On Mon, Sep 23, 2019 at 11:11:53AM +0200, Michael Kerrisk (man-pages) wrote: > Hello Christian and all, > > Below, I have the rendered version of the current draft of > the pidfd_open(2) manual page that I have written. > The page source can be found in a Git branch at: > https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/log/?h=draft_pidfd > > I would be pleased to receive corrections and notes on any > details that should be added. (For example, are there error > cases that I have missed?) > > Would you be able to review please? Again, thank you Michael for doing this! > > Thanks, > > Michael > > > NAME > pidfd_open - obtain a file descriptor that refers to a process > > SYNOPSIS > int pidfd_open(pid_t pid, unsigned int flags); > > DESCRIPTION > The pidfd_open() system creates a file descriptor that refers to s/system/system call/ > the process whose PID is specified in pid. The file descriptor is > returned as the function result; the close-on-exec flag is set on > the file descriptor. > > The flags argument is reserved for future use; currently, this > argument must be specified as 0. > > RETURN VALUE > On success, pidfd_open() returns a nonnegative file descriptor. > On success, -1 is returned and errno is set to indicate the cause s/On success/On error/g > of the error. > > ERRORS > EINVAL flags is not 0. > > EINVAL pid is not valid. > > ESRCH The process specified by pid does not exist. > > VERSIONS > pidfd_open() first appeared in Linux 5.3. > > CONFORMING TO > pidfd_open() is Linux specific. > > NOTES > Currently, there is no glibc wrapper for this system call; call it > using syscall(2). > > The pidfd_send_signal(2) system call can be used to send a signal > to the process referred to by a PID file descriptor. > > 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 Not a native English speaker but should this be "indicates it is readable"? > current implementation, nothing can be read from the file descrip‐ > tor. > > 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. I mentioned this already in the CLONE_PIDFD manpage, we should probably not make a big deal out of this and not mention /proc/<pid> here at all. (Crazy idea, but we could also have a config option that allows you to turn of proc-pid-dirfds as pidfds if we start to feel really strongly about this or a sysctl whatever...) > > See also the discussion of the CLONE_PIDFD flag in clone(2). > > EXAMPLE > The program below opens a PID file descriptor for the process > whose PID is specified as its command-line argument. It then mon‐ > itors the file descriptor for readability (POLLIN) using poll(2). Yeah, maybe say "monitors the file descriptor for process exit indicated by an EPOLLIN event" or something. Readability might be confusing. > When the process with the specified by PID terminates, poll(2) > returns, and indicates that the file descriptor is readable. See comment above "readable". (I'm on my phone and I think someone pointed this out already.) > > Program source > > #define _GNU_SOURCE > #include <sys/syscall.h> > #include <unistd.h> > #include <poll.h> > #include <stdlib.h> > #include <stdio.h> > > #ifndef __NR_pidfd_open > #define __NR_pidfd_open 434 > #endif Alpha is special... (and not in a good way). So you would need to special case Alpha since that's the only arch where we haven't been able to unify syscall numbering. :D But it's not super important. I like the program example. > > static > int pidfd_open(pid_t pid, unsigned int flags) > { > return syscall(__NR_pidfd_open, pid, flags); > } > > int > main(int argc, char *argv[]) > { > struct pollfd pollfd; > int pidfd, ready; > > if (argc != 2) { > fprintf(stderr, "Usage: %s <pid>\n", argv[0]); > exit(EXIT_SUCCESS); > } > > pidfd = pidfd_open(atoi(argv[1]), 0); > if (pidfd == -1) { > perror("pidfd_open"); > exit(EXIT_FAILURE); > } > > pollfd.fd = pidfd; > pollfd.events = POLLIN; > > ready = poll(&pollfd, 1, -1); > if (ready == -1) { > perror("poll"); > exit(EXIT_FAILURE); > } > > printf("Events (0x%x): POLLIN is %sset\n", pollfd.revents, > (pollfd.revents & POLLIN) ? "" : "not "); > > exit(EXIT_SUCCESS); > } > > SEE ALSO > clone(2), kill(2), pidfd_send_signal(2), poll(2), select(2), > epoll(7) > > > -- > Michael Kerrisk > Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/ > Linux/UNIX System Programming Training: http://man7.org/training/
