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(libc.info)Users and Groups

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Users and Groups
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   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.