CREDENTIALS(7) Linux Programmer's Manual CREDENTIALS(7)
NAME
credentials - process identifiers
DESCRIPTION
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: 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).
Parent process ID (PPID)
A process's parent process ID identifies the process that created this process using fork(2). A process can obtain its PPID using getp-
pid(2). A PPID is represented using the type pid_t.
A process's PPID is preserved across an execve(2).
Process group ID and session ID
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 pre-
served 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 collec-
tion 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(3), 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).
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>).
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 correspond-
ing 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)).
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.
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 setuid(2), setresuid(2)) is carried through to all of the POSIX threads in a process.
See nptl(7) for further details.
SEE ALSO
bash(1), csh(1), groups(1), id(1), newgrp(1), ps(1), runuser(1), setpriv(1), sg(1), su(1), access(2), execve(2), faccessat(2), fork(2),
getgroups(2), getpgrp(2), getpid(2), getppid(2), getsid(2), kill(2), setegid(2), seteuid(2), setfsgid(2), setfsuid(2), setgid(2), set-
groups(2), setpgid(2), setresgid(2), setresuid(2), setsid(2), setuid(2), waitpid(2), euidaccess(3), initgroups(3), killpg(3), tcgetpgrp(3),
tcsetpgrp(3), group(5), passwd(5), shadow(5), capabilities(7), namespaces(7), path_resolution(7), pid_namespaces(7), pthreads(7), sig-
nal(7), unix(7), user_namespaces(7), sudo(8)
COLOPHON
This page is part of release 4.15 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 https://www.kernel.org/doc/man-pages/.
Linux 2016-12-12 CREDENTIALS(7)