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PIDFD_GETFD(2) Linux Programmer's Manual PIDFD_GETFD(2)
NAME
pidfd_getfd - obtain a duplicate of another process's file descriptor
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).
VERSIONS
pidfd_getfd() first appeared in Linux 5.6.
CONFORMING TO
pidfd_getfd() is Linux specific.
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)
Linux 2021-03-22 PIDFD_GETFD(2)