.\" Copyright (c) 2007 by Michael Kerrisk <mtk.manpages@gmail.com>

.\"

.\" %%%LICENSE_START(VERBATIM)

.\" Permission is granted to make and distribute verbatim copies of this

.\" manual provided the copyright notice and this permission notice are

.\" preserved on all copies.

.\"

.\" Permission is granted to copy and distribute modified versions of this

.\" manual under the conditions for verbatim copying, provided that the

.\" entire resulting derived work is distributed under the terms of a

.\" permission notice identical to this one.

.\"

.\" Since the Linux kernel and libraries are constantly changing, this

.\" manual page may be incorrect or out-of-date.  The author(s) assume no

.\" responsibility for errors or omissions, or for damages resulting from

.\" the use of the information contained herein.  The author(s) may not

.\" have taken the same level of care in the production of this manual,

.\" which is licensed free of charge, as they might when working

.\" professionally.

.\"

.\" Formatted or processed versions of this manual, if unaccompanied by

.\" the source, must acknowledge the copyright and authors of this work.

.\" %%%LICENSE_END

.\"

.\" 2007-06-13 Creation

.\"

.TH CREDENTIALS 7 2016-12-12 "Linux" "Linux Programmer's Manual"

.SH NAME

credentials \- process identifiers

.SH DESCRIPTION

.SS Process ID (PID)

Each process has a unique nonnegative integer identifier

that is assigned when the process is created using

.BR fork (2).

A process can obtain its PID using

.BR getpid (2).

A PID is represented using the type

.I pid_t

(defined in

.IR <sys/types.h> ).

.PP

PIDs are used in a range of system calls to identify the process

affected by the call, for example:

.BR kill (2),

.BR ptrace (2),

.BR setpriority (2)

.\" .BR sched_rr_get_interval (2),

.\" .BR sched_getaffinity (2),

.\" .BR sched_setaffinity (2),

.\" .BR sched_getparam (2),

.\" .BR sched_setparam (2),

.\" .BR sched_setscheduler (2),

.\" .BR sched_getscheduler (2),

.BR setpgid (2),

.\" .BR getsid (2),

.BR setsid (2),

.BR sigqueue (3),

and

.BR waitpid (2).

.\" .BR waitid (2),

.\" .BR wait4 (2),

.PP

A process's PID is preserved across an

.BR execve (2).

.SS Parent process ID (PPID)

A process's parent process ID identifies the process that created

this process using

.BR fork (2).

A process can obtain its PPID using

.BR getppid (2).

A PPID is represented using the type

.IR pid_t .

.PP

A process's PPID is preserved across an

.BR execve (2).

.SS Process group ID and session ID

Each process has a session ID and a process group ID,

both represented using the type

.IR pid_t .

A process can obtain its session ID using

.BR getsid (2),

and its process group ID using

.BR getpgrp (2).

.PP

A child created by

.BR 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

.BR execve (2).

.PP

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

.BR setpgid (2).

The process whose process ID is the same as its process group ID is the

\fIprocess group leader\fP for that group.

.PP

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

.BR setsid (2),

which creates a new session whose session ID is the same

as the PID of the process that called

.BR setsid (2).

The creator of the session is called the \fIsession leader\fP.

.PP

All of the processes in a session share a

.IR "controlling terminal" .

The controlling terminal is established when the session leader

first opens a terminal (unless the

.BR O_NOCTTY

flag is specified when calling

.BR open (2)).

A terminal may be the controlling terminal of at most one session.

.PP

At most one of the jobs in a session may be the

.IR "foreground job" ;

other jobs in the session are

.IR "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

.BR SIGTTIN

signal, which suspends the job.

If the

.BR TOSTOP

flag has been set for the terminal (see

.BR termios (3)),

then only the foreground job may write to the terminal;

writes from background job cause a

.BR SIGTTOU

signal to be generated, which suspends the job.

When terminal keys that generate a signal (such as the

.I interrupt

key, normally control-C)

are pressed, the signal is sent to the processes in the foreground job.

.PP

Various system calls and library functions

may operate on all members of a process group,

including

.BR kill (2),

.BR killpg (3),

.BR getpriority (2),

.BR setpriority (2),

.BR ioprio_get (2),

.BR ioprio_set (2),

.BR waitid (2),

and

.BR waitpid (2).

See also the discussion of the

.BR F_GETOWN ,

.BR F_GETOWN_EX ,

.BR F_SETOWN ,

and

.BR F_SETOWN_EX

operations in

.BR fcntl (2).

.SS User and group identifiers

Each process has various associated user and group IDs.

These IDs are integers, respectively represented using the types

.I uid_t

and

.I gid_t

(defined in

.IR <sys/types.h> ).

.PP

On Linux, each process has the following user and group identifiers:

.IP * 3

Real user ID and real group ID.

These IDs determine who owns the process.

A process can obtain its real user (group) ID using

.BR getuid (2)

.RB ( getgid (2)).

.IP *

Effective user ID and effective group ID.

These IDs are used by the kernel to determine the permissions

that the process will have when accessing shared resources such

as message queues, shared memory, and semaphores.

On most UNIX systems, these IDs also determine the

permissions when accessing files.

However, Linux uses the filesystem IDs described below

for this task.

A process can obtain its effective user (group) ID using

.BR geteuid (2)

.RB ( getegid (2)).

.IP *

Saved set-user-ID and saved set-group-ID.

These IDs are used in set-user-ID and set-group-ID programs to save

a copy of the corresponding effective IDs that were set when

the program was executed (see

.BR execve (2)).

A set-user-ID program can assume and drop privileges by

switching its effective user ID back and forth between the values

in its real user ID and saved set-user-ID.

This switching is done via calls to

.BR seteuid (2),

.BR setreuid (2),

or

.BR setresuid (2).

A set-group-ID program performs the analogous tasks using

.BR setegid (2),

.BR setregid (2),

or

.BR setresgid (2).

A process can obtain its saved set-user-ID (set-group-ID) using

.BR getresuid (2)

.RB ( getresgid (2)).

.IP *

Filesystem user ID and filesystem group ID (Linux-specific).

These IDs, in conjunction with the supplementary group IDs described

below, are used to determine permissions for accessing files; see

.BR path_resolution (7)

for details.

Whenever a process's effective user (group) ID is changed,

the kernel also automatically changes the filesystem user (group) ID

to the same value.

Consequently, the filesystem IDs normally have the same values

as the corresponding effective ID, and the semantics for file-permission

checks are thus the same on Linux as on other UNIX systems.

The filesystem IDs can be made to differ from the effective IDs

by calling

.BR setfsuid (2)

and

.BR setfsgid (2).

.IP *

Supplementary group IDs.

This is a set of additional group IDs that are used for permission

checks when accessing files and other shared resources.

On Linux kernels before 2.6.4,

a process can be a member of up to 32 supplementary groups;

since kernel 2.6.4,

a process can be a member of up to 65536 supplementary groups.

The call

.I sysconf(_SC_NGROUPS_MAX)

can be used to determine the number of supplementary groups

of which a process may be a member.

.\" Since kernel 2.6.4, the limit is visible via the read-only file

.\" /proc/sys/kernel/ngroups_max.

.\" As at 2.6.22-rc2, this file is still read-only.

A process can obtain its set of supplementary group IDs using

.BR getgroups (2),

and can modify the set using

.BR setgroups (2).

.PP

A child process created by

.BR fork (2)

inherits copies of its parent's user and groups IDs.

During an

.BR 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

.BR execve (2).

.PP

Aside from the purposes noted above,

a process's user IDs are also employed in a number of other contexts:

.IP * 3

when determining the permissions for sending signals (see

.BR kill (2));

.IP *

when determining the permissions for setting

process-scheduling parameters (nice value, real time

scheduling policy and priority, CPU affinity, I/O priority) using

.BR setpriority (2),

.BR sched_setaffinity (2),

.BR sched_setscheduler (2),

.BR sched_setparam (2),

.BR sched_setattr (2),

and

.BR ioprio_set (2);

.IP *

when checking resource limits (see

.BR getrlimit (2));

.IP *

when checking the limit on the number of inotify instances

that the process may create (see

.BR inotify (7)).

.SH CONFORMING TO

Process IDs, parent process IDs, process group IDs, and session IDs

are specified in POSIX.1.

The real, effective, and saved set user and groups IDs,

and the supplementary group IDs, are specified in POSIX.1.

The filesystem user and group IDs are a Linux extension.

.SH NOTES

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

.BR setuid (2),

.BR setresuid (2))

is carried through to all of the POSIX threads in a process.

See

.BR nptl (7)

for further details.

.SH SEE ALSO

.BR bash (1),

.BR csh (1),

.BR groups (1),

.BR id (1),

.BR newgrp (1),

.BR ps (1),

.BR runuser (1),

.BR setpriv (1),

.BR sg (1),

.BR su (1),

.BR access (2),

.BR execve (2),

.BR faccessat (2),

.BR fork (2),

.BR getgroups (2),

.BR getpgrp (2),

.BR getpid (2),

.BR getppid (2),

.BR getsid (2),

.BR kill (2),

.BR setegid (2),

.BR seteuid (2),

.BR setfsgid (2),

.BR setfsuid (2),

.BR setgid (2),

.BR setgroups (2),

.BR setpgid (2),

.BR setresgid (2),

.BR setresuid (2),

.BR setsid (2),

.BR setuid (2),

.BR waitpid (2),

.BR euidaccess (3),

.BR initgroups (3),

.BR killpg (3),

.BR tcgetpgrp (3),

.BR tcsetpgrp (3),

.BR group (5),

.BR passwd (5),

.BR shadow (5),

.BR capabilities (7),

.BR namespaces (7),

.BR path_resolution (7),

.BR pid_namespaces (7),

.BR pthreads (7),

.BR signal (7),

.BR unix (7),

.BR user_namespaces (7),

.BR sudo (8)

.SH COLOPHON

This page is part of release 4.15 of the Linux

.I man-pages

project.

A description of the project,

information about reporting bugs,

and the latest version of this page,

can be found at

\%https://www.kernel.org/doc/man\-pages/.