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 2008-06-29 "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.
  99 .BR getlogin_r ()
 100 returns 0 when successful, and nonzero on failure.
 101 .SH ERRORS
 102 POSIX specifies
 103 .TP
 104 .B EMFILE
 105 The calling process already has the maximum allowed number of open files.
 106 .TP
 107 .B ENFILE
 108 The system already has the maximum allowed number of open files.
 109 .TP
 110 .B ENXIO
 111 The calling process has no controlling terminal.
 112 .TP
 113 .B ERANGE
 114 (getlogin_r)
 115 The length of the username, including the terminating null byte,
 116 is larger than
 117 .IR bufsize .
 118 .LP
 119 Linux/glibc also has
 120 .TP
 121 .B ENOENT
 122 There was no corresponding entry in the utmp-file.
 123 .TP
 124 .B ENOMEM
 125 Insufficient memory to allocate passwd structure.
 126 .TP
 127 .B ENOTTY
 128 Standard input didn't refer to a terminal.
 129 (See BUGS.)
 130 .SH FILES
 131 .TP
 132 \fI/etc/passwd\fP
 133 password database file
 134 .TP
 135 \fI/var/run/utmp\fP
 136 (traditionally \fI/etc/utmp\fP;
 137 some libc versions used \fI/var/adm/utmp\fP)
 138 .SH CONFORMING TO
 139 .BR getlogin ()
 140 and
 141 .BR getlogin_r ()
 142 specified in POSIX.1-2001.
 143
 144 System V has a
 145 .BR cuserid ()
 146 function which uses the real
 147 user ID rather than the effective user ID.
 148 The
 149 .BR cuserid ()
 150 function
 151 was included in the 1988 version of POSIX,
 152 but removed from the 1990 version.
 153 It was present in SUSv2, but removed in POSIX.1-2001.
 154 .LP
 155 OpenBSD has
 156 .BR getlogin ()
 157 and
 158 .BR setlogin (),
 159 and a username
 160 associated with a session, even if it has no controlling terminal.
 161 .SH BUGS
 162 Unfortunately, it is often rather easy to fool
 163 .BR getlogin ().
 164 Sometimes it does not work at all, because some program messed up
 165 the utmp file.
 166 Often, it gives only the first 8 characters of
 167 the login name.
 168 The user currently logged in on the controlling terminal
 169 of our program need not be the user who started it.
 170 Avoid
 171 .BR getlogin ()
 172 for security-related purposes.
 173 .LP
 174 Note that glibc does not follow the POSIX specification and uses
 175 .I stdin
 176 instead of
 177 .IR /dev/tty .
 178 A bug.
 179 (Other recent systems, like SunOS 5.8 and HP-UX 11.11 and FreeBSD 4.8
 180 all return the login name also when
 181 .I stdin
 182 is redirected.)
 183 .LP
 184 Nobody knows precisely what
 185 .BR cuserid ()
 186 does; avoid it in portable programs.
 187 Or avoid it altogether: use
 188 .I getpwuid(geteuid())
 189 instead, if that is
 190 what you meant.
 191 .B Do not use
 192 .BR cuserid ().
 193 .SH SEE ALSO
 194 .BR geteuid (2),
 195 .BR getuid (2),
 196 .BR utmp (5)