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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 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 get-
       sid(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 collec-
       tion of processes that share the	same process group ID; the shell  cre-
       ates  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  ses-
       sion 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	termi-
       nal (see	termios(3)), then only the foreground job  may	write  to  the
       terminal;  writes from background job cause a SIGTTOU signal to be gen-
       erated, 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), set-
       priority(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 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  filesys-
	  tem  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	corre-
	  sponding effective IDs that were set when the	program	 was  executed
	  (see	execve(2)).   A	set-user-ID program can	assume and drop	privi-
	  leges	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 sup-
	  plementary 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_setaffin-
	  ity(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.1-2001.  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), sete-
       gid(2), seteuid(2), setfsgid(2),	setfsuid(2), setgid(2),	 setgroups(2),
       setresgid(2), setresuid(2), setuid(2), waitpid(2), euidaccess(3), init-
       groups(3), tcgetpgrp(3),	tcsetpgrp(3), capabilities(7),	namespaces(7),
       path_resolution(7),  pid_namespaces(7),	signal(7), user_namespaces(7),
       unix(7)

COLOPHON
       This page is part of release 3.74 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/.

Linux				  2014-09-21			CREDENTIALS(7)

NAME | DESCRIPTION | CONFORMING TO | NOTES | SEE ALSO | COLOPHON

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