man3/login.3

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  25 .TH LOGIN 3 2017-09-15 "GNU" "Linux Programmer's Manual"
  26 .SH NAME
  27 login, logout \- write utmp and wtmp entries
  28 .SH SYNOPSIS
  29 .B #include <utmp.h>
  30 .PP
  31 .BI "void login(const struct utmp *" ut );
  32 .PP
  33 .BI "int logout(const char *" ut_line );
  34 .PP
  35 Link with \fI\-lutil\fP.
  36 .SH DESCRIPTION
  37 The utmp file records who is currently using the system.
  38 The wtmp file records all logins and logouts.
  39 See
  40 .BR utmp (5).
  41 .PP
  42 The function
  43 .BR login ()
  44 takes the supplied
  45 .IR "struct utmp" ,
  46 .IR ut ,
  47 and writes it to both the utmp and the wtmp file.
  48 .PP
  49 The function
  50 .BR logout ()
  51 clears the entry in the utmp file again.
  52 .SS GNU details
  53 More precisely,
  54 .BR login ()
  55 takes the argument
  56 .I ut
  57 struct, fills the field
  58 .I ut\->ut_type
  59 (if there is such a field) with the value
  60 .BR USER_PROCESS ,
  61 and fills the field
  62 .I ut\->ut_pid
  63 (if there is such a field) with the process ID of the calling process.
  64 Then it tries to fill the field
  65 .IR ut\->ut_line .
  66 It takes the first of
  67 .IR stdin ,
  68 .IR stdout ,
  69 .I stderr
  70 that is a terminal, and
  71 stores the corresponding pathname minus a possible leading
  72 .I /dev/
  73 into this field, and then writes the struct to the utmp file.
  74 On the other hand,
  75 if no terminal name was found, this field is filled with "???"
  76 and the struct is not written to the utmp file.
  77 After this, the struct is written to the wtmp file.
  78 .PP
  79 The
  80 .BR logout ()
  81 function searches the utmp file for an entry matching the
  82 .I ut_line
  83 argument.
  84 If a record is found, it is updated by zeroing out the
  85 .I ut_name
  86 and
  87 .I ut_host
  88 fields, updating the
  89 .I ut_tv
  90 timestamp field and setting
  91 .I ut_type
  92 (if there is such a field) to
  93 .BR DEAD_PROCESS .
  94 .SH RETURN VALUE
  95 The
  96 .BR logout ()
  97 function returns 1 if the entry was successfully written to the
  98 database, or 0 if an error occurred.
  99 .SH FILES
 100 .TP
 101 .I /var/run/utmp
 102 user accounting database, configured through
 103 .B _PATH_UTMP
 104 in
 105 .I <paths.h>
 106 .TP
 107 .I /var/log/wtmp
 108 user accounting log file, configured through
 109 .B _PATH_WTMP
 110 in
 111 .I <paths.h>
 112 .SH ATTRIBUTES
 113 For an explanation of the terms used in this section, see
 114 .BR attributes (7).
 115 .TS
 116 allbox;
 117 lb lb lbw20
 118 l l l.
 119 Interface       Attribute       Value
 120 T{
 121 .BR login (),
 122 .br
 123 .BR logout ()
 124 T}      Thread safety   T{
 125 MT-Unsafe race:utent
 126 .br
 127 sig:ALRM timer
 128 T}
 129 .TE
 130 .sp 1
 131 In the above table,
 132 .I utent
 133 in
 134 .I race:utent
 135 signifies that if any of the functions
 136 .BR setutent (3),
 137 .BR getutent (3),
 138 or
 139 .BR endutent (3)
 140 are used in parallel in different threads of a program,
 141 then data races could occur.
 142 .BR login ()
 143 and
 144 .BR logout ()
 145 calls those functions,
 146 so we use race:utent to remind users.
 147 .SH CONFORMING TO
 148 Not in POSIX.1.
 149 Present on the BSDs.
 150 .SH NOTES
 151 Note that the
 152 member
 153 .I ut_user
 154 of
 155 .I struct utmp
 156 is called
 157 .I ut_name
 158 in BSD.
 159 Therefore,
 160 .I ut_name
 161 is defined as an alias for
 162 .I ut_user
 163 in
 164 .IR <utmp.h> .
 165 .SH SEE ALSO
 166 .BR getutent (3),
 167 .BR utmp (5)