NAME | DESCRIPTION | CONFORMING TO | NOTES | SEE ALSO | COLOPHON
CREDENTIALS(7) Linux Programmer's Manual CREDENTIALS(7)
credentials - process identifiers
Each process has a unique non-negative 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(2), 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.
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 file system 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)).
* File system user ID and file system 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 file system user (group) ID to the
same value. Consequently, the file system 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
file system 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),
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 file
system 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), etc.) 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),
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), path_resolution(7), unix(7)
This page is part of release 3.23 of the Linux man-pages project. A
description of the project, and information about reporting bugs, can be found
at http://www.kernel.org/doc/man-pages/.
Linux 2008-06-03 CREDENTIALS(7)