1CREDENTIALS(7) Linux Programmer's Manual CREDENTIALS(7)
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6 credentials - process identifiers
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9 Process ID (PID)
10 Each process has a unique nonnegative integer identifier that is
11 assigned when the process is created using fork(2). A process can
12 obtain its PID using getpid(2). A PID is represented using the type
13 pid_t (defined in <sys/types.h>).
14 PIDs are used in a range of system calls to identify the process
16 affected by the call, for example: kill(2), ptrace(2), setpriority(2)
17 setpgid(2), setsid(2), sigqueue(3), and waitpid(2).
18 A process's PID is preserved across an execve(2).
20 Parent process ID (PPID)
22 A process's parent process ID identifies the process that created this
23 process using fork(2). A process can obtain its PPID using getppid(2).
24 A PPID is represented using the type pid_t.
25 A process's PPID is preserved across an execve(2).
27 Process group ID and session ID
29 Each process has a session ID and a process group ID, both represented
30 using the type pid_t. A process can obtain its session ID using get‐
31 sid(2), and its process group ID using getpgrp(2).
32 A child created by fork(2) inherits its parent's session ID and process
34 group ID. A process's session ID and process group ID are preserved
35 across an execve(2).
36 Sessions and process groups are abstractions devised to support shell
38 job control. A process group (sometimes called a "job") is a collec‐
39 tion of processes that share the same process group ID; the shell cre‐
40 ates a new process group for the process(es) used to execute single
41 command or pipeline (e.g., the two processes created to execute the
42 command "ls | wc" are placed in the same process group). A process's
43 group membership can be set using setpgid(2). The process whose
44 process ID is the same as its process group ID is the process group
45 leader for that group.
46 A session is a collection of processes that share the same session ID.
48 All of the members of a process group also have the same session ID
49 (i.e., all of the members of a process group always belong to the same
50 session, so that sessions and process groups form a strict two-level
51 hierarchy of processes.) A new session is created when a process calls
52 setsid(2), which creates a new session whose session ID is the same as
53 the PID of the process that called setsid(2). The creator of the ses‐
54 sion is called the session leader.
55 All of the processes in a session share a controlling terminal. The
57 controlling terminal is established when the session leader first opens
58 a terminal (unless the O_NOCTTY flag is specified when calling
59 open(2)). A terminal may be the controlling terminal of at most one
60 session.
61 At most one of the jobs in a session may be the foreground job; other
63 jobs in the session are background jobs. Only the foreground job may
64 read from the terminal; when a process in the background attempts to
65 read from the terminal, its process group is sent a SIGTTIN signal,
66 which suspen