(libc.info.gz) Users and Groups

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 29 Users and Groups
 *******************
 
 Every user who can log in on the system is identified by a unique number
 called the "user ID".  Each process has an effective user ID which says
 which user's access permissions it has.
 
    Users are classified into "groups" for access control purposes.  Each
 process has one or more "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
 "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.

 
* 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 Accounting Database    Keeping information about users and various
                                  actions in databases.
 
* 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 the use of database
 				 inquiry functions.
* Netgroup Database           Functions for accessing the netgroup database.

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