getlogin(3)

1GETLOGIN(3)                Linux Programmer's Manual               GETLOGIN(3)
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NAME

6       getlogin, getlogin_r, cuserid - get user name
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SYNOPSIS

9       #include <unistd.h>
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11       char *getlogin(void);
12       int getlogin_r(char *buf, size_t bufsize);
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14       #include <stdio.h>
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16       char *cuserid(char *string);
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DESCRIPTION

19       getlogin()  returns  a  pointer  to a string containing the name of the
20       user logged in on the controlling terminal of the process,  or  a  null
21       pointer if this information cannot be determined.  The string is stati‐
22       cally allocated and might be overwritten on subsequent  calls  to  this
23       function or to cuserid().
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25       getlogin_r()  returns this same user name in the array buf of size buf‐
26       size.
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28       cuserid() returns a pointer to a string containing a user name  associ‐
29       ated  with  the  effective  user ID of the process.  If string is not a
30       null pointer, it should be an array that can hold  at  least  L_cuserid
31       characters; the string is returned in this array.  Otherwise, a pointer
32       to a string in a static area is returned.  This  string  is  statically
33       allocated and might be overwritten on subsequent calls to this function
34       or to getlogin().
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36       The macro L_cuserid is an integer constant that indicates how  long  an
37       array  you  might  need to store a user name.  L_cuserid is declared in
38       stdio.h.
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40       These functions let your program identify positively the  user  who  is
41       running  (cuserid())  or  the  user  who logged in this session (getlo‐
42       gin()).  (These can differ when set-user-ID programs are involved.)
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44       For most purposes, it is more useful to use  the  environment  variable
45       LOGNAME  to  find out who the user is.  This is more flexible precisely
46       because the user can set LOGNAME arbitrarily.
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RETURN VALUE

49       getlogin() returns a pointer to the user name when successful, and NULL
50       on  failure.   getlogin_r()  returns 0 when successful, and non-zero on
51       failure.
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ERRORS

54       POSIX specifies
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56       EMFILE The calling process already has the maximum  allowed  number  of
57              open files.
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59       ENFILE The system already has the maximum allowed number of open files.
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61       ENXIO  The calling process has no controlling tty.
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63       ERANGE (getlogin_r)  The  length of the user name, including the termi‐
64              nating null byte, is larger than bufsize.
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66       Linux/glibc also has
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68       ENOENT There was no corresponding entry in the utmp-file.
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70       ENOMEM Insufficient memory to allocate passwd structure.
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FILES

73       /etc/passwd    password database file
74       /var/run/utmp  (traditionally /etc/utmp;
75                      some libc versions used /var/adm/utmp)
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CONFORMING TO

78       getlogin() and getlogin_r() specified in POSIX.1-2001.
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80       System V has a cuserid() function which uses the real  user  ID  rather
81       than the effective user ID.  The cuserid() function was included in the
82       1988 version of POSIX, but removed  from  the  1990  version.   It  was
83       present in SUSv2, but removed in POSIX.1-2001.
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85       OpenBSD has getlogin() and setlogin(), and a username associated with a
86       session, even if it has no controlling tty.
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BUGS

89       Unfortunately, it is often rather easy to fool  getlogin().   Sometimes
90       it  does not work at all, because some program messed up the utmp file.
91       Often, it gives only the first 8 characters of the login name. The user
92       currently  logged  in on the controlling tty of our program need not be
93       the user who started it.  Avoid getlogin()  for  security-related  pur‐
94       poses.
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96       Note  that  glibc does not follow the POSIX spec and uses stdin instead
97       of /dev/tty.  A bug. (Other recent systems, like SunOS  5.8  and  HP-UX
98       11.11  and  FreeBSD  4.8  all  return the login name also when stdin is
99       redirected.)
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101       Nobody knows precisely what cuserid() does; avoid it in  portable  pro‐
102       grams.   Or  avoid  it  altogether: use getpwuid(geteuid()) instead, if
103       that is what you meant.  DO NOT USE cuserid().
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