int getlogin_r(char *buf, size_t bufsize);
char *cuserid(char *string);
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
getlogin_r(): _REENTRANT || _POSIX_C_SOURCE >= 199506L
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 bufsize.
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 getlogin().
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 <stdio.h>.
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.
getlogin() returns a pointer to the username when successful, and NULL on failure, with errno set to indicate the cause of the error. getlogin_r() returns 0 when successful, and nonzero on failure.
The calling process already has the maximum allowed number of open files.
The system already has the maximum allowed number of open files.
The calling process has no controlling terminal.
(getlogin_r) The length of the username, including the terminating null byte ('\0'), is larger than bufsize.
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
(traditionally /etc/utmp; some libc versions used /var/adm/utmp)
The getlogin() function is not thread-safe.
The getlogin_r() function is thread-safe.
The cuserid() function is thread-safe with exceptions. It is not thread-safe if called with a NULL parameter.
getlogin() and getlogin_r() specified in POSIX.1-2001.
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.
OpenBSD has getlogin() and setlogin(), and a username associated with a session, even if it has no controlling terminal.
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.)
Nobody knows precisely what cuserid() does; avoid it in portable programs. Or avoid it altogether: use getpwuid(geteuid()) instead, if that is what you meant. Do not use cuserid().
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