_thread();
    struct interrupt_data *data=thread->interrupt_data;
    sigset_t block;

    if(maybe_defer_handler(sig_stop_for_gc_handler,data,
                           signal,info,context)){
        return;
    }
    sigemptyset(&block);
    sigaddset_blockable(&block);
    sigprocmask(SIG_BLOCK, &block, 0);
    get_spinlock(&all_threads_lock,thread->pid);
    countdown_to_gc--;
    release_spinlock(&all_threads_lock);
    /* need the context stored so it can have registers scavenged */
    fake_foreign_function_call(context); 
    kill(getpid(),SIGSTOP);
    undo_fake_foreign_function_call(context);
}

void
interrupt_handle_now_handler(int signal, siginfo_t *info, void *void_context)
{
    os_context_t *context = arch_os_get_context(&void_context);
    interrupt_handle_now(signal, info, context);
}

/*
 * stuff to detect and handle hitting the GC trigger
 */

#ifndef LISP_FEATURE_GENCGC 
/* since GENCGC has its own way to record trigger */
static boolean
gc_trigger_hit(int signal, siginfo_t *info, os_context_t *context)
{
    if (current_auto_gc_trigger == NULL)
        return 0;
    else{
        void *badaddr=arch_get_bad_addr(signal,info,context);
        return (badaddr >= (void *)current_auto_gc_trigger &&
                badaddr <((void *)current_dynamic_space + DYNAMIC_SPACE_SIZE));
    }
}
#endif

/* manipulate the signal context and stack such that when the handler
 * returns, it will call function instead of whatever it was doing
 * previously
 */

extern lispobj call_into_lisp(lispobj fun, lispobj *args, int nargs);
extern void post_signal_tramp(void);
void arrange_return_to_lisp_function(os_context_t *context, lispobj function)
{
    void * fun=native_pointer(function);
    char *code = &(((struct simple_fun *) fun)->code);
    
    /* Build a stack frame showing `interrupted' so that the
     * user's backtrace makes (as much) sense (as usual) */
#ifdef LISP_FEATURE_X86
    /* Suppose the existence of some function that saved all
     * registers, called call_into_lisp, then restored GP registers and
     * returned.  We shortcut this: fake the stack that call_into_lisp
     * would see, then arrange to have it called directly.  post_signal_tramp
     * is the second half of this function
     */
    u32 *sp=(u32 *)*os_context_register_addr(context,reg_ESP);

    *(sp-14) = post_signal_tramp; /* return address for call_into_lisp */
    *(sp-13) = function;        /* args for call_into_lisp : function*/
    *(sp-12) = 0;               /*                           arg array */
    *(sp-11) = 0;               /*                           no. args */
    /* this order matches that used in POPAD */
    *(sp-10)=*os_context_register_addr(context,reg_EDI);
    *(sp-9)=*os_context_register_addr(context,reg_ESI);
    /* this gets overwritten again before it's used, anyway */
    *(sp-8)=*os_context_register_addr(context,reg_EBP);
    *(sp-7)=0 ; /* POPAD doesn't set ESP, but expects a gap for it anyway */
    *(sp-6)=*os_context_register_addr(context,reg_EBX);

    *(sp-5)=*os_context_register_addr(context,reg_EDX);
    *(sp-4)=*os_context_register_addr(context,reg_ECX);
    *(sp-3)=*os_context_register_addr(context,reg_EAX);
    *(sp-2)=*os_context_register_addr(context,reg_EBP);
    *(sp-1)=*os_context_pc_addr(context);

#else 
    struct thread *th=arch_os_get_current_thread();
    build_fake_control_stack_frames(th,context);
#endif

#ifdef LISP_FEATURE_X86
    *os_context_pc_addr(context) = call_into_lisp;
    *os_context_register_addr(context,reg_ECX) = 0; 
    *os_context_register_addr(context,reg_EBP) = sp-2;
    *os_context_register_addr(context,reg_ESP) = sp-14;
#else
    /* this much of the calling convention is common to all
       non-x86 ports */
    *os_context_pc_addr(context) = code;
    *os_context_register_addr(context,reg_NARGS) = 0; 
    *os_context_register_addr(context,reg_LIP) = code;
    *os_context_register_addr(context,reg_CFP) = 
        current_control_frame_pointer;
#endif
#ifdef ARCH_HAS_NPC_REGISTER
    *os_context_npc_addr(context) =
        4 + *os_context_pc_addr(context);
#endif
#ifdef LISP_FEATURE_SPARC
    *os_context_register_addr(context,reg_CODE) = 
        fun + FUN_POINTER_LOWTAG;
#endif
}

#ifdef LISP_FEATURE_SB_THREAD
void handle_rt_signal(int num, siginfo_t *info, void *v_context)
{
    os_context_t *context = (os_context_t*)arch_os_get_context(&v_context);
    struct thread *th=arch_os_get_current_thread();
    struct interrupt_data *data=
        th ? th->interrupt_data : global_interrupt_data;
    if(maybe_defer_handler(handle_rt_signal,data,num,info,context)){
        return ;
    }
    arrange_return_to_lisp_function(context,info->si_value.sival_int);
}
#endif

boolean handle_control_stack_guard_triggered(os_context_t *context,void *addr){
    struct thread *th=arch_os_get_current_thread();
    /* note the os_context hackery here.  When the signal handler returns, 
     * it won't go back to what it was doing ... */
    if(addr>=(void *)CONTROL_STACK_GUARD_PAGE(th) && 
       addr<(void *)(CONTROL_STACK_GUARD_PAGE(th)+os_vm_page_size)) {
        /* we hit the end of the control stack.  disable protection
         * temporarily so the error handler has some headroom */
        protect_control_stack_guard_page(th->pid,0L);
        
        arrange_return_to_lisp_function
            (context, SymbolFunction(CONTROL_STACK_EXHAUSTED_ERROR));
        return 1;
    }
    else return 0;
}

#ifndef LISP_FEATURE_GENCGC
/* This function gets called from the SIGSEGV (for e.g. Linux or
 * OpenBSD) or SIGBUS (for e.g. FreeBSD) handler. Here we check
 * whether the signal was due to treading on the mprotect()ed zone -
 * and if so, arrange for a GC to happen. */
extern unsigned long bytes_consed_between_gcs; /* gc-common.c */

boolean
interrupt_maybe_gc(int signal, siginfo_t *info, void *void_context)
{
    os_context_t *context=(os_context_t *) void_context;
    struct thread *th=arch_os_get_current_thread();
    struct interrupt_data *data=
        th ? th->interrupt_data : global_interrupt_data;

    if(!foreign_function_call_active && gc_trigger_hit(signal, info, context)){
        clear_auto_gc_trigger();
        if(!maybe_defer_handler
           (interrupt_maybe_gc_int,data,signal,info,void_context))
            interrupt_maybe_gc_int(signal,info,void_context);
        return 1;
    }
    return 0;
}

#endif

/* this is also used by from gencgc.c alloc() */
boolean
interrupt_maybe_gc_int(int signal, siginfo_t *info, void *void_context)
{
    os_context_t *context=(os_context_t *) void_context;
    fake_foreign_function_call(context);
    /* SUB-GC may return without GCing if *GC-INHIBIT* is set, in
     * which case we will be running with no gc trigger barrier
     * thing for a while.  But it shouldn't be long until the end
     * of WITHOUT-GCING. */
    funcall0(SymbolFunction(SUB_GC));
    undo_fake_foreign_function_call(context);
    return 1;
}

\f
/*
 * noise to install handlers
 */

void
undoably_install_low_level_interrupt_handler (int signal,
                                              void handler(int,
                                                           siginfo_t*,
                                                           void*))
{
    struct sigaction sa;
    struct thread *th=arch_os_get_current_thread();
    struct interrupt_data *data=
        th ? th->interrupt_data : global_interrupt_data;

    if (0 > signal || signal >= NSIG) {
        lose("bad signal number %d", signal);
    }

    sa.sa_sigaction = handler;
    sigemptyset(&sa.sa_mask);
    sigaddset_blockable(&sa.sa_mask);
    sa.sa_flags = SA_SIGINFO | SA_RESTART;
#ifdef LISP_FEATURE_C_STACK_IS_CONTROL_STACK
    if((signal==SIG_MEMORY_FAULT) 
#ifdef SIG_INTERRUPT_THREAD
       || (signal==SIG_INTERRUPT_THREAD)
#endif
       )
        sa.sa_flags|= SA_ONSTACK;
#endif
    
    sigaction(signal, &sa, NULL);
    data->interrupt_low_level_handlers[signal] =
        (ARE_SAME_HANDLER(handler, SIG_DFL) ? 0 : handler);
}

/* This is called from Lisp. */
unsigned long
install_handler(int signal, void handler(int, siginfo_t*, void*))
{
    struct sigaction sa;
    sigset_t old, new;
    union interrupt_handler oldhandler;
    struct thread *th=arch_os_get_current_thread();
    struct interrupt_data *data=
        th ? th->interrupt_data : global_interrupt_data;

    FSHOW((stderr, "/entering POSIX install_handler(%d, ..)\n", signal));

    sigemptyset(&new);
    sigaddset(&new, signal);
    sigprocmask(SIG_BLOCK, &new, &old);

    sigemptyset(&new);
    sigaddset_blockable(&new);

    FSHOW((stderr, "/interrupt_low_level_handlers[signal]=%d\n",
           interrupt_low_level_handlers[signal]));
    if (data->interrupt_low_level_handlers[signal]==0) {
        if (ARE_SAME_HANDLER(handler, SIG_DFL) ||
            ARE_SAME_HANDLER(handler, SIG_IGN)) {
            sa.sa_sigaction = handler;
        } else if (sigismember(&new, signal)) {
            sa.sa_sigaction = maybe_now_maybe_later;
        } else {
            sa.sa_sigaction = interrupt_handle_now_handler;
        }

        sigemptyset(&sa.sa_mask);
        sigaddset_blockable(&sa.sa_mask);
        sa.sa_flags = SA_SIGINFO | SA_RESTART;
        sigaction(signal, &sa, NULL);
    }

    oldhandler = data->interrupt_handlers[signal];
    data->interrupt_handlers[signal].c = handler;

    sigprocmask(SIG_SETMASK, &old, 0);

    FSHOW((stderr, "/leaving POSIX install_handler(%d, ..)\n", signal));

    return (unsigned long)oldhandler.lisp;
}

void
interrupt_init()
{
    int i;
    SHOW("entering interrupt_init()");
    global_interrupt_data=calloc(sizeof(struct interrupt_data), 1);

    /* Set up high level handler information. */
    for (i = 0; i < NSIG; i++) {
        global_interrupt_data->interrupt_handlers[i].c =
            /* (The cast here blasts away the distinction between
             * SA_SIGACTION-style three-argument handlers and
             * signal(..)-style one-argument handlers, which is OK
             * because it works to call the 1-argument form where the
             * 3-argument form is expected.) */
            (void (*)(int, siginfo_t*, void*))SIG_DFL;
    }

    SHOW("returning from interrupt_init()");
}