man3/getlogin.3

   1 .\" Copyright 1995  James R. Van Zandt <jrv@vanzandt.mv.com>
   2 .\"
   3 .\" %%%LICENSE_START(VERBATIM)
   4 .\" Permission is granted to make and distribute verbatim copies of this
   5 .\" manual provided the copyright notice and this permission notice are
   6 .\" preserved on all copies.
   7 .\"
   8 .\" Permission is granted to copy and distribute modified versions of this
   9 .\" manual under the conditions for verbatim copying, provided that the
  10 .\" entire resulting derived work is distributed under the terms of a
  11 .\" permission notice identical to this one.
  12 .\"
  13 .\" Since the Linux kernel and libraries are constantly changing, this
  14 .\" manual page may be incorrect or out-of-date.  The author(s) assume no
  15 .\" responsibility for errors or omissions, or for damages resulting from
  16 .\" the use of the information contained herein.  The author(s) may not
  17 .\" have taken the same level of care in the production of this manual,
  18 .\" which is licensed free of charge, as they might when working
  19 .\" professionally.
  20 .\"
  21 .\" Formatted or processed versions of this manual, if unaccompanied by
  22 .\" the source, must acknowledge the copyright and authors of this work.
  23 .\" %%%LICENSE_END
  24 .\"
  25 .\" Changed Tue Sep 19 01:49:29 1995, aeb: moved from man2 to man3
  26 .\"  added ref to /etc/utmp, added BUGS section, etc.
  27 .\" modified 2003 Walter Harms, aeb - added getlogin_r, note on stdin use
  28 .TH GETLOGIN 3 2016-10-08 "GNU" "Linux Programmer's Manual"
  29 .SH NAME
  30 getlogin, getlogin_r, cuserid \- get username
  31 .SH SYNOPSIS
  32 .B #include <unistd.h>
  33 .sp
  34 .B "char *getlogin(void);"
  35 .br
  36 .BI "int getlogin_r(char *" buf ", size_t " bufsize );
  37 .sp
  38 .B #include <stdio.h>
  39 .sp
  40 .BI "char *cuserid(char *" string );
  41 .sp
  42 .in -4n
  43 Feature Test Macro Requirements for glibc (see
  44 .BR feature_test_macros (7)):
  45 .in
  46 .sp
  47 .BR getlogin_r ():
  48 _REENTRANT || _POSIX_C_SOURCE\ >=\ 199506L
  49 .sp
  50 .BR cuserid ():
  51 .nf
  52     Since glibc 2.24:
  53         (_XOPEN_SOURCE && ! (_POSIX_C_SOURCE >= 200112L)
  54         || __GNU_SOURCE
  55     Up to and including glibc 2.23:
  56         _XOPEN_SOURCE
  57 .fi
  58 .SH DESCRIPTION
  59 .BR getlogin ()
  60 returns a pointer to a string containing the name of
  61 the user logged in on the controlling terminal of the process, or a
  62 null pointer if this information cannot be determined.
  63 The string is
  64 statically allocated and might be overwritten on subsequent calls to
  65 this function or to
  66 .BR cuserid ().
  67 .PP
  68 .BR getlogin_r ()
  69 returns this same username in the array
  70 .I buf
  71 of size
  72 .IR bufsize .
  73 .PP
  74 .BR cuserid ()
  75 returns a pointer to a string containing a username
  76 associated with the effective user ID of the process.
  77 If \fIstring\fP
  78 is not a null pointer, it should be an array that can hold at least
  79 \fBL_cuserid\fP characters; the string is returned in this array.
  80 Otherwise, a pointer to a string in a static area is returned.
  81 This
  82 string is statically allocated and might be overwritten on subsequent
  83 calls to this function or to
  84 .BR getlogin ().
  85 .PP
  86 The macro \fBL_cuserid\fP is an integer constant that indicates how
  87 long an array you might need to store a username.
  88 \fBL_cuserid\fP is declared in \fI<stdio.h>\fP.
  89 .PP
  90 These functions let your program identify positively the user who is
  91 running
  92 .RB ( cuserid ())
  93 or the user who logged in this session
  94 .RB ( getlogin ()).
  95 (These can differ when set-user-ID programs are involved.)
  96 .PP
  97 For most purposes, it is more useful to use the environment variable
  98 \fBLOGNAME\fP to find out who the user is.
  99 This is more flexible
 100 precisely because the user can set \fBLOGNAME\fP arbitrarily.
 101 .SH RETURN VALUE
 102 .BR getlogin ()
 103 returns a pointer to the username when successful,
 104 and NULL on failure, with
 105 .I errno
 106 set to indicate the cause of the error.
 107 .BR getlogin_r ()
 108 returns 0 when successful, and nonzero on failure.
 109 .SH ERRORS
 110 POSIX specifies
 111 .TP
 112 .B EMFILE
 113 The per-process limit on the number of open file descriptors has been reached.
 114 .TP
 115 .B ENFILE
 116 The system-wide limit on the total number of open files has been reached.
 117 .TP
 118 .B ENXIO
 119 The calling process has no controlling terminal.
 120 .TP
 121 .B ERANGE
 122 (getlogin_r)
 123 The length of the username, including the terminating null byte (\(aq\\0\(aq),
 124 is larger than
 125 .IR bufsize .
 126 .LP
 127 Linux/glibc also has
 128 .TP
 129 .B ENOENT
 130 There was no corresponding entry in the utmp-file.
 131 .TP
 132 .B ENOMEM
 133 Insufficient memory to allocate passwd structure.
 134 .TP
 135 .B ENOTTY
 136 Standard input didn't refer to a terminal.
 137 (See BUGS.)
 138 .SH FILES
 139 .TP
 140 \fI/etc/passwd\fP
 141 password database file
 142 .TP
 143 \fI/var/run/utmp\fP
 144 (traditionally \fI/etc/utmp\fP;
 145 some libc versions used \fI/var/adm/utmp\fP)
 146 .SH ATTRIBUTES
 147 For an explanation of the terms used in this section, see
 148 .BR attributes (7).
 149 .TS
 150 allbox;
 151 lb lb lb
 152 l l l.
 153 Interface       Attribute       Value
 154 T{
 155 .BR getlogin ()
 156 T}      Thread safety   T{
 157 MT-Unsafe race:getlogin race:utent
 158 .br
 159 sig:ALRM timer locale
 160 T}
 161 T{
 162 .BR getlogin_r ()
 163 T}      Thread safety   T{
 164 MT-Unsafe race:utent sig:ALRM timer
 165 .br
 166 locale
 167 T}
 168 T{
 169 .BR cuserid ()
 170 T}      Thread safety   MT-Unsafe race:cuserid/!string locale
 171 .TE
 172
 173 In the above table,
 174 .I utent
 175 in
 176 .I race:utent
 177 signifies that if any of the functions
 178 .BR setutent (3),
 179 .BR getutent (3),
 180 or
 181 .BR endutent (3)
 182 are used in parallel in different threads of a program,
 183 then data races could occur.
 184 .BR getlogin ()
 185 and
 186 .BR getlogin_r ()
 187 call those functions,
 188 so we use race:utent to remind users.
 189 .SH CONFORMING TO
 190 .BR getlogin ()
 191 and
 192 .BR getlogin_r ():
 193 POSIX.1-2001, POSIX.1-2008.
 194
 195 System V has a
 196 .BR cuserid ()
 197 function which uses the real
 198 user ID rather than the effective user ID.
 199 The
 200 .BR cuserid ()
 201 function
 202 was included in the 1988 version of POSIX,
 203 but removed from the 1990 version.
 204 It was present in SUSv2, but removed in POSIX.1-2001.
 205 .LP
 206 OpenBSD has
 207 .BR getlogin ()
 208 and
 209 .BR setlogin (),
 210 and a username
 211 associated with a session, even if it has no controlling terminal.
 212 .SH BUGS
 213 Unfortunately, it is often rather easy to fool
 214 .BR getlogin ().
 215 Sometimes it does not work at all, because some program messed up
 216 the utmp file.
 217 Often, it gives only the first 8 characters of
 218 the login name.
 219 The user currently logged in on the controlling terminal
 220 of our program need not be the user who started it.
 221 Avoid
 222 .BR getlogin ()
 223 for security-related purposes.
 224 .LP
 225 Note that glibc does not follow the POSIX specification and uses
 226 .I stdin
 227 instead of
 228 .IR /dev/tty .
 229 A bug.
 230 (Other recent systems, like SunOS 5.8 and HP-UX 11.11 and FreeBSD 4.8
 231 all return the login name also when
 232 .I stdin
 233 is redirected.)
 234 .LP
 235 Nobody knows precisely what
 236 .BR cuserid ()
 237 does; avoid it in portable programs.
 238 Or avoid it altogether: use
 239 .I getpwuid(geteuid())
 240 instead, if that is
 241 what you meant.
 242 .B Do not use
 243 .BR cuserid ().
 244 .SH SEE ALSO
 245 .BR logname (1),
 246 .BR geteuid (2),
 247 .BR getuid (2),
 248 .BR utmp (5)