How is GNU `yes` so fast?

Re:How is GNU `yes` so fast? by wolfgang42 on Tuesday June 13, 2017 @10:18AM (#14544839)

When I started this comment, I didn't think you were measuring what you thought you were measuring with those straces. 'strace yes > test2' only watches 'yes', not '> test2' (which is handled by the shell). Here's what your command outputs:

    $ strace yes > /tmp/test2
    [various initialization steps]
    write(1, "y\ny\ny\ny\ny\ny\ny\ny\ny\ny\ny\ny\ny\ny\ny\ny\n"..., 8192) = 8192
    write(1, "y\ny\ny\ny\ny\ny\ny\ny\ny\ny\ny\ny\ny\ny\ny\ny\n"..., 8192) = 8192
    write(1, "y\ny\ny\ny\ny\ny\ny\ny\ny\ny\ny\ny\ny\ny\ny\ny\n"..., 8192) = 8192
    write(1, "y\ny\ny\ny\ny\ny\ny\ny\ny\ny\ny\ny\ny\ny\ny\ny\n"..., 8192) = 8192
    [...]

To measure everything, I started up a whole new shell, expecting to see a 'write(1, ...)', a 'read(1)', and a 'write(f, ...)':

    $ strace -f sh -c 'yes > /tmp/test2'
    [various initialization steps]
    [pid 15839] write(1, "y\ny\ny\ny\ny\ny\ny\ny\ny\ny\ny\ny\ny\ny\ny\ny\n"..., 8192) = 8192
    [pid 15839] write(1, "y\ny\ny\ny\ny\ny\ny\ny\ny\ny\ny\ny\ny\ny\ny\ny\n"..., 8192) = 8192
    [pid 15839] write(1, "y\ny\ny\ny\ny\ny\ny\ny\ny\ny\ny\ny\ny\ny\ny\ny\n"..., 8192) = 8192
    [pid 15839] write(1, "y\ny\ny\ny\ny\ny\ny\ny\ny\ny\ny\ny\ny\ny\ny\ny\n"..., 8192) = 8192
    [...]

How does this possibly work? File descriptor 1 is supposed to be the terminal, not a file! Of course, the magic of file redirection:

    open("/tmp/test2", O_WRONLY|O_CREAT|O_TRUNC, 0666) = 3 # Open the file, get FD 3
    fcntl(1, F_DUPFD, 10)                   = 10 # Copy FD 1 (the terminal, STDOUT) to FD 10 temporarily
    close(1)                                = 0  # Close original FD 1
    fcntl(10, F_SETFD, FD_CLOEXEC)          = 0  # Close FD 10 (the terminal, copy of STDOUT) when exec() is called
    dup2(3, 1)                              = 1  # Copy FD 3 (the file) to FD 1 (STDOUT)
    close(3)                                = 0  # Close FD 3 (the file's original descrptor)

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Re:How is GNU `yes` so fast? by kccqzy on Tuesday June 13, 2017 @02:41PM (#14547410)

I had the impression that you thought "yes" pipes the output to shell, and then the shell writes to disk. That's incorrect. " > file" means redirecting to a file, and therefore all write system calls actually write to disk.

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Re:How is GNU `yes` so fast? by wolfgang42 on Tuesday June 13, 2017 @03:22PM (#14547781)

That's exactly what I thought. Now, had you asked me how redirection worked I would have said "the redirection operators cause the shell to attach a file to the descriptor," but I'd never actually thought through the implications of that in terms of what syscalls get made, and the output of strace presented a rather visceral demonstration of the implications of this clever bit of design.

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