Advanced Programming in the UNIX® Environment

ptg10805159

Section 9.5 Sessions 295

9.5 Sessions

Asession is a collection of one or moreprocess groups. For example, we could have the

arrangement shown in Figure9.6. Here we have three process groups in a single

session.

login shell proc1 proc2 proc3 proc4

proc5

process group process group

process group

session

Figure 9.6Arrangement of processes into process groups and sessions

The processes in a process group areusually placed there by a shell pipeline. For

example, the arrangement shown in Figure9.6 could have been generated by shell

commands of the form

proc1 | proc2 &

proc3 | proc4 | proc5

Aprocess establishes a new session by calling thesetsidfunction.

#include <unistd.h>

pid_t setsid(void);

Returns: process group ID if OK,−1 on error

If the calling process is not a process group leader,this function creates a new session.

Three things happen.

1. The process becomes thesession leaderof this new session. (A session leader is
   the process that creates a session.) The process is the only process in this new
   session.


2. The process becomes the process group leader of a new process group. The new
   process group ID is the process ID of the calling process.


3. The process has no controlling terminal. (We’ll discuss controlling terminals in
   the next section.) If the process had a controlling terminal beforecalling
   setsid,that association is broken.