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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.