inotify_add_watch() adds a new watch, or modifies an existing watch, for the file whose location is specified in pathname; the caller must have read permission for this file. The fd argument is a file descriptor referring to the inotify instance whose watch list is to be modified. The events to be monitored for pathname are specified in the mask bit-mask argument. See inotify(7) for a description of the bits that can be set in mask.
A successful call to inotify_add_watch() returns a unique watch descriptor for this inotify instance, for the filesystem object (inode) that corresponds to pathname. If the filesystem object was not previously being watched by this inotify instance, then the watch descriptor is newly allocated. If the filesystem object was already being watched (perhaps via a different link to the same object), then the descriptor for the existing watch is returned.
The watch descriptor is returned by later read(2)s from the inotify file descriptor. These reads fetch inotify_event structures (see inotify(7)) indicating filesystem events; the watch descriptor inside this structure identifies the object for which the event occurred.