D.3 Multiple sessions per process on some platforms

Some ports of PuTTY - notably the in-progress Mac port - are constrained by the operating system to run as a single process potentially managing multiple sessions.

Therefore, the platform-independent parts of PuTTY never use global variables to store per-session data. The global variables that do exist are tolerated because they are not specific to a particular login session: flags defines properties that are expected to apply equally to all the sessions run by a single PuTTY process, the random number state in sshrand.c and the timer list in timing.c serve all sessions equally, and so on. But most data is specific to a particular network session, and is therefore stored in dynamically allocated data structures, and pointers to these structures are passed around between functions.

Platform-specific code can reverse this decision if it likes. The Windows code, for historical reasons, stores most of its data as global variables. That's OK, because on Windows we know there is only one session per PuTTY process, so it's safe to do that. But changes to the platform-independent code should avoid introducing global variables, unless they are genuinely cross-session.