getlogin(3)

1GETLOGIN(3)                Linux Programmer's Manual               GETLOGIN(3)
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NAME

6       getlogin, getlogin_r, cuserid - get username
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SYNOPSIS

9       #include <unistd.h>
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11       char *getlogin(void);
12       int getlogin_r(char *buf, size_t bufsize);
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14       #include <stdio.h>
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16       char *cuserid(char *string);
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18   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
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20       getlogin_r(): _REENTRANT || _POSIX_C_SOURCE >= 199506L
21       cuserid(): _XOPEN_SOURCE
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DESCRIPTION

24       getlogin()  returns  a  pointer  to a string containing the name of the
25       user logged in on the controlling terminal of the process,  or  a  null
26       pointer if this information cannot be determined.  The string is stati‐
27       cally allocated and might be overwritten on subsequent  calls  to  this
28       function or to cuserid().
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30       getlogin_r()  returns  this same username in the array buf of size buf‐
31       size.
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33       cuserid() returns a pointer to a string containing a  username  associ‐
34       ated  with  the  effective  user ID of the process.  If string is not a
35       null pointer, it should be an array that can hold  at  least  L_cuserid
36       characters; the string is returned in this array.  Otherwise, a pointer
37       to a string in a static area is returned.  This  string  is  statically
38       allocated and might be overwritten on subsequent calls to this function
39       or to getlogin().
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41       The macro L_cuserid is an integer constant that indicates how  long  an
42       array  you  might  need  to store a username.  L_cuserid is declared in
43       <stdio.h>.
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45       These functions let your program identify positively the  user  who  is
46       running  (cuserid())  or  the  user  who logged in this session (getlo‐
47       gin()).  (These can differ when set-user-ID programs are involved.)
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49       For most purposes, it is more useful to use  the  environment  variable
50       LOGNAME  to  find out who the user is.  This is more flexible precisely
51       because the user can set LOGNAME arbitrarily.
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RETURN VALUE

54       getlogin() returns a pointer to the username when successful, and  NULL
55       on  failure.   getlogin_r()  returns 0 when successful, and non-zero on
56       failure.
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ERRORS

59       POSIX specifies
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61       EMFILE The calling process already has the maximum  allowed  number  of
62              open files.
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64       ENFILE The system already has the maximum allowed number of open files.
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66       ENXIO  The calling process has no controlling tty.
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68       ERANGE (getlogin_r) The length of the username, including the terminat‐
69              ing null byte, is larger than bufsize.
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71       Linux/glibc also has
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73       ENOENT There was no corresponding entry in the utmp-file.
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75       ENOMEM Insufficient memory to allocate passwd structure.
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77       ENOTTY Standard input didn't refer to a terminal.  (See BUGS.)
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FILES

80       /etc/passwd
81              password database file
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83       /var/run/utmp
84              (traditionally /etc/utmp; some libc versions used /var/adm/utmp)
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CONFORMING TO

87       getlogin() and getlogin_r() specified in POSIX.1-2001.
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89       System V has a cuserid() function which uses the real  user  ID  rather
90       than the effective user ID.  The cuserid() function was included in the
91       1988 version of POSIX, but removed  from  the  1990  version.   It  was
92       present in SUSv2, but removed in POSIX.1-2001.
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94       OpenBSD has getlogin() and setlogin(), and a username associated with a
95       session, even if it has no controlling tty.
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BUGS

98       Unfortunately, it is often rather easy to fool  getlogin().   Sometimes
99       it  does not work at all, because some program messed up the utmp file.
100       Often, it gives only the first 8 characters of  the  login  name.   The
101       user currently logged in on the controlling tty of our program need not
102       be the user who started it.  Avoid getlogin() for security-related pur‐
103       poses.
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105       Note  that glibc does not follow the POSIX specification and uses stdin
106       instead of /dev/tty.  A bug.  (Other recent systems, like SunOS 5.8 and
107       HP-UX  11.11  and FreeBSD 4.8 all return the login name also when stdin
108       is redirected.)
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110       Nobody knows precisely what cuserid() does; avoid it in  portable  pro‐
111       grams.   Or  avoid  it  altogether: use getpwuid(geteuid()) instead, if
112       that is what you meant.  Do not use cuserid().
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COLOPHON

118       This page is part of release 3.22 of the Linux  man-pages  project.   A
119       description  of  the project, and information about reporting bugs, can
120       be found at http://www.kernel.org/doc/man-pages/.
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124GNU                               2008-06-29                       GETLOGIN(3)