CREDENTIALS(7)

credentials - process identifiers

NOTE:

一、"credential"的中文意思是"凭证"，它其实描述的是各种process identifiers

二、从下面的内容可以看出，Linux process的"credential"比较多

Process ID (PID)

Each process has a unique nonnegative integer identifier that is assigned when the process is created using fork(2). A process can obtain its PID using getpid(2). A PID is represented using the type pid_t (defined in <sys/types.h>).

PIDs are used in a range of system calls to identify the process affected by the call, for example: , ptrace(2), setpriority(2) setpgid(2), setsid(2), sigqueue(3), and waitpid(2).

A process's PID is preserved across an execve(2).

Parent process ID (PPID)

NOTE: 父进程ID

Process group ID and session ID

NOTE:

关于此，APUE中，讲得最好，并且还有图片

Each process has a session ID and a process group ID, both represented using the type pid_t. A process can obtain its session ID using getsid(2), and its process group ID using getpgrp(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.

NOTE: 如果session leader不去open terminal，则这个session就没有controlling terminal了；

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.

System call on process group

Various system calls and library functions may operate on all members of a process group, including:

1、kill(2),

2、killpg(3)

...

User and group identifiers

Each process has various associated user and group IDs. These IDs are integers, respectively represented using the types uid_t and gid_t (defined in <sys/types.h>).

Modifying process user and group IDs

NOTE:

介绍了修改 process user and group IDs的system call