proc_pid_fd(5) - Linux manual page (original) (raw)

procpidfd(5) File Formats Manual procpidfd(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)](../man2/readlink.2.html) 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)](../man2/bpf.2.html),
          [epoll_create(2)](../man2/epoll%5Fcreate.2.html), [eventfd(2)](../man2/eventfd.2.html), [inotify_init(2)](../man2/inotify%5Finit.2.html),
          [perf_event_open(2)](../man2/perf%5Fevent%5Fopen.2.html), [signalfd(2)](../man2/signalfd.2.html), [timerfd_create(2)](../man2/timerfd%5Fcreate.2.html), and
          [userfaultfd(2)](../man2/userfaultfd.2.html)), 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 _anoninode:[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)](../man3/pthread%5Fexit.3.html)).

          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)](../man2/readlink.2.html)) the
          symbolic links in this directory is governed by a ptrace
          access mode **PTRACE_MODE_READ_FSCREDS** check; see [ptrace(2)](../man2/ptrace.2.html).

          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)](../man5/proc.5.html)

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Linux man-pages 6.10 2024-11-17 procpidfd(5)