proc_pid_fd(5) File Formats Manual proc_pid_fd(5)
NAME
/proc/pid/fd/ - file descriptors
DESCRIPTION
/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 in-
formation 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 sym-
bolic 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 (typi-
cally 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 argu-
ment, but don't send their output to standard output if no argu-
ment 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 designat-
ing 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 symboli-
cally 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
proc(5)
Linux man-pages 6.7 2023-08-15 proc_pid_fd(5)
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