Manpages - pidfd_getfd.2

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

  #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).

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)).

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

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).

*pidfd_getfd*() first appeared in Linux 5.6.

*pidfd_getfd*() is Linux specific.

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.

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

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