chvt_exec.c

#include <errno.h>
#include <fcntl.h>
#include <linux/limits.h> // PATH_MAX
#include <linux/vt.h> // VT_*
#include <stdio.h>
#include <stdlib.h>
#include <string.h> // strerror
#include <sys/ioctl.h>
#include <sys/stat.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <termios.h> // TIO*
#include <unistd.h>

void die(const char* message)
{
    fprintf(stderr, "%s - %s\n", message, strerror(errno));
    exit(1);
}

int main(int argc, char* argv[])
{
    if(argc < 2)
    {
        printf("Usage: chvt_exec <command> [args...]");
        return 0;
    }

    int fd = open("/dev/console", O_RDWR);
    if(fd < 0) die("Failed to open console");

    struct vt_stat st;
    if(ioctl(fd, VT_GETSTATE, &st) < 0) die("Failed to get active vt");

    int vt = -1;
    if(ioctl(fd, VT_OPENQRY, &vt) < 0) die("Failed to get free vt");

    char vt_name[PATH_MAX + 1];
    snprintf(vt_name, PATH_MAX, "/dev/tty%d", vt);

    printf("Found free vt: %s\n", vt_name);

    pid_t pid = fork();
    if(pid == 0)
    {
        // child
        if(setsid() < 0) die("Failed to create new session");

        int fd_c = open(vt_name, O_RDWR);
        if(fd_c < 0) die("Failed to open new vt");

        if(ioctl(fd_c, TIOCSCTTY, 1) < 0) die("Failed to set controlling terminal");

        if(ioctl(fd_c, VT_ACTIVATE, vt) < 0) die("Failed to activate new vt");
        if(ioctl(fd_c, VT_WAITACTIVE, vt) < 0) die("Failed to wait for new vt to become active");

        if(dup2(fd_c, 0) < 0 || dup2(fd_c, 1) < 0 || dup2(fd_c, 2) < 0)
            die("Failed to redirect stdin, stdout or stderr to new vt");

        close(fd);
        execvp(argv[1], &argv[1]);

        exit(1);
    }
    else if(pid < 0) die("Failed to fork child process");

    // parent
    int status = 0;

    printf("Waiting for child process\n");
    wait(NULL);
    waitpid(pid, &status, 0);

    if(ioctl(fd, VT_ACTIVATE, st.v_active) < 0) die("Failed to activate old vt");
    if(ioctl(fd, VT_WAITACTIVE, st.v_active) < 0) die("Failed to wait for old vt to become active");

    if(ioctl(fd, VT_DISALLOCATE, vt) < 0) die("Failed to deallocate new vt");

    close(fd);
    return status;
}