test-vt.c

#include <assert.h>
#include <fcntl.h>
#include <stdbool.h>
#include <stdint.h>
#include <stdlib.h>
#include <stdio.h>
#include <termios.h>
#include <unistd.h>
#include <sys/ioctl.h>
#include <sys/timerfd.h>
#include <sys/wait.h>

#include <linux/vt.h>

static inline void err(char *str)
{ perror(str); exit(1); }

static inline void errif(bool condition, char *str)
{ if (condition) err(str); }


// signal handlers for VT switching

enum
{ FB_ACTIVE
, FB_RELEASE_REQUESTED
, FB_INACTIVE
, FB_ACQUIRED
} fb_state = FB_INACTIVE;

void release_vt(int)
{ assert (FB_ACQUIRED == fb_state | FB_ACTIVE == fb_state);
  fb_state = FB_RELEASE_REQUESTED;
}

void acquire_vt(int)
{ assert (FB_INACTIVE == fb_state);
  fb_state = FB_ACQUIRED;
}


int main(int argc, char **argv)
{
  { // go fork yourself
    struct termios termios;
    errif (-1 == tcgetattr(0, &termios), "tcgetattr");
    pid_t pid = fork();
    errif (-1 == pid, "fork");
    if (pid)
    { int wstatus;
      signal(SIGINT, SIG_IGN);
      waitpid(pid, &wstatus, 0);
      // cleanup
      tcsetattr(0, TCSAFLUSH, &termios);
      write(1, "\e[?25h", 6);
      ioctl(0, VT_SETMODE, &(struct vt_mode){ VT_AUTO });
      if (WIFSIGNALED(wstatus))
        raise (WTERMSIG(wstatus));
      return WIFEXITED(wstatus) ? WEXITSTATUS(wstatus) : -1;
    }
  }

  { // keep the terminal off the framebuffer
    // kernel doesn't clean up after KDSETMODE, so avoid it here
    struct termios termios;
    errif (-1 == tcgetattr(0, &termios), "tcgetattr");
    termios.c_lflag &= ~(ECHO | ICANON);
    errif (-1 == tcsetattr(0, TCSANOW, &termios), "tcsetattr");
    write(1, "\e[?25l", 6);
  }

  int timer;
  { // limit 60 frames per second
    timer = timerfd_create(CLOCK_MONOTONIC, 0);
    errif (-1 == timer, "timerfd_create");
    struct timespec timespec = { 0, 1000000000 / 60 };
    struct itimerspec itimerspec = { timespec, timespec };
    errif (-1 == timerfd_settime(timer, 0, &itimerspec, 0), "timerfd_settime");
  }

  { // set up VT switching
    struct vt_mode vt_mode =
    { .mode = VT_PROCESS
    , .relsig = SIGUSR1
    , .acqsig = SIGUSR2
    };
    errif(SIG_ERR == signal(SIGUSR1, release_vt), "signal");
    errif(SIG_ERR == signal(SIGUSR2, acquire_vt), "signal");
    fb_state = FB_ACQUIRED;
    errif(-1 == ioctl(0, VT_SETMODE, &vt_mode), "ioctl vt");
  }

  fcntl(0, F_SETFL, O_NONBLOCK);

  for (;;)
  {
    switch (fb_state)
    { case FB_ACQUIRED:
        // redraw screen
        fb_state = FB_ACTIVE;
        printf("active\n");
        break;
      case FB_RELEASE_REQUESTED:
        errif(-1 == ioctl(0, VT_RELDISP, 1), "ioctl vt");
        fb_state = FB_INACTIVE;
        printf("inactive\n");
        break;
    }

    { char buffer;
      ssize_t n = read(0, &buffer, 1);
      if (1 == n) break;
    }

    { // delay
      uint64_t buffer;
      errif(-1 == read(timer, &buffer, sizeof(buffer)), "read");
    }
  }

  return 0;
}