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 2015-03-02 "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 .br
  50 .BR cuserid ():
  51 _XOPEN_SOURCE
  52 .SH DESCRIPTION
  53 .BR getlogin ()
  54 returns a pointer to a string containing the name of
  55 the user logged in on the controlling terminal of the process, or a
  56 null pointer if this information cannot be determined.
  57 The string is
  58 statically allocated and might be overwritten on subsequent calls to
  59 this function or to
  60 .BR cuserid ().
  61 .PP
  62 .BR getlogin_r ()
  63 returns this same username in the array
  64 .I buf
  65 of size
  66 .IR bufsize .
  67 .PP
  68 .BR cuserid ()
  69 returns a pointer to a string containing a username
  70 associated with the effective user ID of the process.
  71 If \fIstring\fP
  72 is not a null pointer, it should be an array that can hold at least
  73 \fBL_cuserid\fP characters; the string is returned in this array.
  74 Otherwise, a pointer to a string in a static area is returned.
  75 This
  76 string is statically allocated and might be overwritten on subsequent
  77 calls to this function or to
  78 .BR getlogin ().
  79 .PP
  80 The macro \fBL_cuserid\fP is an integer constant that indicates how
  81 long an array you might need to store a username.
  82 \fBL_cuserid\fP is declared in \fI<stdio.h>\fP.
  83 .PP
  84 These functions let your program identify positively the user who is
  85 running
  86 .RB ( cuserid ())
  87 or the user who logged in this session
  88 .RB ( getlogin ()).
  89 (These can differ when set-user-ID programs are involved.)
  90 .PP
  91 For most purposes, it is more useful to use the environment variable
  92 \fBLOGNAME\fP to find out who the user is.
  93 This is more flexible
  94 precisely because the user can set \fBLOGNAME\fP arbitrarily.
  95 .SH RETURN VALUE
  96 .BR getlogin ()
  97 returns a pointer to the username when successful,
  98 and NULL on failure, with
  99 .I errno
 100 set to indicate the cause of the error.
 101 .BR getlogin_r ()
 102 returns 0 when successful, and nonzero on failure.
 103 .SH ERRORS
 104 POSIX specifies
 105 .TP
 106 .B EMFILE
 107 The calling process already has the maximum allowed number of open files.
 108 .TP
 109 .B ENFILE
 110 The system already has the maximum allowed number of open files.
 111 .TP
 112 .B ENXIO
 113 The calling process has no controlling terminal.
 114 .TP
 115 .B ERANGE
 116 (getlogin_r)
 117 The length of the username, including the terminating null byte (\(aq\\0\(aq),
 118 is larger than
 119 .IR bufsize .
 120 .LP
 121 Linux/glibc also has
 122 .TP
 123 .B ENOENT
 124 There was no corresponding entry in the utmp-file.
 125 .TP
 126 .B ENOMEM
 127 Insufficient memory to allocate passwd structure.
 128 .TP
 129 .B ENOTTY
 130 Standard input didn't refer to a terminal.
 131 (See BUGS.)
 132 .SH FILES
 133 .TP
 134 \fI/etc/passwd\fP
 135 password database file
 136 .TP
 137 \fI/var/run/utmp\fP
 138 (traditionally \fI/etc/utmp\fP;
 139 some libc versions used \fI/var/adm/utmp\fP)
 140 .SH ATTRIBUTES
 141 For an explanation of the terms used in this section, see
 142 .BR attributes (7).
 143 .TS
 144 allbox;
 145 lb lb lb
 146 l l l.
 147 Interface       Attribute       Value
 148 T{
 149 .BR getlogin ()
 150 T}      Thread safety   T{
 151 MT-Unsafe race:getlogin race:utent
 152 .br
 153 sig:ALRM timer locale
 154 T}
 155 T{
 156 .BR getlogin_r ()
 157 T}      Thread safety   T{
 158 MT-Unsafe race:utent sig:ALRM timer
 159 .br
 160 locale
 161 T}
 162 T{
 163 .BR cuserid ()
 164 T}      Thread safety   MT-Unsafe race:cuserid/!string locale
 165 .TE
 166
 167 In the above table,
 168 .I utent
 169 in
 170 .I race:utent
 171 signifies that if any of the functions
 172 .BR setutent (3),
 173 .BR getutent (3),
 174 or
 175 .BR endutent (3)
 176 are used in parallel in different threads of a program,
 177 then data races could occur.
 178 .BR getlogin ()
 179 and
 180 .BR getlogin_r ()
 181 call those functions,
 182 so we use race:utent to remind users.
 183 .SH CONFORMING TO
 184 .BR getlogin ()
 185 and
 186 .BR getlogin_r ():
 187 POSIX.1-2001, POSIX.1-2008.
 188
 189 System V has a
 190 .BR cuserid ()
 191 function which uses the real
 192 user ID rather than the effective user ID.
 193 The
 194 .BR cuserid ()
 195 function
 196 was included in the 1988 version of POSIX,
 197 but removed from the 1990 version.
 198 It was present in SUSv2, but removed in POSIX.1-2001.
 199 .LP
 200 OpenBSD has
 201 .BR getlogin ()
 202 and
 203 .BR setlogin (),
 204 and a username
 205 associated with a session, even if it has no controlling terminal.
 206 .SH BUGS
 207 Unfortunately, it is often rather easy to fool
 208 .BR getlogin ().
 209 Sometimes it does not work at all, because some program messed up
 210 the utmp file.
 211 Often, it gives only the first 8 characters of
 212 the login name.
 213 The user currently logged in on the controlling terminal
 214 of our program need not be the user who started it.
 215 Avoid
 216 .BR getlogin ()
 217 for security-related purposes.
 218 .LP
 219 Note that glibc does not follow the POSIX specification and uses
 220 .I stdin
 221 instead of
 222 .IR /dev/tty .
 223 A bug.
 224 (Other recent systems, like SunOS 5.8 and HP-UX 11.11 and FreeBSD 4.8
 225 all return the login name also when
 226 .I stdin
 227 is redirected.)
 228 .LP
 229 Nobody knows precisely what
 230 .BR cuserid ()
 231 does; avoid it in portable programs.
 232 Or avoid it altogether: use
 233 .I getpwuid(geteuid())
 234 instead, if that is
 235 what you meant.
 236 .B Do not use
 237 .BR cuserid ().
 238 .SH SEE ALSO
 239 .BR geteuid (2),
 240 .BR getuid (2),
 241 .BR utmp (5)