Hi Emanuele, On 2023-08-25 01:25, Emanuele Torre wrote: > Hi. > > Today, I was reading pidfd_send_signal(2), and I was suprised to see it > mentioning that you can get a PID file descriptor by opening a /proc/pid > directory. > > NOTES > PID file descriptors > The pidfd argument is a PID file descriptor, a file descriptor > that refers to process. Such a file descriptor can be obtained > in any of the following ways: > . by opening a /proc/pid directory; > . using pidfd_open(2); or > . via the PID file descriptor that is returned by a call to > clone(2) or clone3(2) that specifies the CLONE_PIDFD flag. > > Unfortunately, if you open /proc/123, you don't get a pidfd for the > process with pid 123; as expected, you will just get a directory file > descriptor for /proc/123. > > And that directory file descriptor won't be usable as a PID file > descriptor either. > (openpidfd, and pidfdgetfd are just simple wrappers for pidfd_open, and > pidfd_getfd) > > $ pidfdgetfd 9 0 0 echo hello 9</proc/1584616 > pidfd_getfd: Bad file descriptor > $ # you must use pidfd_open > $ openpidfd 9 1584616 pidfdgetfd 9 0 0 echo hello > hello > > I also wrote a test program that uses a /proc/pid directory file > descriptor as pidfd for waitid(P_PID) and that also didnd't work > (waitid fails with EINVAL). > > But those directory file descriptors do work as alternative to actual > pidfds for pidfd_send_signal(2) specifically. > > I think the documentation should be changed to say that > pidfd_send_signal accepts either a PID file descriptor (obtainable using > pidfd_open or CLONE_PIDFD), or, alternatively, a file descriptor for a > /proc/pid directory to avoid confusions. Would you mind sending a patch? If you do, please include in the commit message the source code _and_ the execution of all of the programs you mentioned above. Thanks, Alex > > Thank you. > > o/ > emanuele6 -- <http://www.alejandro-colomar.es/> GPG key fingerprint: A9348594CE31283A826FBDD8D57633D441E25BB5
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