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GETLOGIN(3P) POSIX Programmer's Manual GETLOGIN(3P)
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.
getlogin, getlogin_r — get login name
#include <unistd.h>
char *getlogin(void);
int getlogin_r(char *name, size_t namesize);
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.
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().
If successful, the getlogin_r() function shall return zero;
otherwise, an error number shall be returned to indicate the error.
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.
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);
}
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.
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.
None.
getpwnam(3p), getpwuid(3p), geteuid(3p), getuid(3p)
The Base Definitions volume of POSIX.1‐2008, limits.h(0p),
unistd.h(0p)
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2013 Edition, Standard for Information
Technology -- Portable Operating System Interface (POSIX), The Open
Group Base Specifications Issue 7, Copyright (C) 2013 by the
Institute of Electrical and Electronics Engineers, Inc and The Open
Group. (This is POSIX.1-2008 with the 2013 Technical Corrigendum 1
applied.) 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.unix.org/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 2013 GETLOGIN(3P)
Pages that refer to this page: unistd.h(0p), logname(1p), endgrent(3p), endpwent(3p), getpwuid(3p)