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Linux 2.6 - man page for credentials (linux section 7)

CREDENTIALS(7)			    Linux Programmer's Manual			   CREDENTIALS(7)

       credentials - process identifiers

   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  repre-
       sented 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  wait-

       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  exe-
       cute  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 set-
       sid(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, respec-
       tively 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 deter-
	  mine 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  per-
	  mission  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	setprior-
	  ity(2),    sched_setaffinity(2),    sched_setscheduler(2),	sched_setparam(2),    and

       *  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  supplemen-
       tary  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 cre-
       dentials 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 car-
       ried 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),  get-
       pid(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_resolu-
       tion(7), unix(7)

       This page is part of release 3.55 of the Linux man-pages project.  A  description  of  the
       project,     and    information	  about    reporting	bugs,	 can	be    found    at

Linux					    2008-06-03				   CREDENTIALS(7)

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