signals/nonatomic_uint64.c

/* nonatomic_uint64.c

   On x86-32, stores and fetches of a 64-bit integer are not atomic. This
   can result in "memory tearing" on fetches and stores, where the memory
   access requires two bus transactions. Consequently, a fetch of an integer
   that is being modified on another CPU might receive half of the old value
   and half of the new value. Thus, for example, a 64-bit integer can not be
   safely used as the flag variable set by a signal handler and checked by
   the main program.

   This program demonstrates the behavior. The signal handler inverts a
   64-bit integer whose initial value is zero, so that the value constantly
   switches between all bits zero and all bits one. The main program
   repeatedly fetches a copy of that integer and prints out a message
   whenever the fetched value is not all zeros or all ones.

   Compile this program with -m32, and run it:

        $ cc -o nonatomic_uint64 -m32 nonatomic_uint64.c
        $ ./nonatomic_uint64
        Unexpected: ffffffff00000000 (loop 48533)
        Unexpected: ffffffff00000000 (loop 171488)
        Unexpected: 00000000ffffffff (loop 272321)
        Unexpected: 00000000ffffffff (loop 297708)

   On Ubuntu and Debian, you may need to install the package 'gcc-multilib'
   in order to build this program as described above.
*/
#include <inttypes.h>
#include <stdlib.h>
#include <stdio.h>
#include <unistd.h>
#include <signal.h>

#define errExit(msg)    do { perror(msg); exit(EXIT_FAILURE); } while (0)

#define SIG SIGUSR1

static volatile uint64_t ip = 0;  /* 'volatile' included here just to show
                                      that it really doesn't help. */

/* Signal handler inverts bits in '*ip'; this may not be atomic. */

static void
handler(int sig)
{
    ip = ~ip;
}

int
main(int argc, char *argv[])
{
    /* Set up signal handler. */

    struct sigaction sa;

    sa.sa_handler = handler;
    sa.sa_flags = 0;
    sigemptyset(&sa.sa_mask);
    if (sigaction(SIG, &sa, NULL) == -1)
        errExit("sigaction");

    /* Create child that will blast signals at parent. */

    pid_t childPid = fork();
    if (childPid == -1)
        errExit("fork");

    if (childPid == 0) {
        while (1) {
            if (kill(getppid(), SIG) == -1)
                errExit("kill() failed in child");
        }
    }

    /* Parent falls through */

    /* Loop, fetching the value of '*ip' (the fetch may not be atomic)
       and checking whether the result is all bits zero or all bits one.
       If it is not, print the fetched value and the loop counter. */

    for (int long long j = 0; ; j++) {
        uint64_t loc = ip;
        if (loc != 0 && loc != ~0)
            printf("Unexpected: %016" PRIx64 " (loop %lld)\n", loc, j);
    }

    exit(EXIT_SUCCESS);
}