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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 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.

   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

       *  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), 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 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 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.

       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 3.54 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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