pidfd_getfd(2) — Linux manual page


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

NAME         top

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

SYNOPSIS         top

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

DESCRIPTION         top

       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

       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

RETURN VALUE         top

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

ERRORS         top

       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

       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

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

VERSIONS         top

       pidfd_getfd() first appeared in Linux 5.6.

CONFORMING TO         top

       pidfd_getfd() is Linux specific.

NOTES         top

       Currently, there is no glibc wrapper for this system call; call it
       using 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
       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         top

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

COLOPHON         top

       This page is part of release 5.09 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

Linux                            2020-03-31                   PIDFD_GETFD(2)

Pages that refer to this page: dup2(2)dup(2)dup3(2)pidfd_open(2)syscalls(2)