ioctl如何知道在linux中调用哪个函数？

SYSCALL_DEFINE3(ioctl,unsigned int,fd,cmd,unsigned long,arg)
{
    struct file *filp;
    int error = -EBADF;
    int fput_needed;

    filp = fget_light(fd,&fput_needed);
    if (!filp)
            goto out;

    error = security_file_ioctl(filp,arg);
    if (error)
            goto out_fput;

    error = do_vfs_ioctl(filp,arg);
 out_fput:
    fput_light(filp,fput_needed);
 out:
    return error;
}

请注意,已经有一个文件描述符fd关联.内核然后调用fget_light()来查找一个filp(大致是文件指针,但是不要把它与标准的IO FILE *文件指针混淆).对security_file_ioctl()的调用检查加载的安全模块是否允许ioctl(无论是按照AppArmor和TOMOYO中的名称,还是按照SMACK和SELinux中的标签),以及用户是否具有正确的功能(功能(7))进行通话.如果调用被允许,那么调用do_vfs_ioctl()来处理常见的ioctl本身：

switch (cmd) {
    case FIOCLEX:
            set_close_on_exec(fd,1);
            break;
    /* ... */

如果没有一个常见的情况是正确的,那么内核调用一个helper例程：

static long vfs_ioctl(struct file *filp,unsigned int cmd,unsigned long arg)
{
    int error = -ENOTTY;

    if (!filp->f_op || !filp->f_op->unlocked_ioctl)
            goto out;

    error = filp->f_op->unlocked_ioctl(filp,arg);
    if (error == -ENOIOCTLCMD)
            error = -EINVAL;
 out:
    return error;
}

驱动程序提供自己的.unlocked_ioctl函数指针,像fs / pipe.c中的这个管道实现：

const struct file_operations rdwr_pipefifo_fops = {
    .llseek         = no_llseek,.read           = do_sync_read,.aio_read       = pipe_read,.write          = do_sync_write,.aio_write      = pipe_write,.poll           = pipe_poll,.unlocked_ioctl = pipe_ioctl,.open           = pipe_rdwr_open,.release        = pipe_rdwr_release,.fasync         = pipe_rdwr_fasync,};