Advanced Programming in the UNIX® Environment

ptg10805159

310 Process Relationships Chapter 9

9.11 FreeBSD Implementation

Having talked about the various attributes of a process, process group, session, and

controlling terminal, it’s worth looking at how all this can be implemented. We’ll look

briefly at the implementation used by FreeBSD. Some details of the SVR4

implementation of these features can be found in Williams[ 1989 ].Figure9.13 shows the

various data structures used by FreeBSD.

p_pglist

p_pid

p_pptr

p_pgrp

procstructure

p_pglist

p_pid

p_pptr

p_pgrp

procstructure

p_pglist

p_pid

p_pptr

p_pgrp

procstructure

pg_id

pg_session

pg_members

pgrpstructure

s_count

s_leader

s_ttyvp

s_ttyp

s_sid

sessionstructure

v_data

t_session vnodestructure

t_pgrp

t_termios

t_winsize

ttystructure

foreground

process group

linked list of

process group members

Figure 9.13 FreeBSD implementation of sessions and process groups

Let’s look at all the fields that we’ve labeled, starting with thesessionstructure.

One of these structures is allocated for each session (e.g., each timesetsidis called).

• s_countis the number of process groups in the session. When this counter is
  decremented to 0, the structurecan be freed.