Provided by: manpages-posix-dev_2017a-2_all 
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 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 character. 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.
The getlogin() and getlogin_r() functions may make use of file descriptors 0, 1, and 2 to find the
controlling terminal of the current process, examining each in turn until the terminal is found. If in
this case none of these three file descriptors is open to the controlling terminal, these functions may
fail. The method used to find the terminal associated with a file descriptor may depend on the file
descriptor being open to the actual terminal device, not /dev/tty.
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 application shall not modify the string returned. The returned pointer might be invalidated or the
string content might be overwritten by a subsequent call to getlogin(). The returned pointer and the
string content might also be invalidated if the calling thread is terminated.
If successful, the getlogin_r() function shall return zero; otherwise, an error number shall be returned
to indicate the error.
ERRORS
These functions may fail if:
EMFILE All file descriptors available to the process are currently open.
ENFILE The maximum allowable number of files is currently open in the system.
ENOTTY None of the file descriptors 0, 1, or 2 is open to the controlling terminal of the current
process.
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 S
The following example calls the getlogin() function to obtain the name of the user associated with the
calling process, and passes this information 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: getpwuid(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, particularly 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 originally 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 ALSO
getpwnam(), getpwuid(), geteuid(), getuid()
The Base Definitions volume of POSIX.1‐2017, <limits.h>, <unistd.h>
COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1-2017, Standard
for Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 7, 2018 Edition, Copyright (C) 2018 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 .
Any typographical or formatting errors that appear in this page are most likely to have been introduced
during the conversion of the source files to man page format. To report such errors, see
https://www.kernel.org/doc/man-pages/reporting_bugs.html .
IEEE/The Open Group 2017 GETLOGIN(3POSIX)