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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 getppid(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 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.
User and group identifiers
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), 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.1-2001. 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.
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 ALSO
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)
COLOPHON
This page is part of release 3.54 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/.