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 <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(2), 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  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 <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)).

       *  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 process's 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.   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).

       groups(3),  tcgetpgrp(3),  tcsetpgrp(3),  capabilities(7), path_resolu-
       tion(7), unix(7)

       This page is part of release 2.77 of the Linux  man-pages  project.   A
       description  of  the project, and information about reporting bugs, can
       be found at

Linux                             2007-06-13                    CREDENTIALS(7)
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