getlogin (3p) - Linux Manuals

getlogin: get login name

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 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.

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 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.

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 .