PIDFD_GETFD(2)



PIDFD_GETFD(2)             Linux Programmer's Manual            PIDFD_GETFD(2)

NAME
       pidfd_getfd - obtain a duplicate of another process's file descriptor

SYNOPSIS
       int pidfd_getfd(int pidfd, int targetfd, unsigned int flags);

DESCRIPTION
       The  pidfd_getfd()  system  call allocates a new file descriptor in the
       calling process.  This new file descriptor is a duplicate of an  exist-
       ing  file  descriptor,  targetfd, in the process referred to by the PID
       file descriptor pidfd.

       The duplicate file descriptor refers to the same open file  description
       (see  open(2))  as the original file descriptor in the process referred
       to by pidfd.  The two file descriptors thus share file status flags and
       file  offset.   Furthermore,  operations  on the underlying file object
       (for example, assigning an address to a socket  object  using  bind(2))
       can equally be performed via the duplicate file descriptor.

       The  close-on-exec  flag  (FD_CLOEXEC; see fcntl(2)) is set on the file
       descriptor returned by pidfd_getfd().

       The flags argument is reserved for future use.  Currently, it  must  be
       specified as 0.

       Permission  to  duplicate another process's file descriptor is governed
       by  a  ptrace  access  mode  PTRACE_MODE_ATTACH_REALCREDS  check   (see
       ptrace(2)).

RETURN VALUE
       On  success, pidfd_getfd() returns a file descriptor (a nonnegative in-
       teger).  On error, -1 is returned and errno  is  set  to  indicate  the
       cause of the error.

ERRORS
       EBADF  pidfd is not a valid PID file descriptor.

       EBADF  targetfd  is not an open file descriptor in the process referred
              to by pidfd.

       EINVAL flags is not 0.

       EMFILE The per-process limit on the number of open file descriptors has
              been  reached  (see  the  description  of RLIMIT_NOFILE in getr-
              limit(2)).

       ENFILE The system-wide limit on the total number of open files has been
              reached.

       EPERM  The  calling  process  did not have PTRACE_MODE_ATTACH_REALCREDS
              permissions (see ptrace(2)) over  the  process  referred  to  by
              pidfd.

       ESRCH  The  process  referred  to by pidfd does not exist (i.e., it has
              terminated and been waited on).

VERSIONS
       pidfd_getfd() first appeared in Linux 5.6.

CONFORMING TO
       pidfd_getfd() is Linux specific.

NOTES
       Currently, there is no glibc wrapper for this system call; call it  us-
       ing syscall(2).

       For a description of PID file descriptors, see pidfd_open(2).

       The  effect  of  pidfd_getfd() is similar to the use of SCM_RIGHTS mes-
       sages described in unix(7), but differs in the following respects:

       o In order to pass a file descriptor using an SCM_RIGHTS  message,  the
         two processes must first establish a UNIX domain socket connection.

       o The use of SCM_RIGHTS requires cooperation on the part of the process
         whose file descriptor is being copied.  By contrast, no such coopera-
         tion is necessary when using pidfd_getfd().

       o The  ability  to use pidfd_getfd() is restricted by a PTRACE_MODE_AT-
         TACH_REALCREDS ptrace  access  mode check.

SEE ALSO
       clone3(2), dup(2), kcmp(2), pidfd_open(2)

COLOPHON
       This page is part of release 5.07 of the Linux  man-pages  project.   A
       description  of  the project, information about reporting bugs, and the
       latest    version    of    this    page,    can     be     found     at
       https://www.kernel.org/doc/man-pages/.

Linux                             2020-03-31                    PIDFD_GETFD(2)

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