8.15. User Identification

Any process can find out its real and effective user ID and group ID. Sometimes, however, we want to find out the login name of the user who's running the program. We could call getpwuid(getuid()), but what if a single user has multiple login names, each with the same user ID? (A person might have multiple entries in the password file with the same user ID to have a different login shell for each entry.) The system normally keeps track of the name we log in under (Section 6.8), and the getlogin function provides a way to fetch that login name.

#include <unistd.h> char *getlogin(void);
Returns: pointer to string giving login name if OK, NULL on error

This function can fail if the process is not attached to a terminal that a user logged in to. We normally call these processes daemons. We discuss them in Chapter 13.

Given the login name, we can then use it to look up the user in the password fileto determine the login shell, for exampleusing getpwnam.

To find the login name, UNIX systems have historically called the ttyname function (Section 18.9) and then tried to find a matching entry in the utmp file (Section 6.8). FreeBSD and Mac OS X store the login name in the session structure associated with the process table entry and provide system calls to fetch and store this name.

System V provided the cuserid function to return the login name. This function called getlogin and, if that failed, did a getpwuid(getuid()). The IEEE Standard 1003.11988 specified cuserid, but it called for the effective user ID to be used, instead of the real user ID. The 1990 version of POSIX.1 dropped the cuserid function.

The environment variable LOGNAME is usually initialized with the user's login name by login(1) and inherited by the login shell. Realize, however, that a user can modify an environment variable, so we shouldn't use LOGNAME to validate the user in any way. Instead, getlogin should be used.