/*************************************************************************\
*                  Copyright (C) Michael Kerrisk, 2026.                   *
*                                                                         *
* This program is free software. You may use, modify, and redistribute it *
* under the terms of the GNU General Public License as published by the   *
* Free Software Foundation, either version 3 or (at your option) any      *
* later version. This program is distributed without any warranty.  See   *
* the file COPYING.gpl-v3 for details.                                    *
\*************************************************************************/

/* t_sigaltstack.c

   Demonstrate the use of sigaltstack() to handle a signal on an alternate
   signal stack.
*/
#define _GNU_SOURCE             /* Get strsignal() declaration from <string.h> */
#include <string.h>
#include <signal.h>
#include "tlpi_hdr.h"

static void
sigsegvHandler(int sig)
{
    int x;

    /* UNSAFE: This handler uses non-async-signal-safe functions
       (printf(), strsignal(), fflush(); see Section 21.1.2) */

    printf("Caught signal %d (%s)\n", sig, strsignal(sig));
    printf("Top of handler stack near     %10p\n", (void *) &x);
    fflush(NULL);

    _exit(EXIT_FAILURE);                /* Can't return after SIGSEGV */
}

/* The following stops 'gcc -Wall' complaining that "control reaches
   end of non-void function" because we don't follow the call to
   overflowStack() stack in main() with a call to exit(). */

#ifdef __GNUC__
static void
overflowStack(int callNum) __attribute__ ((__noreturn__));
#endif

static void             /* A recursive function that overflows the stack */
overflowStack(int callNum)
{
    char a[100000];                     /* Make this stack frame large */

    printf("Call %4d - top of stack near %10p\n", callNum, &a[0]);
    overflowStack(callNum+1);
}

int
main(int argc, char *argv[])
{
    stack_t sigstack;
    struct sigaction sa;
    int j;

    printf("Top of standard stack is near %10p\n", (void *) &j);

    /* Allocate alternate stack and inform kernel of its existence */

    sigstack.ss_sp = malloc(SIGSTKSZ);
    if (sigstack.ss_sp == NULL)
        errExit("malloc");

    sigstack.ss_size = SIGSTKSZ;
    sigstack.ss_flags = 0;
    if (sigaltstack(&sigstack, NULL) == -1)
        errExit("sigaltstack");
    printf("Alternate stack is at         %10p-%p\n",
            sigstack.ss_sp, (char *) sbrk(0) - 1);

    sa.sa_handler = sigsegvHandler;     /* Establish handler for SIGSEGV */
    sigemptyset(&sa.sa_mask);
    sa.sa_flags = SA_ONSTACK;           /* Handler uses alternate stack */
    if (sigaction(SIGSEGV, &sa, NULL) == -1)
        errExit("sigaction");

    overflowStack(1);
}