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.
>
> > 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!
