Re: how do I check that an FD is open?

[Paolo Bonzini]

Sam Steingold wrote:
> How do I figure out if the fd (specifically, stdin=0) is open?
> apparently it is closed when the application is run by nohup.
> the only thing I could figure out so far is fstat: when 0 is open,
> st_mode is 8592, when it is closed it is 8630...

Given that 8592 & 0777 == 0620, while 8630 & 0777 == 0666, but neither
case returns EBADF, I would guess that nohup actually opens /dev/null to
0 instead...  and in fact, "strace nohup ls" gives:

open("/dev/null", O_WRONLY|O_LARGEFILE) = 3
dup2(3, 0)                              = 0
close(3)                                = 0
umask(037777777177)                     = 022
open("nohup.out", O_WRONLY|O_CREAT|O_APPEND|O_LARGEFILE, 0600) = 3
dup2(3, 1)                              = 1
close(3)                                = 0
umask(022)                              = 0177


If that's what you want, you could check the major/minor device number
in the result of fstat.  However:

1) I think it's not portable to Windows (except Cygwin), though there
you do not have to worry about nohup.

2) It would not distinguish nohup from "./foo < /dev/null" of course.

Paolo