Name
/proc/pid/fd/ - file descriptorsDescription
/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
- proc(5)