PIDFD_GETFD(2)            (2020-03-31)             PIDFD_GETFD(2)

          pidfd_getfd - obtain a duplicate of another process's file

          int pidfd_getfd(int pidfd, int targetfd, unsigned int flags

          The pidfd_getfd() system call allocates a new file descrip-
          tor in the calling process.  This new file descriptor is a
          duplicate of an existing 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)).

          On success, pidfd_getfd() returns a file descriptor (a non-
          negative integer).  On error, -1 is returned and errno is
          set to indicate the cause of the error.

               pidfd is not a valid PID file descriptor.

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

               flags is not 0.

               The per-process limit on the number of open file
               descriptors has been reached (see the description of

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     PIDFD_GETFD(2)            (2020-03-31)             PIDFD_GETFD(2)

               RLIMIT_NOFILE in getrlimit(2)).

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

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

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

          pidfd_getfd() first appeared in Linux 5.6.

          pidfd_getfd() is Linux specific.

          Currently, there is no glibc wrapper for this system call;
          call it using syscall(2).

          For a description of PID file descriptors, see

          The effect of pidfd_getfd() is similar to the use of
          SCM_RIGHTS messages 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 cooperation is necessary when using

          +o The ability to use pidfd_getfd() is restricted by a
            PTRACE_MODE_ATTACH_REALCREDS ptrace  access  mode check.

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

          This page is part of release 5.10 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

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