Linux Device Drivers, Second Edition

poll and select

Applications that use nonblocking I/O often use the poll and select system calls as well. poll and select have essentially the same functionality: both allow a process to determine whether it can read from or write to one or more open files without blocking. They are thus often used in applications that must use multiple input or output streams without blocking on any one of them. The same functionality is offered by two separate functions because they were implemented in Unix almost at the same time by two different groups: select was introduced in BSD Unix, whereas poll was the System V solution.

Support for either system call requires support from the device driver to function. In version 2.0 of the kernel the device method was modeled on select (and no poll was available to user programs); from version 2.1.23 onward both were offered, and the device method was based on the newly introduced poll system call because poll offered more detailed control than select.

Implementations of the poll method, implementing both the poll and select system calls, have the following prototype:

 unsigned int (*poll) (struct file *, poll_table *);

The driver’s method will be called whenever the user-space program performs a poll or select system call involving a file descriptor associated with the driver. The device method is in charge of these two steps:

1. Call poll_wait on one or more wait queues that could indicate a change in the poll status.

2. Return a bit mask describing ...