man3/getlogin.3

   1 .\" Copyright 1995  James R. Van Zandt <jrv@vanzandt.mv.com>
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  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 2019-03-06 "GNU" "Linux Programmer's Manual"
  29 .SH NAME
  30 getlogin, getlogin_r, cuserid \- get username
  31 .SH SYNOPSIS
  32 .B #include <unistd.h>
  33 .PP
  34 .B "char *getlogin(void);"
  35 .br
  36 .BI "int getlogin_r(char *" buf ", size_t " bufsize );
  37 .PP
  38 .B #include <stdio.h>
  39 .PP
  40 .BI "char *cuserid(char *" string );
  41 .PP
  42 .in -4n
  43 Feature Test Macro Requirements for glibc (see
  44 .BR feature_test_macros (7)):
  45 .in
  46 .PP
  47 .BR getlogin_r ():
  48 .\" Deprecated: _REENTRANT ||
  49 _POSIX_C_SOURCE\ >=\ 199506L
  50 .PP
  51 .BR cuserid ():
  52 .nf
  53     Since glibc 2.24:
  54         (_XOPEN_SOURCE && ! (_POSIX_C_SOURCE >= 200112L)
  55         || _GNU_SOURCE
  56     Up to and including glibc 2.23:
  57         _XOPEN_SOURCE
  58 .fi
  59 .SH DESCRIPTION
  60 .BR getlogin ()
  61 returns a pointer to a string containing the name of
  62 the user logged in on the controlling terminal of the process, or a
  63 null pointer if this information cannot be determined.
  64 The string is
  65 statically allocated and might be overwritten on subsequent calls to
  66 this function or to
  67 .BR cuserid ().
  68 .PP
  69 .BR getlogin_r ()
  70 returns this same username in the array
  71 .I buf
  72 of size
  73 .IR bufsize .
  74 .PP
  75 .BR cuserid ()
  76 returns a pointer to a string containing a username
  77 associated with the effective user ID of the process.
  78 If \fIstring\fP
  79 is not a null pointer, it should be an array that can hold at least
  80 \fBL_cuserid\fP characters; the string is returned in this array.
  81 Otherwise, a pointer to a string in a static area is returned.
  82 This
  83 string is statically allocated and might be overwritten on subsequent
  84 calls to this function or to
  85 .BR getlogin ().
  86 .PP
  87 The macro \fBL_cuserid\fP is an integer constant that indicates how
  88 long an array you might need to store a username.
  89 \fBL_cuserid\fP is declared in \fI<stdio.h>\fP.
  90 .PP
  91 These functions let your program identify positively the user who is
  92 running
  93 .RB ( cuserid ())
  94 or the user who logged in this session
  95 .RB ( getlogin ()).
  96 (These can differ when set-user-ID programs are involved.)
  97 .PP
  98 For most purposes, it is more useful to use the environment variable
  99 \fBLOGNAME\fP to find out who the user is.
 100 This is more flexible
 101 precisely because the user can set \fBLOGNAME\fP arbitrarily.
 102 .SH RETURN VALUE
 103 .BR getlogin ()
 104 returns a pointer to the username when successful,
 105 and NULL on failure, with
 106 .I errno
 107 set to indicate the cause of the error.
 108 .BR getlogin_r ()
 109 returns 0 when successful, and nonzero on failure.
 110 .SH ERRORS
 111 POSIX specifies
 112 .TP
 113 .B EMFILE
 114 The per-process limit on the number of open file descriptors has been reached.
 115 .TP
 116 .B ENFILE
 117 The system-wide limit on the total number of open files has been reached.
 118 .TP
 119 .B ENXIO
 120 The calling process has no controlling terminal.
 121 .TP
 122 .B ERANGE
 123 (getlogin_r)
 124 The length of the username, including the terminating null byte (\(aq\e0\(aq),
 125 is larger than
 126 .IR bufsize .
 127 .PP
 128 Linux/glibc also has
 129 .TP
 130 .B ENOENT
 131 There was no corresponding entry in the utmp-file.
 132 .TP
 133 .B ENOMEM
 134 Insufficient memory to allocate passwd structure.
 135 .TP
 136 .B ENOTTY
 137 Standard input didn't refer to a terminal.
 138 (See BUGS.)
 139 .SH FILES
 140 .TP
 141 \fI/etc/passwd\fP
 142 password database file
 143 .TP
 144 \fI/var/run/utmp\fP
 145 (traditionally \fI/etc/utmp\fP;
 146 some libc versions used \fI/var/adm/utmp\fP)
 147 .SH ATTRIBUTES
 148 For an explanation of the terms used in this section, see
 149 .BR attributes (7).
 150 .TS
 151 allbox;
 152 lb lb lb
 153 l l l.
 154 Interface       Attribute       Value
 155 T{
 156 .BR getlogin ()
 157 T}      Thread safety   T{
 158 MT-Unsafe race:getlogin race:utent
 159 .br
 160 sig:ALRM timer locale
 161 T}
 162 T{
 163 .BR getlogin_r ()
 164 T}      Thread safety   T{
 165 MT-Unsafe race:utent sig:ALRM timer
 166 .br
 167 locale
 168 T}
 169 T{
 170 .BR cuserid ()
 171 T}      Thread safety   MT-Unsafe race:cuserid/!string locale
 172 .TE
 173 .sp 1
 174 In the above table,
 175 .I utent
 176 in
 177 .I race:utent
 178 signifies that if any of the functions
 179 .BR setutent (3),
 180 .BR getutent (3),
 181 or
 182 .BR endutent (3)
 183 are used in parallel in different threads of a program,
 184 then data races could occur.
 185 .BR getlogin ()
 186 and
 187 .BR getlogin_r ()
 188 call those functions,
 189 so we use race:utent to remind users.
 190 .SH CONFORMING TO
 191 .BR getlogin ()
 192 and
 193 .BR getlogin_r ():
 194 POSIX.1-2001, POSIX.1-2008.
 195 .PP
 196 System V has a
 197 .BR cuserid ()
 198 function which uses the real
 199 user ID rather than the effective user ID.
 200 The
 201 .BR cuserid ()
 202 function
 203 was included in the 1988 version of POSIX,
 204 but removed from the 1990 version.
 205 It was present in SUSv2, but removed in POSIX.1-2001.
 206 .PP
 207 OpenBSD has
 208 .BR getlogin ()
 209 and
 210 .BR setlogin (),
 211 and a username
 212 associated with a session, even if it has no controlling terminal.
 213 .SH BUGS
 214 Unfortunately, it is often rather easy to fool
 215 .BR getlogin ().
 216 Sometimes it does not work at all, because some program messed up
 217 the utmp file.
 218 Often, it gives only the first 8 characters of
 219 the login name.
 220 The user currently logged in on the controlling terminal
 221 of our program need not be the user who started it.
 222 Avoid
 223 .BR getlogin ()
 224 for security-related purposes.
 225 .PP
 226 Note that glibc does not follow the POSIX specification and uses
 227 .I stdin
 228 instead of
 229 .IR /dev/tty .
 230 A bug.
 231 (Other recent systems, like SunOS 5.8 and HP-UX 11.11 and FreeBSD 4.8
 232 all return the login name also when
 233 .I stdin
 234 is redirected.)
 235 .PP
 236 Nobody knows precisely what
 237 .BR cuserid ()
 238 does; avoid it in portable programs.
 239 Or avoid it altogether: use
 240 .I getpwuid(geteuid())
 241 instead, if that is
 242 what you meant.
 243 .B Do not use
 244 .BR cuserid ().
 245 .SH SEE ALSO
 246 .BR logname (1),
 247 .BR geteuid (2),
 248 .BR getuid (2),
 249 .BR utmp (5)