pidfd_getfd(2) System Calls Manual pidfd_getfd(2)
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 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 er-
ror.
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).
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 mes-
sages 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_AT-
TACH_REALCREDS ptrace access mode check.
SEE ALSO
clone3(2), dup(2), kcmp(2), pidfd_open(2)
Linux man-pages 6.7 2023-10-31 pidfd_getfd(2)
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