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CREDENTIALS(7)		   Linux Programmers Manual	       CREDENTIALS(7)

       credentials - process identifiers

   Process ID (PID)
       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 ).

       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 processs PID is preserved across an execve(2).

   Parent Process ID (PPID)
       A processs 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 processs 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 parents session ID and process
       group  ID.   A  processs 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 processs
       group membership can be set using setpgid(2).  The process  whose  pro
       cess 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.

   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 ).

       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 sys
	  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)).

       *  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 processs 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

       *  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 parents user
       and  groups  IDs.  During an execve(2), a processs 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 processs 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), 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).

       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

       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), 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),  path_resolu
       tion(7), unix(7)

       This  page  is  part of release 3.05 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)

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