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

6       getlogin, getlogin_r, cuserid - get username
7

SYNOPSIS

9       #include <unistd.h>
10
11       char *getlogin(void);
12       int getlogin_r(char *buf, size_t bufsize);
13
14       #include <stdio.h>
15
16       char *cuserid(char *string);
17
18   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
19
20       getlogin_r(): _POSIX_C_SOURCE >= 199506L
21
22       cuserid():
23           Since glibc 2.24:
24               (_XOPEN_SOURCE && ! (_POSIX_C_SOURCE >= 200112L)
25               || __GNU_SOURCE
26           Up to and including glibc 2.23:
27               _XOPEN_SOURCE
28

DESCRIPTION

30       getlogin()  returns  a  pointer  to a string containing the name of the
31       user logged in on the controlling terminal of the process,  or  a  null
32       pointer if this information cannot be determined.  The string is stati‐
33       cally allocated and might be overwritten on subsequent  calls  to  this
34       function or to cuserid().
35
36       getlogin_r()  returns  this same username in the array buf of size buf‐
37       size.
38
39       cuserid() returns a pointer to a string containing a  username  associ‐
40       ated  with  the  effective  user ID of the process.  If string is not a
41       null pointer, it should be an array that can hold  at  least  L_cuserid
42       characters; the string is returned in this array.  Otherwise, a pointer
43       to a string in a static area is returned.  This  string  is  statically
44       allocated and might be overwritten on subsequent calls to this function
45       or to getlogin().
46
47       The macro L_cuserid is an integer constant that indicates how  long  an
48       array  you  might  need  to store a username.  L_cuserid is declared in
49       <stdio.h>.
50
51       These functions let your program identify positively the  user  who  is
52       running  (cuserid())  or  the  user  who logged in this session (getlo‐
53       gin()).  (These can differ when set-user-ID programs are involved.)
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55       For most purposes, it is more useful to use  the  environment  variable
56       LOGNAME  to  find out who the user is.  This is more flexible precisely
57       because the user can set LOGNAME arbitrarily.
58

RETURN VALUE

60       getlogin() returns a pointer to the username when successful, and  NULL
61       on  failure, with errno set to indicate the cause of the error.  getlo‐
62       gin_r() returns 0 when successful, and nonzero on failure.
63

ERRORS

65       POSIX specifies
66
67       EMFILE The per-process limit on the number of open file descriptors has
68              been reached.
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70       ENFILE The system-wide limit on the total number of open files has been
71              reached.
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73       ENXIO  The calling process has no controlling terminal.
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75       ERANGE (getlogin_r) The length of the username, including the terminat‐
76              ing null byte ('\0'), is larger than bufsize.
77
78       Linux/glibc also has
79
80       ENOENT There was no corresponding entry in the utmp-file.
81
82       ENOMEM Insufficient memory to allocate passwd structure.
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84       ENOTTY Standard input didn't refer to a terminal.  (See BUGS.)
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FILES

87       /etc/passwd
88              password database file
89
90       /var/run/utmp
91              (traditionally /etc/utmp; some libc versions used /var/adm/utmp)
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ATTRIBUTES

94       For   an   explanation   of   the  terms  used  in  this  section,  see
95       attributes(7).
96
97       ┌─────────────┬───────────────┬───────────────────────────────────────┐
98Interface    Attribute     Value                                 
99       ├─────────────┼───────────────┼───────────────────────────────────────┤
100getlogin()   │ Thread safety │ MT-Unsafe race:getlogin race:utent    │
101       │             │               │ sig:ALRM timer locale                 │
102       ├─────────────┼───────────────┼───────────────────────────────────────┤
103getlogin_r() │ Thread safety │ MT-Unsafe race:utent sig:ALRM timer   │
104       │             │               │ locale                                │
105       ├─────────────┼───────────────┼───────────────────────────────────────┤
106cuserid()    │ Thread safety │ MT-Unsafe race:cuserid/!string locale │
107       └─────────────┴───────────────┴───────────────────────────────────────┘
108       In the above table, utent in race:utent signifies that if  any  of  the
109       functions setutent(3), getutent(3), or endutent(3) are used in parallel
110       in different threads of a program, then data races could occur.  getlo‐
111       gin()  and  getlogin_r()  call those functions, so we use race:utent to
112       remind users.
113

CONFORMING TO

115       getlogin() and getlogin_r(): POSIX.1-2001, POSIX.1-2008.
116
117       System V has a cuserid() function which uses the real  user  ID  rather
118       than the effective user ID.  The cuserid() function was included in the
119       1988 version of POSIX, but removed  from  the  1990  version.   It  was
120       present in SUSv2, but removed in POSIX.1-2001.
121
122       OpenBSD has getlogin() and setlogin(), and a username associated with a
123       session, even if it has no controlling terminal.
124

BUGS

126       Unfortunately, it is often rather easy to fool  getlogin().   Sometimes
127       it  does not work at all, because some program messed up the utmp file.
128       Often, it gives only the first 8 characters of  the  login  name.   The
129       user  currently  logged  in  on the controlling terminal of our program
130       need not be the user who started it.  Avoid  getlogin()  for  security-
131       related purposes.
132
133       Note  that glibc does not follow the POSIX specification and uses stdin
134       instead of /dev/tty.  A bug.  (Other recent systems, like SunOS 5.8 and
135       HP-UX  11.11  and FreeBSD 4.8 all return the login name also when stdin
136       is redirected.)
137
138       Nobody knows precisely what cuserid() does; avoid it in  portable  pro‐
139       grams.   Or  avoid  it  altogether: use getpwuid(geteuid()) instead, if
140       that is what you meant.  Do not use cuserid().
141

SEE ALSO

143       logname(1), geteuid(2), getuid(2), utmp(5)
144

COLOPHON

146       This page is part of release 4.15 of the Linux  man-pages  project.   A
147       description  of  the project, information about reporting bugs, and the
148       latest    version    of    this    page,    can     be     found     at
149       https://www.kernel.org/doc/man-pages/.
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153GNU                               2017-09-15                       GETLOGIN(3)
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