How to use "sigaltstack" in signal handler program?

Question

Do anyone know how to use the sigaltstack in a real signal handler program? A simple but complete code may be great help!

Answer 1

Here is a minimal sample program that uses sigaltstack to catch infinite recursion. If you comment out the sigaltstack call or SA_ONSTACK flag, the signal handler will not be able to run because it has no stack left and the program will just crash.

#define _XOPEN_SOURCE 700
#include <signal.h>
#include <unistd.h>
void handler(int sig)
{
    write(2, "stack overflow\n", 15);
    _exit(1);
}
unsigned infinite_recursion(unsigned x) {
    return infinite_recursion(x)+1;
}
int main()
{
    static char stack[SIGSTKSZ];
    stack_t ss = {
        .ss_size = SIGSTKSZ,
        .ss_sp = stack,
    };
    struct sigaction sa = {
        .sa_handler = handler,
        .sa_flags = SA_ONSTACK
    };
    sigaltstack(&ss, 0);
    sigfillset(&sa.sa_mask);
    sigaction(SIGSEGV, &sa, 0);
    infinite_recursion(0);
}

A more sophisticated use might actually perform siglongjmp to jump out of the signal handler and back to a point where the infinite recursion can be avoided. This is not valid if async-signal-unsafe library calls are being used, or if your data might be left in an unsafe/unrecoverable state, but if you're performing pure arithmetic computations, it may be valid.

Perhaps a better task for the signal handler would be performing an emergency dump of any valuable/critical data that wasn't already saved to disk. This could be difficult if you can't call async-signal-unsafe functions, but it's usually possible if you put some effort into it.

Answer 2

#define _GNU_SOURCE
#include <stdio.h>
#include <signal.h>
#include <unistd.h>
#include <stdlib.h>
#include <string.h>

static void handler(int signo)
{
    int x;
    if(signo == SIGSEGV)
    {
        printf("Waoh, caught signal %s\n",strsignal(signo));
        printf("Top of stack is near %10p", (void*)&x);
    }
    _exit(EXIT_FAILURE);
}

static void overflowStack(int i)
{
    char a[8964];
    printf("(%d) Called overflow function. The top of stack is near %10p\n",i ,&a[0]);
    overflowStack(i+1);
}

int main(int argc, char *argv[])
{


    /*(1)specify that the signal handler will be allocated onto the
    alternate signal stack*/
    stack_t sigstack;
    //malloc return the pointer to the allocated memory on success
    //malloc return NULL on error
    sigstack.ss_sp = malloc(SIGSTKSZ);
    if( sigstack.ss_sp == NULL)
    {
        printf("Err: malloc error\n");
        exit(EXIT_FAILURE);
    }
    sigstack.ss_size = SIGSTKSZ;
    sigstack.ss_flags = 0;

    /*(2)Specify that the signal handler will be allocated on the alternate 
    signal stack */
    if(sigaltstack(&sigstack, NULL) == -1)
    {
        printf("Err: sigaltstack error\n");
        exit(EXIT_FAILURE);
    }
    // sbrk() change the location of the program break, which defines the end of the process's data segment 
    //On success, sbrk() returns the previous program break. 
    printf("Now the alternate signal stack is successfully allocated\n");
    printf("The address of signal stack is : %10p - %10p\n",sigstack.ss_sp,(char*)sbrk(0)-1);

    /*(3)define a struct sigaction to deal with the SIGSEGV*/
    struct sigaction act;
    act.sa_flags = SA_ONSTACK;
    sigemptyset(&act.sa_mask);
    act.sa_handler = handler;
    sigaction(SIGSEGV, &act, NULL);

    overflowStack(1);

}