从磁盘和EINTR读取

Question

is it necessary to check for errno == EINTR if you read massive amounts of data? I use the pread() function to read. In all my time I have never seen EINTR returned, but I have seen some code online where it is explicitely checks for it.

so really is it necessary to check for EINTR and maybe repeat the call?

Answer 1

EINTR is returned when as system call is interrupted as a result of your process receiving a signal. If your process was blocked in the kernel, waiting for the read to complete, and a signal is caught, this may wake the kernel; this depends on if the operation is interruptable. The sleeping I/O routine is woken and is expected to return EINTR to user-space.

Just before the kernel returns to user space, it checks for pending signals. If a signal is pending, it will take the action associated with that signal. Possible actions include: dispatching the signal to a signal handler, killing your process, or ignoring the signal. Assuming this does not kill your process and/or your signal handler returns normally, the system call will return EINTR.

If you were not expecting this, you typically want to try the action again, but this can also be used as a way to gracefully abort an I/O operation. For example, alarm(2) can be used to implement a timeout, where SIGALRM is delivered if the I/O does not complete in a timely manner. In your signal handler, you could set a flag indicating a timeout and when your read operation returns EINTR, you can check for your timeout flag.

Answer 2

The reason is - on a busy system, for example, it is possible to have an interrupt on the read. So, on your desktop you may never see it. On an overloaded server, you can.

Se Chapter 5 of Advanced Programming in the UNIX Environment - Stevens and Rago. There is a complete explanation.