path: root/manual/users.texi

@node Users and Groups, System Information, Job Control, Top
@chapter Users and Groups

Every user who can log in on the system is identified by a unique number
called the @dfn{user ID}.  Each process has an effective user ID which
says which user's access permissions it has.

Users are classified into @dfn{groups} for access control purposes.  Each
process has one or more @dfn{group ID values} which say which groups the
process can use for access to files.

The effective user and group IDs of a process collectively form its
@dfn{persona}.  This determines which files the process can access.
Normally, a process inherits its persona from the parent process, but
under special circumstances a process can change its persona and thus
change its access permissions.

Each file in the system also has a user ID and a group ID.  Access
control works by comparing the user and group IDs of the file with those
of the running process.

The system keeps a database of all the registered users, and another
database of all the defined groups.  There are library functions you
can use to examine these databases.

@menu
* User and Group IDs::          Each user has a unique numeric ID;
				 likewise for groups.
* Process Persona::             The user IDs and group IDs of a process.
* Why Change Persona::          Why a program might need to change
				 its user and/or group IDs.
* How Change Persona::          Changing the user and group IDs.
* Reading Persona::             How to examine the user and group IDs.

* Setting User ID::             Functions for setting the user ID.
* Setting Groups::              Functions for setting the group IDs.

* Enable/Disable Setuid::       Turning setuid access on and off.
* Setuid Program Example::      The pertinent parts of one sample program.
* Tips for Setuid::             How to avoid granting unlimited access.

* Who Logged In::               Getting the name of the user who logged in,
				 or of the real user ID of the current process.

* User Database::               Functions and data structures for
                        	 accessing the user database.
* Group Database::              Functions and data structures for
                        	 accessing the group database.
* Database Example::            Example program showing use of database
				 inquiry functions.
@end menu

@node User and Group IDs
@section User and Group IDs

@cindex login name
@cindex user name
@cindex user ID
Each user account on a computer system is identified by a @dfn{user
name} (or @dfn{login name}) and @dfn{user ID}.  Normally, each user name
has a unique user ID, but it is possible for several login names to have
the same user ID.  The user names and corresponding user IDs are stored
in a data base which you can access as described in @ref{User Database}.

@cindex group name
@cindex group ID
Users are classified in @dfn{groups}.  Each user name also belongs to
one or more groups, and h