同步调用可以通过编程来实现，但带有超时设置，如果调用在一定的时间内未返回，调用Web Services可能取消锁定，从而从系统退出。

Synchronous calls can be programmed, but with timeouts so that if the call can't come back in a certain time, the calling Web service can be unlocked to exit from the system.

youdao

步骤二：数据从内核空间缓冲区复制到用户空间缓冲区，之后系统调用read返回，这导致了从内核空间向用户空间的上下文切换。

Step two: data is copied from the kernel buffer into the user buffer, and the read system call returns. The return from the call caused a context switch from kernel back to user mode.

youdao

从系统调用中返回后，最终执行syscall_exit，并调用resume_userspace返回用户空间。

Upon return from the system call, syscall_exit is eventually reached, and a call to resume_userspace transitions back to user-space.

youdao