Every file (or directory, or device entry, or whatever) in the filesystem has an owner and group. The owner and group of a file determine to whom the owner and group permissions apply (read, write, and/or execute). The owner and group of a file are determined at the time the file is created, but under certain circumstances, you can change them. (The exact circumstances depend on the particular flavor of UNIX you are running: see the chown manpage for details.)
The chown function takes a user ID number (UID), a group ID number (GID), and a list of filenames, and attempts to change the ownership of each of the listed files as specified. A success is indicated by a nonzero return value equal to the number of files successfully changed—just like chmod or unlink. Note that you are changing both the owner and the group at once. Use -1 instead of an actual user or group ID if you do not want to change the ID. Also note that you must use the numeric UID and GID, not the corresponding symbolic names (even though the chmod command accepts the names). For example, if fred is UID 1234 and fred's default group stoners is GID 35, then the following command makes the files slate and granite belong to fred and his default group:
chown(1234, 35, "slate", "granite"); # same as: # chown fred slate granite # chgrp stoners slate granite
In Chapter 16, you'll learn how to convert fred to 1234 and stoners to 35.