# pidfd_open

## NAME



           pidfd_open - obtain a file descriptor that refers to a process
    


## LIBRARY



           Standard C library (_libc_ , _-lc_)
    


## SYNOPSIS



           **#include <sys/syscall.h>      **/* Definition of **SYS_*** constants */
           **#include <unistd.h>**
    
           **int syscall(SYS_pidfd_open, pid_t** _pid_**, unsigned int** _flags_**);**
    
           _Note_ : glibc provides no wrapper for **pidfd_open**(), necessitating
           the use of syscall(2).
    


## DESCRIPTION



           The **pidfd_open**() system call creates a file descriptor that refers
           to 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 either has the value 0, or contains the
           following flag:
    
           **PIDFD_NONBLOCK**(since Linux 5.10)
                  Return a nonblocking file descriptor.  If the process
                  referred to by the file descriptor has not yet terminated,
                  then an attempt to wait on the file descriptor using
                  waitid(2) will immediately return the error **EAGAIN** rather
                  than blocking.
    


## RETURN VALUE



           On success, **pidfd_open**() returns a file descriptor (a nonnegative
           integer).  On error, -1 is returned and _errno_ is set to indicate
           the error.
    


## ERRORS



           **EINVAL** _flags_ is not valid.
    
           **EINVAL** _pid_ is not valid.
    
           **EMFILE** The per-process limit on the number of open file
                  descriptors has been reached (see the description of
                  **RLIMIT_NOFILE** in getrlimit(2)).
    
           **ENFILE** The system-wide limit on the total number of open files has
                  been reached.
    
           **ENODEV** The anonymous inode filesystem is not available in this
                  kernel.
    
           **ENOMEM** Insufficient kernel memory was available.
    
           **ESRCH** The process specified by _pid_ does not exist.
    


## STANDARDS



           Linux.
    


## NOTES



           The following code sequence can be used to obtain a file
           descriptor for the child of fork(2):
    
               pid = fork();
               if (pid > 0) {     /* If parent */
                   pidfd = pidfd_open(pid, 0);
                   ...
               }
    
           Even if the child has already terminated by the time of the
           **pidfd_open**() call, its PID will not have been recycled and the
           returned file descriptor will refer to the resulting zombie
           process.  Note, however, that this is guaranteed only if the
           following conditions hold true:
    
           •  the disposition of **SIGCHLD** has not been explicitly set to
              **SIG_IGN**(see sigaction(2));
    
           •  the **SA_NOCLDWAIT** flag was not specified while establishing a
              handler for **SIGCHLD** or while setting the disposition of that
              signal to **SIG_DFL**(see sigaction(2)); and
    
           •  the zombie process was not reaped elsewhere in the program
              (e.g., either by an asynchronously executed signal handler or
              by wait(2) or similar in another thread).
    
           If any of these conditions does not hold, then the child process
           (along with a PID file descriptor that refers to it) should
           instead be created using clone(2) with the **CLONE_PIDFD** flag.
    
       **Use cases for PID file descriptors**
           A PID file descriptor returned by **pidfd_open**() (or by clone(2)
           with the **CLONE_PID** flag) can be used for the following purposes:
    
           •  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, these interfaces indicate the file descriptor as
              readable.  Note, however, that in the current implementation,
              nothing can be read from the file descriptor (read(2) on the
              file descriptor fails with the error **EINVAL**).
    
           •  If the PID file descriptor refers to a child of the calling
              process, then it can be waited on using waitid(2).
    
           •  The pidfd_getfd(2) system call can be used to obtain a
              duplicate of a file descriptor of another process referred to
              by a PID file descriptor.
    
           •  A PID file descriptor can be used as the argument of setns(2)
              in order to move into one or more of the same namespaces as the
              process referred to by the file descriptor.
    
           •  A PID file descriptor can be used as the argument of
              process_madvise(2) in order to provide advice on the memory
              usage patterns of the process referred to by the file
              descriptor.
    
           The **pidfd_open**() system call is the preferred way of obtaining a
           PID file descriptor for an already existing process.  The
           alternative is to obtain a file descriptor by opening a _/proc/_ pid
           directory.  However, the latter technique is possible only if the
           proc(5) filesystem is mounted; furthermore, the file descriptor
           obtained in this way is _not_ pollable and can't be waited on with
           waitid(2).
    


## EXAMPLES



           The program below opens a PID file descriptor for the process
           whose PID is specified as its command-line argument.  It then uses
           poll(2) to monitor the file descriptor for process exit, as
           indicated by an **EPOLLIN** event.
    
       **Program source**
    
           #define _GNU_SOURCE
           #include <poll.h>
           #include <stdio.h>
           #include <stdlib.h>
           #include <sys/syscall.h>
           #include <sys/types.h>
           #include <unistd.h>
    
           static int
           pidfd_open(pid_t pid, unsigned int flags)
           {
               return syscall(SYS_pidfd_open, pid, flags);
           }
    
           int
           main(int argc, char *argv[])
           {
               int            pidfd, ready;
               struct pollfd  pollfd;
    
               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 (%#x): POLLIN is %sset\n", pollfd.revents,
                      (pollfd.revents & POLLIN) ? "" : "not ");
    
               close(pidfd);
               exit(EXIT_SUCCESS);
           }
    


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