Advanced Programming in the UNIX® Environment

ptg10805159

Section 9.11FreeBSD Implementation 311

• s_leaderis a pointer to theprocstructure of the session leader.


• s_ttyvpis a pointer to thevnodestructure of the controlling terminal.


• s_ttypis a pointer to thettystructure of the controlling terminal.


• s_sidis the session ID. Recall that the concept of a session ID is not part of the
  Single UNIX Specification.

Whensetsidis called, a newsessionstructureisallocated within the kernel.

Nows_countis set to 1,s_leaderis set to point to theprocstructure of the calling

process,s_sidis set to the process ID, ands_ttyvpands_ttypareset to null

pointers, since the new session doesn’t have a controlling terminal.

Let’s move to thettystructure. The kernel contains one of these structures for

each terminal device and each pseudo terminal device. (Wetalk moreabout pseudo

terminals in Chapter 19.)

• t_sessionpoints to the sessionstructurethat has this terminal as its
  controlling terminal. (Note that the tty structurepoints to the session
  structure, and vice versa.) This pointer is used by the terminal to send a hang-
  up signal to the session leader if the terminal loses carrier (Figure9.7).


• t_pgrppoints to thepgrpstructureofthe foreground process group. This field
  is used by the terminal driver to send signals to the foreground process group.
  The three signals generated by entering special characters (interrupt, quit, and
  suspend) aresent to the foreground process group.


• t_termios is a structurecontaining all the special characters and related
  information for this terminal, such as baud rate, whether echo is enabled, and so
  on. We’ll return to this structure in Chapter 18.


• t_winsizeis awinsizestructurethat contains the current size of the terminal
  window.When the size of the terminal window changes, theSIGWINCHsignal
  is sent to the foreground process group. Weshow how to set and fetch the
  terminal’s current window size in Section 18.12.

To find the foreground process group of a particular session, the kernel has to start with

the session structure, follows_ttypto get to the controlling terminal’sttystructure,

and then followt_pgrpto get to the foreground process group’spgrpstructure. The

pgrpstructurecontains the information for a particular process group.

• pg_idis the process group ID.


• pg_sessionpoints to the sessionstructurefor the session to which this
  process group belongs.


• pg_membersis a pointer to the list ofprocstructures that aremembers of this
  process group. Thep_pgliststructureinthatprocstructure is a doubly
  linked list entry that points to both the next process and the previous process in
  the group, and so on, until a null pointer is encountered in theprocstructureof
  the last process in the group.