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>).

          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).

       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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