man3/getlogin.3

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   3 .\" 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 2003-08-24 "Linux 2.4" "Linux Programmer's Manual"
  29 .SH NAME
  30 getlogin, getlogin_r, cuserid \- get user name
  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 .SH DESCRIPTION
  42 \fBgetlogin\fP returns a pointer to a string containing the name of
  43 the user logged in on the controlling terminal of the process, or a
  44 null pointer if this information cannot be determined.  The string is
  45 statically allocated and might be overwritten on subsequent calls to
  46 this function or to \fBcuserid\fP.
  47 .PP
  48 \fBgetlogin_r\fP returns this same user name in the array
  49 .I buf
  50 of size
  51 .IR bufsize .
  52 .PP
  53 \fBcuserid\fP returns a pointer to a string containing a user name
  54 associated with the effective user ID of the process.  If \fIstring\fP
  55 is not a null pointer, it should be an array that can hold at least
  56 \fBL_cuserid\fP characters; the string is returned in this array.
  57 Otherwise, a pointer to a string in a static area is returned. This
  58 string is statically allocated and might be overwritten on subsequent
  59 calls to this function or to \fBgetlogin\fP.
  60 .PP
  61 The macro \fBL_cuserid\fP is an integer constant that indicates how
  62 long an array you might need to store a user name.  \fBL_cuserid\fP is
  63 declared in \fBstdio.h\fP.
  64 .PP
  65 These functions let your program identify positively the user who is
  66 running (\fBcuserid\fP) or the user who logged in this session
  67 (\fBgetlogin\fP).  (These can differ when set-user-ID programs are
  68 involved.)
  69 .PP
  70 For most purposes, it is more useful to use the environment variable
  71 \fBLOGNAME\fP to find out who the user is.  This is more flexible
  72 precisely because the user can set \fBLOGNAME\fP arbitrarily.
  73 .SH "RETURN VALUE"
  74 \fBgetlogin\fP returns a pointer to the user name when successful,
  75 and NULL on failure.
  76 \fBgetlogin_r\fP returns 0 when successful, and non-zero on failure.
  77 .SH ERRORS
  78 POSIX specifies
  79 .TP
  80 .B EMFILE
  81 The calling process already has the maximum allowed number of open files.
  82 .TP
  83 .B ENFILE
  84 The system already has the maximum allowed number of open files.
  85 .TP
  86 .B ENXIO
  87 The calling process has no controlling tty.
  88 .TP
  89 .B ERANGE
  90 (getlogin_r)
  91 The length of the user name, including final NUL, is larger than
  92 .IR bufsize .
  93 .LP
  94 Linux/glibc also has
  95 .TP
  96 .B ENOENT
  97 There was no corresponding entry in the utmp-file.
  98 .TP
  99 .B ENOMEM
 100 Insufficient memory to allocate passwd structure.
 101 .SH FILES
 102 .nf
 103 \fI/etc/passwd\fP       password database file
 104 .br
 105 \fI/var/run/utmp\fP     (traditionally \fI/etc/utmp\fP;
 106                         some libc versions used \fI/var/adm/utmp\fP)
 107 .fi
 108 .SH "CONFORMING TO"
 109 POSIX.1.  System V has a \fBcuserid\fP function which uses the real
 110 user ID rather than the effective user ID. The \fBcuserid\fP function
 111 was included in the 1988 version of POSIX, but removed from the 1990 version.
 112 .LP
 113 OpenBSD has \fBgetlogin\fP and \fBsetlogin\fP, and a username
 114 associated with a session, even if it has no controlling tty.
 115 .SH BUGS
 116 Unfortunately, it is often rather easy to fool getlogin().
 117 Sometimes it does not work at all, because some program messed up
 118 the utmp file. Often, it gives only the first 8 characters of
 119 the login name. The user currently logged in on the controlling tty
 120 of our program need not be the user who started it.
 121 Avoid getlogin() for security-related purposes.
 122 .LP
 123 Note that glibc does not follow the POSIX spec and uses stdin
 124 instead of
 125 .IR /dev/tty .
 126 A bug. (Other recent systems, like SunOS 5.8 and HPUX 11.11 and FreeBSD 4.8
 127 all return the login name also when stdin is redirected.)
 128 .LP
 129 Nobody knows precisely what cuserid() does; avoid it in portable programs.
 130 Or avoid it altogether: use getpwuid(geteuid()) instead, if that is
 131 what you meant.
 132 DO NOT USE cuserid().
 133 .SH "SEE ALSO"
 134 .BR geteuid (2),
 135 .BR getuid (2)