getlogin_r(3) -- Linux man page
NAMEgetlogin, getlogin_r, cuserid - get user name
int getlogin_r(char *buf, size_t bufsize);
DESCRIPTIONgetlogin 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 user name in the array buf of size bufsize.
cuserid returns a pointer to a string containing a user name 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 user name. 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 setuid programs are involved.)
RETURN VALUEgetlogin returns a pointer to the user name when successful, and NULL on failure. getlogin_r returns 0 when successful, and nonzero on failure.
- The calling process has no controlling tty.
- The calling process already has the maximum allowed number of open files.
- The system already has the maximum allowed number of open files.
- (getlogin_r) The length of the user name, including final NUL, is larger than bufsize.
Linux/glibc also has
- Insufficient memory to allocate passwd structure.
- There was no corresponding entry in the utmp-file.
/etc/passwd password database file
/var/run/utmp (traditionally /etc/utmp; some libc versions used /var/adm/utmp)
CONFORMING TOPOSIX.1. 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.
BUGSUnfortunately, 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 tty 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 spec and uses stdin instead of /dev/tty. A bug. (Other recent systems, like SunOS 5.8 and HPUX 11.11 and FreeBSD 4.8 all return the login name also when stdin is redirected.)
SEE ALSOgeteuid(2), getuid(2)