9.5. Sessions

A session is a collection of one or more process groups. For example, we could have the arrangement shown in Figure 9.6. Here we have three process groups in a single session.

Figure 9.6. Arrangement of processes into process groups and sessions

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The processes in a process group are usually placed there by a shell pipeline. For example, the arrangement shown in Figure 9.6 could have been generated by shell commands of the form

   proc1 | proc2 &
   proc3 | proc4 | proc5

A process establishes a new session by calling the setsid function.

#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.

This function returns an error if the caller is already a process group leader. To ensure this is not the case, the usual practice is to call fork and have the parent terminate and the child continue. We are guaranteed that the child is not a process group leader, because the process group ID of the parent is inherited by the child, but the child gets a new process ID. Hence, it is impossible for the child's process ID to equal its inherited process group ID.

The Single UNIX Specification talks only about a "session leader." There is no "session ID" similar to a process ID or a process group ID. Obviously, a session leader is a single process that has a unique process ID, so we could talk about a session ID that is the process ID of the session leader. This concept of a session ID was introduced in SVR4. Historically, BSD-based systems didn't support this notion, but have since been updated to include it. The getsid function returns the process group ID of a process's session leader. The getsid function is included as an XSI extension in the Single UNIX Specification.

Some implementations, such as Solaris, join with the Single UNIX Specification in the practice of avoiding the use of the phrase "session ID," opting instead to refer to this as the "process group ID of the session leader." The two are equivalent, since the session leader is always the leader of a process group.

#include <unistd.h> pid_t getsid(pid_t pid);
Returns: session leader's process group ID if OK, 1 on error

If pid is 0, getsid returns the process group ID of the calling process's session leader. For security reasons, some implementations may restrict the calling process from obtaining the process group ID of the session leader if pid doesn't belong to the same session as the caller.