NAME

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

LIBRARY

       Standard C library (libc, -lc)

SYNOPSIS

       #include <sys/syscall.h>      /* Definition of SYS_* constants */
       #include <unistd.h>

       int syscall(SYS_pidfd_getfd, int pidfd, int targetfd,
                   unsigned int flags);

       Note: glibc provides no wrapper for pidfd_getfd(), necessitating the use of syscall(2).

DESCRIPTION

       The  pidfd_getfd()  system  call  allocates  a new file descriptor 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)).

RETURN VALUE

       On success, pidfd_getfd() returns a file descriptor (a nonnegative integer).  On error, -1
       is returned and errno is set to indicate 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 getrlimit(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).

STANDARDS

       Linux.

HISTORY

       Linux 5.6.

NOTES

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

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

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

       •  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
          pidfd_getfd().

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

SEE ALSO

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