are commonly used in conjunction with GNU Make's automatic macros (like $@) GNU Make provides a modifier syntax. Appending D or F to any automatic macro is equivalent to calling $(dir) or $(notdir) on it. For example, $(@D) is equivalent to $(dir $@) and $(@F) is the same as $(notdir $@).
GNU Make 3.81 added two very useful functions: abspath and realpath.
realpath is a GNU Make wrapper for the C library realpath function which removes ./, resolves ../, removes duplicated / and follows symlinks. The argument to realpath must exist in the file system. The path returned by realpath is absolute. If the path does not exist then the function returns an empty string.
Note that you cannot use realpath as a file exists function because the final part of the path does not have to exist; to do file exists used $(wildcard FILE). For example, the full path of the current directory could be found like this:
current := $(realpath ./)
abspath is similar but it does not follow symlinks and does not have to refer to an existing file or directory.