SIGSEGV experiments

sigsegv-experiments.c

#include <fcntl.h>
#include <signal.h>
#include <stdio.h>
#include <sys/mman.h>
#include <sys/time.h>
#include <unistd.h>

static char *buffer1;
static char *buffer2;
static const int page_size = 1 << 12;

static void handler (const int sig, siginfo_t *const si, void *const unused) {
  mprotect (si->si_addr, page_size, PROT_READ | PROT_WRITE);
  *(char *)(si->si_addr) = 1;
}

typedef struct timings {
  struct timeval start;
  struct timeval mid;
  struct timeval end;
} timings;

static double time_difference (
  struct timeval *const start,
  struct timeval *const end
) {
  return (end->tv_sec + end->tv_usec * 1e-6) - (start->tv_sec + start->tv_usec * 1e-6);
}

static const char *SHARED_OBJECT_NAME = "this_is_my_shared_page";

int main () {
  struct sigaction sa;
  sa.sa_flags = SA_SIGINFO;
  sigemptyset (&sa.sa_mask);
  sa.sa_sigaction = handler;
  sigaction (SIGBUS, &sa, NULL); // OS X
  sigaction (SIGSEGV, &sa, NULL); // Linux
  int sanity = 0;
  enum { SAMPLES = 10000 };
  timings times[SAMPLES];
  for (int i = 0; i < SAMPLES; ++i) {
    gettimeofday (&times [i].start, NULL);
    int buffer_fd;
    buffer_fd = shm_open (SHARED_OBJECT_NAME, O_RDWR | O_CREAT, 0644);
    // shm_unlink (SHARED_OBJECT_NAME);
    ftruncate (buffer_fd, page_size);
    buffer1 = mmap (
      NULL,
      page_size,
      PROT_READ | PROT_WRITE,
      MAP_SHARED,
      buffer_fd,
      (off_t)0
    );
    buffer2 = mmap (
      NULL,
      page_size,
      PROT_NONE,
      MAP_SHARED,
      buffer_fd,
      (off_t)0
    );
    gettimeofday (&times [i].mid, NULL);
    buffer1 [0] = 0;
    sanity += buffer1 [0];
    sanity += buffer2 [0];
    munmap (buffer1, page_size);
    munmap (buffer2, page_size);
    close (buffer_fd);
    gettimeofday (&times [i].end, NULL);
  }
  double total_a = 0.0;
  double total_b = 0.0;
  double total = 0.0;
  for (int i = 0; i < SAMPLES; ++i) {
    struct timeval start_time = times [i].start;
    struct timeval mid_time = times [i].mid;
    struct timeval end_time = times [i].end;
    double a = time_difference (&times [i].start, &times [i].mid);
    double b = time_difference (&times [i].mid, &times [i].end);
    total_a += a;
    total_b += b;
    total += a + b;
  }
  printf (
    "Legend: A = allocate, B = use & free\n"
    "Sanity: %d = %d\n"
    "Totals:\n"
    "A: %f s\n"
    "B: %f s\n"
    "A+B: %f s\n"
    "Means:\n"
    "A: %f μs\n"
    "B: %f μs\n"
    "A+B: %f μs\n",
    SAMPLES,
    sanity,
    total_a, total_b, total,
    total_a / SAMPLES * 1e6, total_b / SAMPLES * 1e6, total / SAMPLES * 1e6
  );
  return 0;
}