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PROLOG | NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | EXAMPLES | APPLICATION USAGE | RATIONALE | FUTURE DIRECTIONS | SEE ALSO | COPYRIGHT |
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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(). 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.
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.1‐2017, limits.h(0p),
unistd.h(0p)
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(3P)
Pages that refer to this page: unistd.h(0p), logname(1p), endgrent(3p), endpwent(3p), getpwuid(3p)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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