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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
       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 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
       session 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
       terminal (see termios(3)), then only the foreground job  may  write  to
       the  terminal;  writes from background job cause a SIGTTOU signal to be
       generated, 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),
       setpriority(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
          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 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
          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     setpriority(2),
          sched_setaffinity(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).

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),
       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_resolution(7), signal(7), unix(7)

COLOPHON

       This page is part of release 3.65 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/.



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