getlogin, getlogin_r, cuserid - get username
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
getlogin() returns a pointer to a string containing the name of the user logged in on the controlling terminal of the process, or a null pointer if this information cannot be determined. The string is statically allocated and might be overwritten on subsequent calls to this function or to cuserid().
getlogin_r() returns this same username in the array
buf of size
cuserid() returns a pointer to a string containing a
username associated with the effective user ID of the process. If
string is not a null pointer, it should be an array that can
hold at least L_cuserid characters; the string is
returned in this array. Otherwise, a pointer to a string in a static
area is returned. This string is statically allocated and might be
overwritten on subsequent calls to this function or to
The macro L_cuserid is an integer constant that
indicates how long an array you might need to store a username.
L_cuserid is declared in
These functions let your program identify positively the user who is running (cuserid()) or the user who logged in this session (getlogin()). (These can differ when set-user-ID programs are involved.)
For most purposes, it is more useful to use the environment variable LOGNAME to find out who the user is. This is more flexible precisely because the user can set LOGNAME arbitrarily.
The per-process limit on the number of open file descriptors has been reached.
The system-wide limit on the total number of open files has been reached.
The calling process has no controlling terminal.
(getlogin_r) The length of the username, including the terminating
null byte ('\0'), is larger than
Linux/glibc also has:
There was no corresponding entry in the utmp-file.
Insufficient memory to allocate passwd structure.
Standard input didn't refer to a terminal. (See BUGS.)
password database file
/etc/utmp; some libc versions used
For an explanation of the terms used in this section, see attributes(7).
|getlogin()||Thread safety||MT-Unsafe race:getlogin race:utent
sig:ALRM timer locale
|getlogin_r()||Thread safety||MT-Unsafe race:utent sig:ALRM timer
|cuserid()||Thread safety||MT-Unsafe race:cuserid/!string locale|
In the above table,
that if any of the functions setutent(3),
getutent(3), or endutent(3) are used
in parallel in different threads of a program, then data races could
occur. getlogin() and getlogin_r()
call those functions, so we use race:utent to remind users.
System V has a cuserid() function which uses the real user ID rather than the effective user ID. The cuserid() function was included in the 1988 version of POSIX, but removed from the 1990 version. It was present in SUSv2, but removed in POSIX.1-2001.
Unfortunately, it is often rather easy to fool getlogin(). Sometimes it does not work at all, because some program messed up the utmp file. Often, it gives only the first 8 characters of the login name. The user currently logged in on the controlling terminal of our program need not be the user who started it. Avoid getlogin() for security-related purposes.
Note that glibc does not follow the POSIX specification and uses
stdin instead of
/dev/tty. A bug. (Other recent
systems, like SunOS 5.8 and HP-UX 11.11 and FreeBSD 4.8 all return the
login name also when
stdin is redirected.)
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