GETLOGIN(P) POSIX Programmer's Manual GETLOGIN(P)PROLOG
This manual page is part of the POSIX Programmer's Manual. The Linux
implementation of this interface may differ (consult the corresponding
Linux manual page for details of Linux behavior), or the interface may
not be implemented on Linux.
NAME
getlogin, getlogin_r - get login name
SYNOPSIS
#include <unistd.h>
char *getlogin(void);
int getlogin_r(char *name, size_t namesize);
DESCRIPTION
The getlogin() function shall return a pointer to a string containing
the user name associated by the login activity with the controlling
terminal of the current process. If getlogin() returns a non-null
pointer, then that pointer points to the name that the user logged in
under, even if there are several login names with the same user ID.
The getlogin() function need not be reentrant. A function that is not
required to be reentrant is not required to be thread-safe.
The getlogin_r() function shall put the name associated by the login
activity with the controlling terminal of the current process in the
character array pointed to by name. The array is namesize characters
long and should have space for the name and the terminating null char‐
acter. The maximum size of the login name is {LOGIN_NAME_MAX}.
If getlogin_r() is successful, name points to the name the user used at
login, even if there are several login names with the same user ID.
RETURN VALUE
Upon successful completion, getlogin() shall return a pointer to the
login name or a null pointer if the user's login name cannot be found.
Otherwise, it shall return a null pointer and set errno to indicate the
error.
The return value from getlogin() may point to static data whose content
is overwritten by each call.
If successful, the getlogin_r() function shall return zero; otherwise,
an error number shall be returned to indicate the error.
ERRORS
The getlogin() and getlogin_r() functions may fail if:
EMFILE {OPEN_MAX} file descriptors are currently open in the calling
process.
ENFILE The maximum allowable number of files is currently open in the
system.
ENXIO The calling process has no controlling terminal.
The getlogin_r() function may fail if:
ERANGE The value of namesize is smaller than the length of the string
to be returned including the terminating null character.
The following sections are informative.
EXAMPLES
Getting the User Login Name
The following example calls the getlogin() function to obtain the name
of the user associated with the calling process, and passes this infor‐
mation to the getpwnam() function to get the associated user database
information.
#include <unistd.h>
#include <sys/types.h>
#include <pwd.h>
#include <stdio.h>
...
char *lgn;
struct passwd *pw;
...
if ((lgn = getlogin()) == NULL || (pw = getpwnam(lgn)) == NULL) {
fprintf(stderr, "Get of user information failed.\n"); exit(1);
}
APPLICATION USAGE
Three names associated with the current process can be determined: get‐
pwuid( geteuid()) shall return the name associated with the effective
user ID of the process; getlogin() shall return the name associated
with the current login activity; and getpwuid( getuid()) shall return
the name associated with the real user ID of the process.
The getlogin_r() function is thread-safe and returns values in a user-
supplied buffer instead of possibly using a static data area that may
be overwritten by each call.
RATIONALE
The getlogin() function returns a pointer to the user's login name. The
same user ID may be shared by several login names. If it is desired to
get the user database entry that is used during login, the result of
getlogin() should be used to provide the argument to the getpwnam()
function. (This might be used to determine the user's login shell, par‐
ticularly where a single user has multiple login shells with distinct
login names, but the same user ID.)
The information provided by the cuserid() function, which was origi‐
nally defined in the POSIX.1-1988 standard and subsequently removed,
can be obtained by the following:
getpwuid(geteuid())
while the information provided by historical implementations of
cuserid() can be obtained by:
getpwuid(getuid())
The thread-safe version of this function places the user name in a
user-supplied buffer and returns a non-zero value if it fails. The non-
thread-safe version may return the name in a static data area that may
be overwritten by each call.
FUTURE DIRECTIONS
None.
SEE ALSOgetpwnam() , getpwuid() , geteuid() , getuid() , the Base Definitions
volume of IEEE Std 1003.1-2001, <limits.h>, <unistd.h>
COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 6, Copyright (C) 2001-2003 by the Institute of
Electrical and Electronics Engineers, Inc and The Open Group. In the
event of any discrepancy between this version and the original IEEE and
The Open Group Standard, the original IEEE and The Open Group Standard
is the referee document. The original Standard can be obtained online
at http://www.opengroup.org/unix/online.html .
IEEE/The Open Group 2003 GETLOGIN(P)
