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/proc/pid/fdinfo/

information about file descriptors

/proc/pid/fdinfo/ (since Linux 2.6.22)
This is a subdirectory containing one entry for each file which the process has open, named by its file descriptor. The files in this directory are readable only by the owner of the process. The contents of each file can be read to obtain information about the corresponding file descriptor. The content depends on the type of file referred to by the corresponding file descriptor.
For regular files and directories, we see something like:

$ cat /proc/12015/fdinfo/4
pos:    1000
flags:  01002002
mnt_id: 21

The fields are as follows:
This is a decimal number showing the file offset.
This is an octal number that displays the file access mode and file status flags (see open(2)). If the close-on-exec file descriptor flag is set, then flags will also include the value O_CLOEXEC.
Before Linux 3.1, this field incorrectly displayed the setting of O_CLOEXEC at the time the file was opened, rather than the current setting of the close-on-exec flag.
mnt_id
This field, present since Linux 3.15, is the ID of the mount containing this file. See the description of /proc/pid/mountinfo.
For eventfd file descriptors (see eventfd(2)), we see (since Linux 3.8) the following fields:

pos:	0
flags:	02
mnt_id:	10
eventfd-count:               40

eventfd-count is the current value of the eventfd counter, in hexadecimal.
For epoll file descriptors (see epoll(7)), we see (since Linux 3.8) the following fields:

pos:	0
flags:	02
mnt_id:	10
tfd:        9 events:       19 data: 74253d2500000009
tfd:        7 events:       19 data: 74253d2500000007

Each of the lines beginning tfd describes one of the file descriptors being monitored via the epoll file descriptor (see epoll_ctl(2) for some details). The tfd field is the number of the file descriptor. The events field is a hexadecimal mask of the events being monitored for this file descriptor. The data field is the data value associated with this file descriptor.
For signalfd file descriptors (see signalfd(2)), we see (since Linux 3.8) the following fields:

pos:	0
flags:	02
mnt_id:	10
sigmask:	0000000000000006

sigmask is the hexadecimal mask of signals that are accepted via this signalfd file descriptor. (In this example, bits 2 and 3 are set, corresponding to the signals SIGINT and SIGQUIT; see signal(7).)
For inotify file descriptors (see inotify(7)), we see (since Linux 3.8) the following fields:

pos:	0
flags:	00
mnt_id:	11
inotify wd:2 ino:7ef82a sdev:800001 mask:800afff ignored_mask:0 fhandle-bytes:8 fhandle-type:1 f_handle:2af87e00220ffd73
inotify wd:1 ino:192627 sdev:800001 mask:800afff ignored_mask:0 fhandle-bytes:8 fhandle-type:1 f_handle:27261900802dfd73

Each of the lines beginning with "inotify" displays information about one file or directory that is being monitored. The fields in this line are as follows:
A watch descriptor number (in decimal).
The inode number of the target file (in hexadecimal).
The ID of the device where the target file resides (in hexadecimal).
The mask of events being monitored for the target file (in hexadecimal).
If the kernel was built with exportfs support, the path to the target file is exposed as a file handle, via three hexadecimal fields: fhandle-bytes, fhandle-type, and f_handle.
For fanotify file descriptors (see fanotify(7)), we see (since Linux 3.8) the following fields:

pos:	0
flags:	02
mnt_id:	11
fanotify flags:0 event-flags:88002
fanotify ino:19264f sdev:800001 mflags:0 mask:1 ignored_mask:0 fhandle-bytes:8 fhandle-type:1 f_handle:4f261900a82dfd73

The fourth line displays information defined when the fanotify group was created via fanotify_init(2):
The flags argument given to fanotify_init(2) (expressed in hexadecimal).
The event_f_flags argument given to fanotify_init(2) (expressed in hexadecimal).
Each additional line shown in the file contains information about one of the marks in the fanotify group. Most of these fields are as for inotify, except:
The flags associated with the mark (expressed in hexadecimal).
The events mask for this mark (expressed in hexadecimal).
The mask of events that are ignored for this mark (expressed in hexadecimal).
For details on these fields, see fanotify_mark(2).
For timerfd file descriptors (see timerfd(2)), we see (since Linux 3.17) the following fields:

pos:    0
flags:  02004002
mnt_id: 13
clockid: 0
ticks: 0
settime flags: 03
it_value: (7695568592, 640020877)
it_interval: (0, 0)

This is the numeric value of the clock ID (corresponding to one of the CLOCK_* constants defined via <time.h>) that is used to mark the progress of the timer (in this example, 0 is CLOCK_REALTIME).
This is the number of timer expirations that have occurred, (i.e., the value that read(2) on it would return).
This field lists the flags with which the timerfd was last armed (see timerfd_settime(2)), in octal (in this example, both TFD_TIMER_ABSTIME and TFD_TIMER_CANCEL_ON_SET are set).
This field contains the amount of time until the timer will next expire, expressed in seconds and nanoseconds. This is always expressed as a relative value, regardless of whether the timer was created using the TFD_TIMER_ABSTIME flag.
This field contains the interval of the timer, in seconds and nanoseconds. (The it_value and it_interval fields contain the values that timerfd_gettime(2) on this file descriptor would return.)

proc(5)