1getlogin(3) Library Functions Manual getlogin(3)
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6 getlogin, getlogin_r, cuserid - get username
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9 Standard C library (libc, -lc)
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12 #include <unistd.h>
13 char *getlogin(void);
15 int getlogin_r(char buf[.bufsize], size_t bufsize);
16 #include <stdio.h>
18 char *cuserid(char *string);
20 Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
22 getlogin_r():
24 _POSIX_C_SOURCE >= 199506L
25 cuserid():
27 Since glibc 2.24:
28 (_XOPEN_SOURCE && ! (_POSIX_C_SOURCE >= 200112L)
29 || _GNU_SOURCE
30 Up to and including glibc 2.23:
31 _XOPEN_SOURCE
32
34 getlogin() returns a pointer to a string containing the name of the
35 user logged in on the controlling terminal of the process, or a null
36 pointer if this information cannot be determined. The string is stati‐
37 cally allocated and might be overwritten on subsequent calls to this
38 function or to cuserid().
39 getlogin_r() returns this same username in the array buf of size buf‐
41 size.
42 cuserid() returns a pointer to a string containing a username associ‐
44 ated with the effective user ID of the process. If string is not a
45 null pointer, it should be an array that can hold at least L_cuserid
46 characters; the string is returned in this array. Otherwise, a pointer
47 to a string in a static area is returned. This string is statically
48 allocated and might be overwritten on subsequent calls to this function
49 or to getlogin().
50 The macro L_cuserid is an integer constant that indicates how long an
52 array you might need to store a username. L_cuserid is declared in
53 <stdio.h>.
54 These functions let your program identify positively the user who is
56 running (cuserid()) or the user who logged in this session (getlo‐
57 gin()). (These can differ when set-user-ID programs are involved.)
58 For most purposes, it is more useful to use the environment variable
60 LOGNAME to find out who the user is. This is more flexible precisely
61 because the user can set LOGNAME arbitrarily.
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64 getlogin() returns a pointer to the username when successful, and NULL
65 on failure, with errno set to indicate the error. getlogin_r() returns
66 0 when successful, and nonzero on failure.
67
69 POSIX specifies:
70 EMFILE The per-process limit on the number of open file descriptors has
72 been reached.
73 ENFILE The system-wide limit on the total number of open files has been
75 reached.
76 ENXIO The calling process has no controlling terminal.
78 ERANGE (getlogin_r) The length of the username, including the terminat‐
80 ing null byte ('\0'), is larger than bufsize.
81 Linux/glibc also has:
83 ENOENT There was no corresponding entry in the utmp-file.
85 ENOMEM Insufficient memory to allocate passwd structure.
87 ENOTTY Standard input didn't refer to a terminal. (See BUGS.)
89
91 /etc/passwd
92 password database file
93 /var/run/utmp
95 (traditionally /etc/utmp; some libc versions used /var/adm/utmp)
96
98 For an explanation of the terms used in this section, see at‐
99 tributes(7).
100 ┌─────────────┬───────────────┬────────────────────────────────────────┐
102 │Interface │ Attribute │ Value │
103 ├─────────────┼───────────────┼────────────────────────────────────────┤
104 │getlogin() │ Thread safety │ MT-Unsafe race:getlogin race:utent │
105 │ │ │ sig:ALRM timer locale │
106 ├─────────────┼───────────────┼────────────────────────────────────────┤
107 │getlogin_r() │ Thread safety │ MT-Unsafe race:utent sig:ALRM timer │
108 │ │ │ locale │
109 ├─────────────┼───────────────┼────────────────────────────────────────┤
110 │cuserid() │ Thread safety │ MT-Unsafe race:cuserid/!string locale │
111 └─────────────┴───────────────┴────────────────────────────────────────┘
112 In the above table, utent in race:utent signifies that if any of the
113 functions setutent(3), getutent(3), or endutent(3) are used in parallel
114 in different threads of a program, then data races could occur. getlo‐
115 gin() and getlogin_r() call those functions, so we use race:utent to
116 remind users.
117
119 OpenBSD has getlogin() and setlogin(), and a username associated with a
120 session, even if it has no controlling terminal.
121
123 getlogin()
124 getlogin_r()
125 POSIX.1-2008.
126 cuserid()
128 None.
129
131 getlogin()
132 getlogin_r():
133 POSIX.1-2001. OpenBSD.
134 cuserid()
136 System V, POSIX.1-1988. Removed in POSIX.1-1990. SUSv2. Re‐
137 moved in POSIX.1-2001.
138 System V has a cuserid() function which uses the real user ID
140 rather than the effective user ID.
141
143 Unfortunately, it is often rather easy to fool getlogin(). Sometimes
144 it does not work at all, because some program messed up the utmp file.
145 Often, it gives only the first 8 characters of the login name. The us‐
146 er currently logged in on the controlling terminal of our program need
147 not be the user who started it. Avoid getlogin() for security-related
148 purposes.
149 Note that glibc does not follow the POSIX specification and uses stdin
151 instead of /dev/tty. A bug. (Other recent systems, like SunOS 5.8 and
152 HP-UX 11.11 and FreeBSD 4.8 all return the login name also when stdin
153 is redirected.)
154 Nobody knows precisely what cuserid() does; avoid it in portable pro‐
156 grams. Or avoid it altogether: use getpwuid(geteuid()) instead, if
157 that is what you meant. Do not use cuserid().
158
160 logname(1), geteuid(2), getuid(2), utmp(5)
161Linux man-pages 6.04 2023-03-30 getlogin(3)