proc_pid_fd(5) — Linux manual page

NAME | DESCRIPTION | SEE ALSO | COLOPHON

proc_pid_fd(5)             File Formats Manual            proc_pid_fd(5)

NAME         top

       /proc/pid/fd/ - file descriptors

DESCRIPTION         top

       /proc/pid/fd/
              This is a subdirectory containing one entry for each file
              which the process has open, named by its file descriptor,
              and which is a symbolic link to the actual file.  Thus, 0
              is standard input, 1 standard output, 2 standard error,
              and so on.

              For file descriptors for pipes and sockets, the entries
              will be symbolic links whose content is the file type with
              the inode.  A readlink(2) call on this file returns a
              string in the format:

                  type:[inode]

              For example, socket:[2248868] will be a socket and its
              inode is 2248868.  For sockets, that inode can be used to
              find more information in one of the files under
              /proc/net/.

              For file descriptors that have no corresponding inode
              (e.g., file descriptors produced by bpf(2),
              epoll_create(2), eventfd(2), inotify_init(2),
              perf_event_open(2), signalfd(2), timerfd_create(2), and
              userfaultfd(2)), the entry will be a symbolic link with
              contents of the form

                  anon_inode:file-type

              In many cases (but not all), the file-type is surrounded
              by square brackets.

              For example, an epoll file descriptor will have a symbolic
              link whose content is the string anon_inode:[eventpoll].

              In a multithreaded process, the contents of this directory
              are not available if the main thread has already
              terminated (typically by calling pthread_exit(3)).

              Programs that take a filename as a command-line argument,
              but don't take input from standard input if no argument is
              supplied, and programs that write to a file named as a
              command-line argument, but don't send their output to
              standard output if no argument is supplied, can
              nevertheless be made to use standard input or standard
              output by using /proc/pid/fd files as command-line
              arguments.  For example, assuming that -i is the flag
              designating an input file and -o is the flag designating
              an output file:

                  $ foobar -i /proc/self/fd/0 -o /proc/self/fd/1 ...

              and you have a working filter.

              /proc/self/fd/N is approximately the same as /dev/fd/N in
              some UNIX and UNIX-like systems.  Most Linux MAKEDEV
              scripts symbolically link /dev/fd to /proc/self/fd, in
              fact.

              Most systems provide symbolic links /dev/stdin,
              /dev/stdout, and /dev/stderr, which respectively link to
              the files 0, 1, and 2 in /proc/self/fd.  Thus the example
              command above could be written as:

                  $ foobar -i /dev/stdin -o /dev/stdout ...

              Permission to dereference or read (readlink(2)) the
              symbolic links in this directory is governed by a ptrace
              access mode PTRACE_MODE_READ_FSCREDS check; see ptrace(2).

              Note that for file descriptors referring to inodes (pipes
              and sockets, see above), those inodes still have
              permission bits and ownership information distinct from
              those of the /proc/pid/fd entry, and that the owner may
              differ from the user and group IDs of the process.  An
              unprivileged process may lack permissions to open them, as
              in this example:

                  $ echo test | sudo -u nobody cat
                  test
                  $ echo test | sudo -u nobody cat /proc/self/fd/0
                  cat: /proc/self/fd/0: Permission denied

              File descriptor 0 refers to the pipe created by the shell
              and owned by that shell's user, which is not nobody, so
              cat does not have permission to create a new file
              descriptor to read from that inode, even though it can
              still read from its existing file descriptor 0.

SEE ALSO         top

       proc(5)

COLOPHON         top

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Linux man-pages 6.9.1          2024-05-02                 proc_pid_fd(5)