1GETLOGIN(3) Linux Programmer's Manual GETLOGIN(3)
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6 getlogin, getlogin_r, cuserid - get username
7
9 #include <unistd.h>
10 char *getlogin(void);
12 int getlogin_r(char *buf, size_t bufsize);
13 #include <stdio.h>
15 char *cuserid(char *string);
17 Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
19 getlogin_r(): _REENTRANT || _POSIX_C_SOURCE >= 199506L
21 cuserid(): _XOPEN_SOURCE
22
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().
29 getlogin_r() returns this same username in the array buf of size buf‐
31 size.
32 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().
40 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>.
44 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.)
48 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.
52
54 getlogin() returns a pointer to the username when successful, and NULL
55 on failure, with errno set to indicate the cause of the error. getlo‐
56 gin_r() returns 0 when successful, and nonzero on failure.
57
59 POSIX specifies
60 EMFILE The calling process already has the maximum allowed number of
62 open files.
63 ENFILE The system already has the maximum allowed number of open files.
65 ENXIO The calling process has no controlling terminal.
67 ERANGE (getlogin_r) The length of the username, including the terminat‐
69 ing null byte ('\0'), is larger than bufsize.
70 Linux/glibc also has
72 ENOENT There was no corresponding entry in the utmp-file.
74 ENOMEM Insufficient memory to allocate passwd structure.
76 ENOTTY Standard input didn't refer to a terminal. (See BUGS.)
78
80 /etc/passwd
81 password database file
82 /var/run/utmp
84 (traditionally /etc/utmp; some libc versions used /var/adm/utmp)
85
87 Multithreading (see pthreads(7))
88 The getlogin() function is not thread-safe.
89 The getlogin_r() function is thread-safe.
91 The cuserid() function is thread-safe with exceptions. It is not
93 thread-safe if called with a NULL parameter.
94
96 getlogin() and getlogin_r() specified in POSIX.1-2001.
97 System V has a cuserid() function which uses the real user ID rather
99 than the effective user ID. The cuserid() function was included in the
100 1988 version of POSIX, but removed from the 1990 version. It was
101 present in SUSv2, but removed in POSIX.1-2001.
102 OpenBSD has getlogin() and setlogin(), and a username associated with a
104 session, even if it has no controlling terminal.
105
107 Unfortunately, it is often rather easy to fool getlogin(). Sometimes
108 it does not work at all, because some program messed up the utmp file.
109 Often, it gives only the first 8 characters of the login name. The
110 user currently logged in on the controlling terminal of our program
111 need not be the user who started it. Avoid getlogin() for security-
112 related purposes.
113 Note that glibc does not follow the POSIX specification and uses stdin
115 instead of /dev/tty. A bug. (Other recent systems, like SunOS 5.8 and
116 HP-UX 11.11 and FreeBSD 4.8 all return the login name also when stdin
117 is redirected.)
118 Nobody knows precisely what cuserid() does; avoid it in portable pro‐
120 grams. Or avoid it altogether: use getpwuid(geteuid()) instead, if
121 that is what you meant. Do not use cuserid().
122
124 geteuid(2), getuid(2), utmp(5)
125
127 This page is part of release 3.53 of the Linux man-pages project. A
128 description of the project, and information about reporting bugs, can
129 be found at http://www.kernel.org/doc/man-pages/.
130GNU 2013-04-19 GETLOGIN(3)