From man 5 proc:
/proc/locks
This file shows current file locks (flock(2) and fcntl(2)) and leases (fcntl(2)).
An example of the content shown in this file is the following:
0 1 2 3 4 5 6 7
1: POSIX ADVISORY READ 5433 08:01:7864448 128 128
2: FLOCK ADVISORY WRITE 2001 08:01:7864554 0 EOF
3: FLOCK ADVISORY WRITE 1568 00:2f:32388 0 EOF
4: POSIX ADVISORY WRITE 699 00:16:28457 0 EOF
5: POSIX ADVISORY WRITE 764 00:16:21448 0 0
6: POSIX ADVISORY READ 3548 08:01:7867240 1 1
7: POSIX ADVISORY READ 3548 08:01:7865567 1826 2335
8: OFDLCK ADVISORY WRITE -1 08:01:8713209 128 191
The fields shown in each line are as follows:
(1) The ordinal position of the lock in the list.
(2) The lock type. Values that may appear here include:
FLOCK This is a BSD file lock created using flock(2).
OFDLCK This is an open file description (OFD) lock created using fcntl(2).
POSIX This is a POSIX byte-range lock created using fcntl(2).
(3) Among the strings that can appear here are the following:
ADVISORY
This is an advisory lock.
MANDATORY
This is a mandatory lock.
(4) The type of lock. Values that can appear here are:
READ This is a POSIX or OFD read lock, or a BSD shared lock.
WRITE This is a POSIX or OFD write lock, or a BSD exclusive lock.
(5) The PID of the process that owns the lock.
Because OFD locks are not owned by a single process (since multiple processes may have file descriptors that refer to the same open file description), the value -1 is displayed in this field for OFD locks. (Before kernel 4.14, a bug meant that the PID of the process that initially acquired the lock was displayed instead of the value -1.)
(6) Three colon-separated subfields that identify the major and minor device ID of the device containing the filesystem
where the locked file resides, followed by the inode number of the locked file.
(7) The byte offset of the first byte of the lock. For BSD locks, this value is always 0.
(8) The byte offset of the last byte of the lock. EOF in this field means that the lock extends to the end of the file.
For BSD locks, the value shown is always EOF.
Since Linux 4.9, the list of locks shown in /proc/locks is filtered to show just the locks for the processes in the PID
namespace (see pid_namespaces(7)) for which the /proc filesystem was mounted. (In the initial PID namespace, there is
no filtering of the records shown in this file.)
The lslocks(8) command provides a bit more information about each lock.