TTYs
Overview
Terminals are one of the most important parts of how programs interact with users. Usually programs print output into them and read user input from. For example when one executes any program from a shell, the shell provides terminal peer into it, so the program output will be seen in a calling shell and a user can do additional processing (say grepping and etc).
Supported terminal types
There are wide range of terminals exist in linux world: unix98 psesudoterminals (pty), bsd terminals, virtual terminals (vt) and etc. In CRIU we support the pty completely and vt in a way full enough for work. By full support we mean complete save of pty internal state including queued data. In turn for vt only plain restoration supported which means it is complete enough to do a traditional operations with terminals after restore but if there was some data queued and not yet delivered to a terminal user it will be lost. It is worth to mention that such situation should not be treated as any kind of error - the terminal transport layer never ever be one with guaranteed data delivery.
Overall CRIU supports the following terminals:
- console
- current
- virtual
- external
- serial
- unix98
Console terminal
console terminal is the most simple one. Its restore is just literally open(/dev/console).
Current terminal
current terminal is rather an abstraction over real terminal an application uses. Upon its opening the kernel simply provides back the reference to the real one thus the same way as for console its restore is just open(/dev/tty) with one exception - it must be restored last, ie after all other terminals are restored.
Virtual terminal
vt stands for ttyN devices for which restore we simply do open(/dev/ttyN), where N is a number.
External terminal
external terminals stands for cases when file descriptors known to be changing between checkpoint/restore cycles and passed from command line options, see Inheriting FDs on restore for details. For this kind of terminals we are relying the file descriptors already opened and passed available so on restore we simply reuse them.
Serial terminal
serial terminals are supported for debug purpose mostly, in particular some developers pass through into virtual machine with this terminals. Its restore as simple as plain open() call.
Unix98 terminal
pty terminals are most commonly used over all other kinds. The pty represent a pair of peers: upon open(/dev/ptmx) the kernel automatically create /dev/pts/N slave peer, where N is a numeric index of the pair opened. Thus on restore we simply need ptmx device and give process master and slave descriptors.