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

6       login, logout - write utmp and wtmp entries
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SYNOPSIS

9       #include <utmp.h>
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11       void login(const struct utmp *ut);
12       int logout(const char *ut_line);
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14       Link with -lutil.
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DESCRIPTION

17       The utmp file records who is currently using the system.  The wtmp file
18       records all logins and logouts.  See utmp(5).
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20       The function login() takes the supplied struct utmp, ut, and writes  it
21       to both the utmp and the wtmp file.
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23       The function logout() clears the entry in the utmp file again.
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25   GNU details
26       More  precisely,  login() takes the argument ut struct, fills the field
27       ut->ut_type (if there is such a field) with the value USER_PROCESS, and
28       fills  the field ut->ut_pid (if there is such a field) with the process
29       ID  of  the  calling  process.   Then  it  tries  to  fill  the   field
30       ut->ut_line.   It  takes  the  first of stdin, stdout, stderr that is a
31       terminal, and stores the corresponding pathname minus a possible  lead‐
32       ing /dev/ into this field, and then writes the struct to the utmp file.
33       On the other hand, if no terminal name was found, this field is  filled
34       with  "???"   and  the  struct  is not written to the utmp file.  After
35       this, the struct is written to the wtmp file.
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37       The logout() function searches the utmp file for an entry matching  the
38       ut_line  argument.   If a record is found, it is updated by zeroing out
39       the ut_name and ut_host fields, updating the ut_tv timestamp field  and
40       setting ut_type (if there is such a field) to DEAD_PROCESS.
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RETURN VALUE

43       The  logout()  function returns 1 if the entry was successfully written
44       to the database, or 0 if an error occurred.
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FILES

47       /var/run/utmp
48              user  accounting  database,  configured  through  _PATH_UTMP  in
49              <paths.h>
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51       /var/log/wtmp
52              user  accounting  log  file,  configured  through  _PATH_WTMP in
53              <paths.h>
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ATTRIBUTES

56       For an  explanation  of  the  terms  used  in  this  section,  see  at‐
57       tributes(7).
58
59       ┌──────────────────┬───────────────┬───────────────────────────────────┐
60Interface         Attribute     Value                             
61       ├──────────────────┼───────────────┼───────────────────────────────────┤
62login(), logout() │ Thread safety │ MT-Unsafe race:utent sig:ALRM     │
63       │                  │               │ timer                             │
64       └──────────────────┴───────────────┴───────────────────────────────────┘
65       In the above table, utent in race:utent signifies that if  any  of  the
66       functions setutent(3), getutent(3), or endutent(3) are used in parallel
67       in different threads of a program, then data races  could  occur.   lo‐
68       gin()  and  logout() calls those functions, so we use race:utent to re‐
69       mind users.
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CONFORMING TO

72       Not in POSIX.1.  Present on the BSDs.
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NOTES

75       Note that the member ut_user of struct utmp is called ut_name  in  BSD.
76       Therefore, ut_name is defined as an alias for ut_user in <utmp.h>.
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SEE ALSO

79       getutent(3), utmp(5)
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COLOPHON

82       This  page  is  part of release 5.12 of the Linux man-pages project.  A
83       description of the project, information about reporting bugs,  and  the
84       latest     version     of     this    page,    can    be    found    at
85       https://www.kernel.org/doc/man-pages/.
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89GNU                               2021-03-22                          LOGIN(3)
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