1 | NAME |
1 | NAME |
2 | libptytty - OS independent and secure pty/tty and utmp/wtmp/lastlog |
2 | libptytty - OS independent and secure pty/tty and utmp/wtmp/lastlog |
3 | handling |
3 | handling |
4 | |
4 | |
5 | SYNOPSIS |
5 | SYNOPSIS |
6 | -lptytty |
6 | cc ... -lptytty |
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7 | |
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8 | #include <libptytty.h> |
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9 | |
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10 | |
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11 | // C++ |
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12 | ptytty *pty = ptytty::create (); |
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13 | |
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14 | if (!pty->get ()) |
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15 | // error allocating pty |
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16 | |
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17 | if (we want utmp) |
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18 | pty->login (process_pid, 0, "remote.host"); |
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19 | else if (we want utmp AND wtmp/lastlog) |
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20 | pty->login (process_pid, 1, "remote.host"); |
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21 | |
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22 | // we are done with it |
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23 | delete pty; |
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24 | |
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25 | |
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26 | // C |
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27 | PTYTTY pty = ptytty_create (); |
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28 | |
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29 | if (!ptytty_get (pty)) |
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30 | // error allocating pty |
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31 | |
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32 | if (we want utmp) |
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33 | ptytty_login (pty, process_pid, 0, "remote.host"); |
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34 | else if (we want utmp AND wtmp/lastlog) |
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35 | ptytty_login (pty, process_pid, 1, "remote.host"); |
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36 | |
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37 | // we are done with it |
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38 | ptytty_delete (pty); |
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39 | |
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40 | See also the eg/ directory, which currently contains the c-sample.c file |
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41 | that spawns a login shell from C using libptytty. |
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42 | |
8 | DESCRIPTION |
43 | DESCRIPTION |
9 | THE ptytty CLASS |
44 | Libptytty is a small library that offers pseudo-tty management in an |
10 | new ptytty |
45 | OS-independent way. It was created out of frustration over the many |
11 | ... |
46 | differences of pty/tty handling in different operating systems for the |
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47 | use inside "rxvt-unicode". |
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48 | |
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49 | In addition to offering mere pty/tty management, it also offers session |
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50 | database support (utmp and optional wtmp/lastlog updates for login |
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51 | shells). |
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52 | |
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53 | It also supports fork'ing after startup and dropping privileges in the |
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54 | calling process, so in case the calling process gets compromised by the |
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55 | user starting the program there is less to gain, as only the helper |
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56 | process runs with privileges (e.g. setuid/setgid), which reduces the |
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57 | area of attack immensely. |
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58 | |
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59 | Libptytty is written in C++, but it also offers a C-only API. |
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60 | |
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61 | SECURITY CONSIDERATIONS |
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62 | *It is of paramount importance that you at least read the following |
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63 | paragraph!* |
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64 | |
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65 | If you write a typical terminal-like program that just wants one or more |
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66 | ptys, you should call the "ptytty::init ()" method (C: "ptytty_init ()" |
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67 | function) as the very first thing in your program: |
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68 | |
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69 | int main (int argc, char *argv[]) |
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70 | { |
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71 | // do nothing here |
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72 | ptytty::init (); |
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73 | // in C: ptytty_init (); |
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74 | |
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75 | // initialise, parse arguments, etc. |
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76 | } |
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77 | |
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78 | This checks whether the program runs setuid or setgid. If yes then it |
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79 | will fork a helper process and drop privileges. |
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80 | |
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81 | Some programs need finer control over if and when this helper process is |
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82 | started, and if and how to drop privileges. For those programs, the |
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83 | methods "ptytty::use_helper" and "ptytty::drop_privileges" (and possibly |
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84 | "ptytty::sanitise_stdfd") are more useful. |
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85 | |
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86 | C++ INTERFACE: THE ptytty CLASS |
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87 | STATIC METHODS |
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88 | ptytty::init () |
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89 | The default way to initialise libptytty. Must be called immediately |
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90 | as the first thing in the "main" function, or earlier e.g. during |
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91 | static construction time. The earlier, the better. |
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92 | |
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93 | This method calls "sanitise_stdfd" and then checks whether the |
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94 | program runs with setuid/setgid permissions and, if yes, spawns a |
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95 | helper process for pty/tty management. It then drops the privileges |
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96 | completely, so the actual program runs without setuid/setgid |
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97 | privileges. |
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98 | |
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99 | ptytty::use_helper () |
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100 | Tries to start a helper process that retains privileges even when |
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101 | the calling process does not. This is usually called from |
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102 | "ptytty::init" when it detects that the program is running setuid or |
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103 | setgid, but can be called manually if it is inconvenient to drop |
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104 | privileges at startup, or when you are not running setuid/setgid but |
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105 | want to drop privileges (e.g. when running as a root-started |
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106 | daemon). |
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107 | |
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108 | This method will try not to start more than one helper process. The |
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109 | same helper process can usually be used both from the process |
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110 | starting it and all its fork'ed (not exec'ed) children. |
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111 | |
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112 | ptytty::drop_privileges () |
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113 | Drops privileges completely, i.e. sets real, effective and saved |
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114 | user id to the real user id. Also aborts if this cannot be achieved. |
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115 | Useful to make sure that the process doesn't run with special |
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116 | privileges. |
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117 | |
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118 | ptytty::sanitise_stdfd () |
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119 | Checks whether file descriptors 0, 1 and 2 (stdin, stdout and |
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120 | stderr) are valid (open) and, if not, connects them to /dev/tty or |
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121 | /dev/null if possible (and aborts otherwise). This is necessary |
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122 | because libptytty might want to output error messages to those |
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123 | descriptors, which at the time of outputting the error message, |
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124 | might be connected to something unsuitable opened by the |
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125 | unsuspecting program itself (this can be a security issue). |
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126 | |
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127 | bool success = ptytty::send_fd (int socket, int fd) |
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128 | Utility method to send a file descriptor over a unix domain socket. |
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129 | Returns true if successful, false otherwise. This method is only |
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130 | exposed for your convenience and is not required for normal |
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131 | operation. |
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132 | |
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133 | int fd = ptytty::recv_fd (int socket) |
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134 | Utility method to receive a file descriptor over a unix domain |
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135 | socket. Returns the fd if successful and -1 otherwise. This method |
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136 | is only exposed for your convenience and is not required for normal |
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137 | operation. |
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138 | |
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139 | ptytty *pty = ptytty::create () |
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140 | Creates new ptytty object. Creation does not yet do anything besides |
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141 | allocating the structure. |
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142 | |
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143 | A static method is used because the actual ptytty implementation can |
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144 | differ at runtime, so you need a dynamic object creation facility. |
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145 | |
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146 | DYNAMIC/SESSION-RELATED DATA MEMBERS AND METHODS |
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147 | int pty_fd = pty->pty |
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148 | int tty_fd = pty->tty |
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149 | These members contain the pty and tty file descriptors, |
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150 | respectively. They initially contain -1 until a successful to |
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151 | "ptytty::get". |
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152 | |
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153 | bool success = pty->get () |
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154 | Tries to find, allocate and initialise a new pty/tty pair. Returns |
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155 | "true" when successful. |
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156 | |
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157 | pty->login (int cmd_pid, bool login_shell, const char *hostname) |
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158 | Creates an entry in the systems session database(s) (utmp, wtmp, |
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159 | lastlog). "cmd_pid" must be the pid of the process representing the |
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160 | session (such as the login shell), "login_shell" defines whether the |
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161 | session is associated with a login, which influences whether wtmp |
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162 | and lastlog entries are created, and "hostname" should identify the |
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163 | "hostname" the user logs in from, which often is the value of the |
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164 | "DISPLAY" variable or tty line in case of local logins. |
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165 | |
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166 | Calling this method is optional. A session starts at the time of the |
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167 | login call and extends until the ptytty object is destroyed. |
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168 | |
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169 | pty->close_tty () |
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170 | Closes the tty. Useful after forking in the parent/pty process. |
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171 | |
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172 | bool success = pty->make_controlling_tty () |
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173 | Tries to make the pty/tty pair the controlling terminal of the |
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174 | current process. Useful after forking in the child/tty process. |
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175 | |
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176 | pty->set_utf8_mode (bool on) |
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177 | On systems supporting special UTF-8 line disciplines (e.g. Linux), |
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178 | this tries to enable this discipline for the given pty. Can be |
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179 | called at any time to change the mode. |
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180 | |
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181 | C INTERFACE: THE ptytty FAMILY OF FUNCTIONS |
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182 | ptytty_init () |
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183 | See "ptytty::init ()". |
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184 | |
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185 | PTYTTY ptytty_create () |
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186 | Creates a new opaque PTYTTY object and returns it. Do not try to |
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187 | access it in any way except by testing it for truthness (e.g. "if |
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188 | (pty) ...."). See "ptytty::create ()". |
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189 | |
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190 | int ptytty_pty (PTYTTY ptytty) |
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191 | Return the pty file descriptor. See "pty->pty". |
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192 | |
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193 | int ptytty_tty (PTYTTY ptytty) |
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194 | Return the tty file descriptor. See "pty->tty". |
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195 | |
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196 | void ptytty_delete (PTYTTY ptytty) |
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197 | Destroys the PTYTTY object, freeing the pty/tty pair and cleaning up |
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198 | the utmp/wtmp/lastlog databases, if initialised/used. Same as |
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199 | "delete pty" in C++. |
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200 | |
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201 | int ptytty_get (PTYTTY ptytty) |
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202 | See "pty->get", returns 0 in case of an error, non-zero otherwise. |
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203 | |
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204 | void ptytty_login (PTYTTY ptytty, int cmd_pid, bool login_shell, const |
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205 | char *hostname) |
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206 | See "pty->login". |
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207 | |
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208 | void ptytty_close_tty (PTYTTY ptytty) |
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209 | See "pty->close_tty". |
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210 | |
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211 | int ptytty_make_controlling_tty (PTYTTY ptytty) |
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212 | See "pty->make_controlling_tty". |
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213 | |
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214 | void ptytty_set_utf8_mode (PTYTTY ptytty, int on) |
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215 | See "pty->set_utf8_mode". |
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216 | |
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217 | void ptytty_drop_privileges () |
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218 | See "ptytty::drop_privileges". |
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219 | |
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220 | void ptytty_use_helper () |
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221 | See "ptytty::use_helper". |
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13 | BUGS |
223 | BUGS |
14 | You kiddin'? |
224 | You kiddin'? |
15 | |
225 | |
16 | AUTHORS |
226 | AUTHORS |