CREDENTIALS(7) manual page
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credentials - process identifiers
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: 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
parent process ID identifies the process that created this process using
fork(2)
. A process can obtain its PPID using getppid(2)
. A PPID is represented
using the type pid_t.
A process’s PPID is preserved across an execve(2)
.
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.
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(2)
, 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)
.
Each
process has various associated user and groups IDs. These IDs are integers,
respectively represented using the types uid_t and gid_t (defined in <sys/types.h>).
On Linux, each process has the following user and group identifiers:
- *
- Real
user ID and real group ID. These IDs determine who owns the process. A process
can obtain its real user (group) ID using getuid(2)
(getgid(2)
).
- *
- 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 geteuid(2)
(getegid(2)
).
- *
- 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 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 seteuid(2)
, setreuid(2)
, or setresuid(2)
. A set-group-ID program
performs the analogous tasks using setegid(2)
, setregid(2)
, or setresgid(2)
.
A process can obtain its saved set-user-ID (set-group-ID) using getresuid(2)
(getresgid(2)
).
- *
- 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 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 setfsuid(2)
and setfsgid(2)
.
- *
- 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 sysconf(_SC_NGROUPS_MAX)
can be used to determine the number of supplementary groups of which a
process may be a member. A process can obtain its set of supplementary
group IDs using getgroups(2)
, and can modify the set using setgroups(2)
.
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:
- *
- when determining the permissions for sending signals (see kill(2)
);
- *
- when
determining the permissions for setting process-scheduling parameters (nice
value, real time scheduling policy and priority, CPU affinity, I/O priority)
using setpriority(2)
, sched_setaffinity(2)
, sched_setscheduler(2)
, sched_setparam(2)
,
sched_setattr(2)
, and ioprio_set(2)
;
- *
- when checking resource limits (see
getrlimit(2)
);
- *
- when checking the limit on the number of inotify instances
that the process may create (see inotify(7)
).
Process IDs,
parent process IDs, process group IDs, and session IDs are specified in
POSIX.1-2001. The real, effective, and saved set user and groups IDs, and
the supplementary group IDs, are specified in POSIX.1-2001. The filesystem
user and group IDs are a Linux extension.
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.
bash(1)
,
csh(1)
, ps(1)
, access(2)
, execve(2)
, faccessat(2)
, fork(2)
, getgroups(2)
,
getpgrp(2)
, getpid(2)
, getppid(2)
, getsid(2)
, kill(2)
, killpg(2)
, setegid(2)
,
seteuid(2)
, setfsgid(2)
, setfsuid(2)
, setgid(2)
, setgroups(2)
, setresgid(2)
,
setresuid(2)
, setuid(2)
, waitpid(2)
, euidaccess(3)
, initgroups(3)
, tcgetpgrp(3)
,
tcsetpgrp(3)
, capabilities(7)
, namespaces(7)
, path_resolution(7)
, pid_namespaces(7)
,
signal(7)
, user_namespaces(7)
, unix(7)
This page is part of release
3.78 of the Linux man-pages project. A description of the project, information
about reporting bugs, and the latest version of this page, can be found
at http://www.kernel.org/doc/man-pages/.
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