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SubjectRe: For review: pidfd_open(2) manual page
On Mon, Sep 23, 2019 at 3:53 AM Florian Weimer <fw@deneb.enyo.de> 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.
>
> > 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.

The phrase "becomes readable" is simpler than "indicates as readable"
and conveys the same meaning. I agree with Florian's comment on this
point below.

> > 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.

Referring to procfs directory FDs as pidfds will probably confuse
people. I'd just omit this paragraph.

> One question is whether the glibc wrapper should fall back back to the
> /proc subdirectory if it is not available. Probably not.

I'd prefer that glibc not provide this kind of fallback.
posix_fallocate-style emulation is, IMHO, too surprising.

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