login / logout / login_tty (3) - glibc

Split from the chapter glibc: Users and Groups into the per-function layout; see the method page and the glibc ledger for coverage status.

Declaration and gates

#include <utmp.h>

int login_tty(int fd);
void login(const struct utmp *entry);
int logout(const char *ut_line);

All three are unconditional in /usr/include/utmp.h: login_tty at line 41, login at line 45, logout at line 48. No feature-test gate is involved; there are no _r or utmpx twins.

Semantics

login_tty makes fd the controlling terminal of the calling process (setsid plus a TIOCSCTTY-style ioctl per the manual’s implementation notes), redirects stdin, stdout and stderr to it, and closes fd; it returns 0 on success and -1 on error (manual, Logging In and Out). login inserts an entry into the user accounting database: it sets ut_type to USER_PROCESS and ut_pid to the process ID, derives ut_line from the first of stdin/stdout/stderr that is a terminal (stripping a leading /dev/; “???” if none is found, in which case the record is not written to utmp), then writes the struct to utmp and a copy to wtmp. logout finds the utmp entry matching ut_line, zeroes ut_name and ut_host, stamps the time, sets ut_type to DEAD_PROCESS and rewrites the record; it returns 1 on success, 0 on error (login(3)).

Gotchas and portability

• The trio is “Not in POSIX.1. Present on the BSDs.” – where libutil still provides them; on glibc >= 2.34 they are in libc proper and -lutil is no longer needed (nm: dual GLIBC_2.34 / GLIBC_2.2.5 versions).
• BSD spells the ut_user member ut_name; <utmp.h> defines ut_name as an alias for ut_user on glibc (manual, same node).
• login writes to wtmp even when no terminal was found, so utmp and wtmp can disagree for that case (manual, Logging In and Out).
• musl 1.2.5: login_tty is implemented (src/misc/login_tty.c, also declared in musl’s utmp.h), but login and logout are absent entirely – no declaration, no source.

References

• glibc manual: users.texi, node Logging In and Out
• man3: login.3 (covers logout), login_tty.3 (link to openpty.3)
• headers: /usr/include/utmp.h, paths.h
• siblings: updwtmp page, utmp page
• symbols: nm -D --with-symbol-versions libc.so.6