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 suspends the job. If the TOSTOP flag has been set for the termi‐
67 nal (see termios(3)), then only the foreground job may write to the
68 terminal; writes from background job cause a SIGTTOU signal to be gen‐
69 erated, which suspends the job. When terminal keys that generate a
70 signal (such as the interrupt key, normally control-C) are pressed, the
71 signal is sent to the processes in the foreground job.
72 Various system calls and library functions may operate on all members
74 of a process group, including kill(2), killpg(3), getpriority(2), set‐
75 priority(2), ioprio_get(2), ioprio_set(2), waitid(2), and waitpid(2).
76 See also the discussion of the F_GETOWN, F_GETOWN_EX, F_SETOWN, and
77 F_SETOWN_EX operations in fcntl(2).
78 User and group identifiers
80 Each process has various associated user and group IDs. These IDs are
81 integers, respectively represented using the types uid_t and gid_t
82 (defined in <sys/types.h>).
83 On Linux, each process has the following user and group identifiers:
85 * Real user ID and real group ID. These IDs determine who owns the
87 process. A process can obtain its real user (group) ID using
88 getuid(2) (getgid(2)).
89 * Effective user ID and effective group ID. These IDs are used by the
91 kernel to determine the permissions that the process will have when
92 accessing shared resources such as message queues, shared memory,
93 and semaphores. On most UNIX systems, these IDs also determine the
94 permissions when accessing files. However, Linux uses the filesys‐
95 tem IDs described below for this task. A process can obtain its
96 effective user (group) ID using geteuid(2) (getegid(2)).
97 * Saved set-user-ID and saved set-group-ID. These IDs are used in
99 set-user-ID and set-group-ID programs to save a copy of the corre‐
100 sponding effective IDs that were set when the program was executed
101 (see execve(2)). A set-user-ID program can assume and drop privi‐
102 leges by switching its effective user ID back and forth between the
103 values in its real user ID and saved set-user-ID. This switching is
104 done via calls to seteuid(2), setreuid(2), or setresuid(2). A set-
105 group-ID program performs the analogous tasks using setegid(2),
106 setregid(2), or setresgid(2). A process can obtain its saved set-
107 user-ID (set-group-ID) using getresuid(2) (getresgid(2)).
108 * Filesystem user ID and filesystem group ID (Linux-specific). These
110 IDs, in conjunction with the supplementary group IDs described
111 below, are used to determine permissions for accessing files; see
112 path_resolution(7) for details. Whenever a process's effective user
113 (group) ID is changed, the kernel also automatically changes the
114 filesystem user (group) ID to the same value. Consequently, the
115 filesystem IDs normally have the same values as the corresponding
116 effective ID, and the semantics for file-permission checks are thus
117 the same on Linux as on other UNIX systems. The filesystem IDs can
118 be made to differ from the effective IDs by calling setfsuid(2) and
119 setfsgid(2).
120 * Supplementary group IDs. This is a set of additional group IDs that
122 are used for permission checks when accessing files and other shared
123 resources. On Linux kernels before 2.6.4, a process can be a member
124 of up to 32 supplementary groups; since kernel 2.6.4, a process can
125 be a member of up to 65536 supplementary groups. The call
126 sysconf(_SC_NGROUPS_MAX) can be used to determine the number of sup‐
127 plementary groups of which a process may be a member. A process can
128 obtain its set of supplementary group IDs using getgroups(2), and
129 can modify the set using setgroups(2).
130 A child process created by fork(2) inherits copies of its parent's user
132 and groups IDs. During an execve(2), a process's real user and group
133 ID and supplementary group IDs are preserved; the effective and saved
134 set IDs may be changed, as described in execve(2).
135 Aside from the purposes noted above, a process's user IDs are also
137 employed in a number of other contexts:
138 * when determining the permissions for sending signals (see kill(2));
140 * when determining the permissions for setting process-scheduling
142 parameters (nice value, real time scheduling policy and priority,
143 CPU affinity, I/O priority) using setpriority(2), sched_setaffin‐
144 ity(2), sched_setscheduler(2), sched_setparam(2), sched_setattr(2),
145 and ioprio_set(2);
146 * when checking resource limits (see getrlimit(2));
148 * when checking the limit on the number of inotify instances that the
150 process may create (see inotify(7)).
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153 Process IDs, parent process IDs, process group IDs, and session IDs are
154 specified in POSIX.1. The real, effective, and saved set user and
155 groups IDs, and the supplementary group IDs, are specified in POSIX.1.
156 The filesystem user and group IDs are a Linux extension.
157
159 The POSIX threads specification requires that credentials are shared by
160 all of the threads in a process. However, at the kernel level, Linux
161 maintains separate user and group credentials for each thread. The
162 NPTL threading implementation does some work to ensure that any change
163 to user or group credentials (e.g., calls to setuid(2), setresuid(2))
164 is carried through to all of the POSIX threads in a process. See
165 nptl(7) for further details.
166
168 bash(1), csh(1), groups(1), id(1), newgrp(1), ps(1), runuser(1), set‐
169 priv(1), sg(1), su(1), access(2), execve(2), faccessat(2), fork(2),
170 getgroups(2), getpgrp(2), getpid(2), getppid(2), getsid(2), kill(2),
171 setegid(2), seteuid(2), setfsgid(2), setfsuid(2), setgid(2), set‐
172 groups(2), setpgid(2), setresgid(2), setresuid(2), setsid(2),
173 setuid(2), waitpid(2), euidaccess(3), initgroups(3), killpg(3), tcgetp‐
174 grp(3), tcsetpgrp(3), group(5), passwd(5), shadow(5), capabilities(7),
175 namespaces(7), path_resolution(7), pid_namespaces(7), pthreads(7), sig‐
176 nal(7), unix(7), user_namespaces(7), sudo(8)
177
179 This page is part of release 4.16 of the Linux man-pages project. A
180 description of the project, information about reporting bugs, and the
181 latest version of this page, can be found at
182 https://www.kernel.org/doc/man-pages/.
183Linux 2016-12-12 CREDENTIALS(7)