Hello Christian,
On 9/23/19 4:47 PM, Christian Brauner wrote:
> On Mon, Sep 23, 2019 at 12:53:09PM +0200, Florian Weimer wrote:
>> * Michael Kerrisk:
>>
>>> SYNOPSIS
>>> int pidfd_open(pid_t pid, unsigned int flags);
>>
>> Should this mention <sys/types.h> for pid_t?
>>
>>> ERRORS
>>> EINVAL flags is not 0.
>>>
>>> EINVAL pid is not valid.
>>>
>>> ESRCH The process specified by pid does not exist.
>>
>> Presumably, EMFILE and ENFILE are also possible errors, and so is
>> ENOMEM.
>
> So, error codes that could surface are:
> EMFILE: too many open files
> ENODEV: the anon inode filesystem is not available in this kernel (unlikely)
> ENOMEM: not enough memory (to allocate the backing struct file)
> ENFILE: you're over the max_files limit which can be set through proc
>
> I think that should be it.
Thanks. I've added those.
>>> A PID file descriptor can be monitored using poll(2), select(2),
>>> and epoll(7). When the process that it refers to terminates, the
>>> file descriptor indicates as readable. Note, however, that in the
>>> current implementation, nothing can be read from the file descrip‐
>>> tor.
>>
>> “is indicated as readable” or “becomes readable”? Will reading block?
>>
>>> The pidfd_open() system call is the preferred way of obtaining a
>>> PID file descriptor. The alternative is to obtain a file descrip‐
>>> tor by opening a /proc/[pid] directory. However, the latter tech‐
>>> nique is possible only if the proc(5) file system is mounted; fur‐
>>> thermore, the file descriptor obtained in this way is not pol‐
>>> lable.
>>
>> One question is whether the glibc wrapper should fall back back to the
>> /proc subdirectory if it is not available. Probably not.
>
> No, that would not be transparent to userspace. Especially because both
> fds differ in what can be done with them.
>
>>
>>> static
>>> int pidfd_open(pid_t pid, unsigned int flags)
>>> {
>>> return syscall(__NR_pidfd_open, pid, flags);
>>> }
>>
>> Please call this function something else (not pidfd_open), so that the
>> example continues to work if glibc provides the system call wrapper.
>
> Agreed!
See my reply to Florian. (So far, I didn't change anything here.)
Thanks,
Michael
--
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/
