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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.

       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

       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(3), 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

       *  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)).

       Process IDs, parent process IDs, process group IDs, and session IDs are
       specified  in  POSIX.1.	 The  real,  effective, and saved set user and
       groups IDs, and the supplementary group IDs, are specified in  POSIX.1.
       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.   See
       nptl(7) for further details.

       bash(1),	 csh(1),  groups(1), id(1), newgrp(1), ps(1), runuser(1), set-
       priv(1), sg(1), su(1),  access(2),  execve(2),  faccessat(2),  fork(2),
       getgroups(2),  getpgrp(2),  getpid(2),  getppid(2), getsid(2), kill(2),
       setegid(2),  seteuid(2),	 setfsgid(2),  setfsuid(2),  setgid(2),	  set-
       groups(2),    setpgid(2),    setresgid(2),   setresuid(2),   setsid(2),
       setuid(2), waitpid(2), euidaccess(3), initgroups(3), killpg(3), tcgetp-
       grp(3),	tcsetpgrp(3), group(5), passwd(5), shadow(5), capabilities(7),
       namespaces(7), path_resolution(7), pid_namespaces(7), pthreads(7), sig-
       nal(7), unix(7), user_namespaces(7), sudo(8)

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Linux				  2016-12-12			CREDENTIALS(7)