select(), EOF...

(sorry if this is a dupe, I haven't seen it come from the list, so I'm resending as plain ASCII in case majordomo kills messages with strange stuff)

Hi,

I need to do something similar to tail -f.
I was hoping that select() or poll() would block my process after reaching EOF but (as the man says) EOF doesn't cause read() to block so select() and poll() both say I can read. The result is (obviously) that my program waits actively and uses all the CPU.
What's the right way of doing this? I assume the kernel provides facilities to find out if there is new data to read (other than EOF).

Thanks.

Quoted 9 lines

(sorry if this is a dupe, I haven't seen it come from the list, so I'm resending as plain ASCII in case majordomo kills messages with strange stuff)

Hi,

I need to do something similar to tail -f.
I was hoping that select() or poll() would block my process after

Quoted 1 line

EOF but (as the man says) EOF doesn't cause read() to block so select()

Quoted 1 line

poll() both say I can read. The result is (obviously) that my program

Quoted 2 lines

actively and uses all the CPU.
What's the right way of doing this? I assume the kernel provides

Quoted 3 lines

to find out if there is new data to read (other than EOF).

Thanks.

tail -f just alternates between open() and close(), keeping in memory the current byte offest into the file.

--
Ignacio Vazquez-Abrams <[email protected]>

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How come the process is never runnable unless there's new data in the file? If it was opening and closing the file continously it would be using lots of CPU, while it's 0 if there's no data coming.

Quoted 2 lines

How come the process is never runnable unless there's new data in the

Quoted 1 line

If it was opening and closing the file continously it would be using

Quoted 1 line

CPU, while it's 0 if there's no data coming.

It sleep()s between close() and open().

--
Ignacio Vazquez-Abrams <[email protected]>

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a strace shows it works differently

nanosleep({1, 0}, {1, 0}) = 0
fstat(3, {st_mode=S_IFREG|0600, st_size=227128, ...}) = 0 rt_sigprocmask(SIG_BLOCK, [CHLD], [RT_0], 8) = 0 rt_sigaction(SIGCHLD, NULL, {SIG_DFL}, 8) = 0 rt_sigprocmask(SIG_SETMASK, [RT_0], NULL, 8) = 0

the file is opened just once (as I expected), and tail sleeps in nanosleep () until the file grows. I think strace isn't showing more nanosleep() as it should be looping there. BTW what's the reason for the sigprocmask, etc?