credentials(7)



CREDENTIALS(7)             Linux Programmer's Manual            CREDENTIALS(7)

NAME
       credentials - process identifiers

DESCRIPTION
   Process ID (PID)
       Each  process  has  a unique nonnegative integer identifier that is as-
       signed 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  af-
       fected  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
       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 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 group IDs.  These IDs  are
       integers, respectively represented using the types uid_t and gid_t (de-
       fined 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 ge-
          tuid(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 ef-
          fective 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), se-
          tregid(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  be-
          low,  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 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  em-
       ployed in a number of other contexts:

       *  when determining the permissions for sending signals (see kill(2));

       *  when  determining the permissions for setting process-scheduling pa-
          rameters (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), sched_setattr(2), and  io-
          prio_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.   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.

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
       nptl(7) for further details.

SEE ALSO
       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),  se-
       tuid(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)

COLOPHON
       This  page  is  part of release 4.16 of the Linux man-pages project.  A
       description of the project, information about reporting bugs,  and  the
       latest     version     of     this    page,    can    be    found    at
       https://www.kernel.org/doc/man-pages/.

Linux                             2016-12-12                    CREDENTIALS(7)

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