|CREDENTIALS(7)||Linux Programmer's Manual||CREDENTIALS(7)|
PIDs are used in a range of system calls to identify the process affected by the call, for example: kill(2), ptrace(2), setpriority(2) setpgid(2), setsid(2), sigqueue(3), and waitpid(2).
A process's PID is preserved across an execve(2).
A process's PPID is preserved across an execve(2).
A child created by fork(2) inherits its parent's session ID and process group ID. A process's session ID and process group ID are preserved across an execve(2).
Sessions and process groups are abstractions devised to support shell job control. A process group (sometimes called a "job") is a collection of processes that share the same process group ID; the shell creates a new process group for the process(es) used to execute single command or pipeline (e.g., the two processes created to execute the command "ls | wc" are placed in the same process group). A process's group membership can be set using setpgid(2). The process whose process ID is the same as its process group ID is the process group leader for that group.
A session is a collection of processes that share the same session ID. All of the members of a process group also have the same session ID (i.e., all of the members of a process group always belong to the same session, so that sessions and process groups form a strict two-level hierarchy of processes.) A new session is created when a process calls setsid(2), which creates a new session whose session ID is the same as the PID of the process that called setsid(2). The creator of the session is called the session leader.
All of the processes in a session share a controlling terminal. The controlling terminal is established when the session leader first opens a terminal (unless the O_NOCTTY flag is specified when calling open(2)). A terminal may be the controlling terminal of at most one session.
At most one of the jobs in a session may be the foreground job; other jobs in the session are background jobs. Only the foreground job may read from the terminal; when a process in the background attempts to read from the terminal, its process group is sent a SIGTTIN signal, which suspends the job. If the TOSTOP flag has been set for the terminal (see termios(3)), then only the foreground job may write to the terminal; writes from background job cause a SIGTTOU signal to be generated, which suspends the job. When terminal keys that generate a signal (such as the interrupt key, normally control-C) are pressed, the signal is sent to the processes in the foreground job.
Various system calls and library functions may operate on all members of a process group, including kill(2), killpg(3), getpriority(2), setpriority(2), ioprio_get(2), ioprio_set(2), waitid(2), and waitpid(2). See also the discussion of the F_GETOWN, F_GETOWN_EX, F_SETOWN, and F_SETOWN_EX operations in fcntl(2).
On Linux, each process has the following user and group identifiers:
A child process created by fork(2) inherits copies of its parent's user and groups IDs. During an execve(2), a process's real user and group ID and supplementary group IDs are preserved; the effective and saved set IDs may be changed, as described in execve(2).
Aside from the purposes noted above, a process's user IDs are also employed in a number of other contexts:
The POSIX threads specification requires that credentials are shared by all of the threads in a process. However, at the kernel level, Linux maintains separate user and group credentials for each thread. The NPTL threading implementation does some work to ensure that any change to user or group credentials (e.g., calls to setuid(2), setresuid(2)) is carried through to all of the POSIX threads in a process. See nptl(7) for further details.