Question

我正在创建访问设备驱动程序的用户程序。多个线程必须打开设备？（每个线程都有一个设备本身的文件描述符）。或者可以从主体打开，所有线程都获得文件描述符的副本？

访问设备的原子性会发生什么变化？

Answer 1

Linux中作为进程的线程是通过系统调用clone创建的。来自documentation：

 If CLONE_FILES is set, the calling process and the child
          process share the same file descriptor table.  Any file
          descriptor created by the calling process or by the child
          process is also valid in the other process.  Similarly, if one
          of the processes closes a file descriptor, or changes its
          associated flags (using the fcntl(2) F_SETFD operation), the
          other process is also affected.

          If CLONE_FILES is not set, the child process inherits a copy
          of all file descriptors opened in the calling process at the
          time of clone().  (The duplicated file descriptors in the
          child refer to the same open file descriptions (see open(2))
          as the corresponding file descriptors in the calling process.)
          Subsequent operations that open or close file descriptors, or
          change file descriptor flags, performed by either the calling
          process or the child process do not affect the other process.

假设您正在使用库来创建 pthread 之类的线程。使用pthread_create创建的线程与进程中的所有其他线程（不仅仅是父线程）共享文件描述符。这不能改变。使用fork创建的进程获取文件描述符的副本。您必须注意共享文件描述符和拥有副本是两回事。如果您有一个副本（例如，使用fork创建），则必须先关闭所有副本，然后才能关闭文件处理程序。如果文件描述符在不同的线程之间共享，一旦一个线程关闭它，该文件描述符将为所有线程关闭。同样，如果其中一个进程更改了其关联的标志（使用fcntl），则另一个进程也会受到影响。

多线程用户程序（或多个进程）访问设备的正确方法如何？

1 个答案: