The g:file-as-namestring type specifier represents and performs automatic conversion between a Lisp namestring and a g:file object for files on a virtual file system. A Lisp namestring or pathname is converted to the corresponding g:file object using the g:file-parse-name function. The conversion from a g:file object back to a Lisp namestring is done with the g:file-get-parse-name function. Conversion of a Lisp namestring to g:file object and back:

(setf path "/home/lisp/github/glib/gio/gio.file.lisp")
=> "/home/lisp/github/glib/gio/gio.file.lisp"
(cffi:convert-to-foreign path 'g:file-as-namestring)
=> #.(SB-SYS:INT-SAP #X60BCAD6BD5B0)
(cffi:convert-from-foreign * 'g:file-as-namestring)
=> "/home/lisp/github/glib/gio/gio.file.lisp"