Linux下的fd都是Pollable的吗?

weichuang02

例如，文件，socket，timer，pipe，mutex，semaphore，event，pthread，这些都是可以用来给poll或者epoll使用的吗?

还是说，有某些类型的fd不是Pollable的? 怎么区分呢?

谢谢。

linux_c_py_php

给你跟了一下内核代码, 调用链比较长, 给你个线索, 你可以自己探索:

fs/eventpoll.c中的:
SYSCALL_DEFINE4(epoll_ctl, int, epfd, int, op, int, fd,                                                                              
        struct epoll_event __user *, event)

有如下检查:
    /* Get the "struct file *" for the target file */
    tfile = fget(fd);
    if (!tfile)
        goto error_fput;

    /* The target file descriptor must support poll */
    error = -EPERM;
    if (!tfile->f_op || !tfile->f_op->poll)
        goto error_tgt_fput;

fget函数在fs/file_table.c中, 其中调用了:fcheck_files函数进行检查, 进一步到include/linux/fdtable.h , 自己跟.

井蛙夏虫

你看man文档，一般都有说明。

对于poll，posix标准明确支持的有(man -s 3p poll):

1. The poll() function shall support regular files, terminal and pseudo-terminal devices, FIFOs, pipes, sockets and  STREAMS-based files.  The behavior of poll() on elements of fds that refer to other types of file is unspecified.

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mutex，semaphore和pthread似乎与fd无关吧。

epoll: Linux-specific

timer(man timerfd_create): Linux-specific

1. Operating on a timer file descriptor
   
2. The file descriptor returned by timerfd_create() supports the following operations:
   
3. poll(2), select(2) (and similar)
   
4. The file descriptor also supports the other file-descriptor multiplexing APIs: pselect(2), ppoll(2), and epoll(7).

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event(man eventfd): Linux-specific

1. As its return value, eventfd() returns a new file descriptor that can be used to refer to the eventfd object.  The following operations can be performed on the file descriptor:
   
2. poll(2), select(2) (and similar)
   
3. The returned file descriptor supports poll(2) (and analogously epoll(7)) and select(2)
   
4. The eventfd file descriptor also supports the other file-descriptor multiplexing APIs: pselect(2), ppoll(2), and epoll(7)

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