cuserid(3)

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

6       getlogin, getlogin_r, cuserid - get username
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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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18   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
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20       getlogin_r(): _REENTRANT || _POSIX_C_SOURCE >= 199506L
21       cuserid(): _XOPEN_SOURCE
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DESCRIPTION

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

54       getlogin() returns a pointer to the username when successful, and  NULL
55       on  failure, with errno set to indicate the cause of the error.  getlo‐
56       gin_r() returns 0 when successful, and nonzero on failure.
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ERRORS

59       POSIX specifies
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61       EMFILE The calling process already has the maximum  allowed  number  of
62              open files.
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64       ENFILE The system already has the maximum allowed number of open files.
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66       ENXIO  The calling process has no controlling terminal.
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68       ERANGE (getlogin_r) The length of the username, including the terminat‐
69              ing null byte ('\0'), is larger than bufsize.
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71       Linux/glibc also has
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73       ENOENT There was no corresponding entry in the utmp-file.
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75       ENOMEM Insufficient memory to allocate passwd structure.
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77       ENOTTY Standard input didn't refer to a terminal.  (See BUGS.)
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FILES

80       /etc/passwd
81              password database file
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83       /var/run/utmp
84              (traditionally /etc/utmp; some libc versions used /var/adm/utmp)
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ATTRIBUTES

87   Multithreading (see pthreads(7))
88       The getlogin() function is not thread-safe.
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90       The getlogin_r() function is thread-safe.
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92       The cuserid() function is  thread-safe  with  exceptions.   It  is  not
93       thread-safe if called with a NULL parameter.
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CONFORMING TO

96       getlogin() and getlogin_r() specified in POSIX.1-2001.
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98       System  V  has  a cuserid() function which uses the real user ID rather
99       than the effective user ID.  The cuserid() function was included in the
100       1988  version  of  POSIX,  but  removed  from the 1990 version.  It was
101       present in SUSv2, but removed in POSIX.1-2001.
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103       OpenBSD has getlogin() and setlogin(), and a username associated with a
104       session, even if it has no controlling terminal.
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BUGS

107       Unfortunately,  it  is often rather easy to fool getlogin().  Sometimes
108       it does not work at all, because some program messed up the utmp  file.
109       Often,  it  gives  only  the first 8 characters of the login name.  The
110       user currently logged in on the controlling  terminal  of  our  program
111       need  not  be  the user who started it.  Avoid getlogin() for security-
112       related purposes.
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114       Note that glibc does not follow the POSIX specification and uses  stdin
115       instead of /dev/tty.  A bug.  (Other recent systems, like SunOS 5.8 and
116       HP-UX 11.11 and FreeBSD 4.8 all return the login name also  when  stdin
117       is redirected.)
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119       Nobody  knows  precisely what cuserid() does; avoid it in portable pro‐
120       grams.  Or avoid it altogether:  use  getpwuid(geteuid())  instead,  if
121       that is what you meant.  Do not use cuserid().
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COLOPHON

127       This  page  is  part of release 3.53 of the Linux man-pages project.  A
128       description of the project, and information about reporting  bugs,  can
129       be found at http://www.kernel.org/doc/man-pages/.
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133GNU                               2013-04-19                       GETLOGIN(3)