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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 assigned when the process
       is created using fork(2).  A process can obtain its PID using getpid(2).  A PID is  repre‐
       sented using the type pid_t (defined in ).

       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  wait‐
       pid(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  exe‐
       cute  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 set‐
       sid(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  back‐
       ground  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,  respec‐
       tively represented using the types uid_t and gid_t (defined in ).

       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  deter‐
          mine 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 per‐
          mission 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 setprior‐
          ity(2),      sched_setaffinity(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)).

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 exten‐
       sion.

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  cre‐
       dentials 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  car‐
       ried through to all of the POSIX threads in a process.  See nptl(7) for further details.

SEE ALSO
       bash(1),  csh(1),  ps(1), access(2), execve(2), faccessat(2), fork(2), getgroups(2), getp‐
       grp(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),
       namespaces(7),  path_resolution(7), pid_namespaces(7), pthreads(7), signal(7), user_names‐
       paces(7), unix(7)

COLOPHON
       This page is part of release 4.04 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 http://www.kernel.org/doc/man-pages/.

Linux                                       2015-03-29                             CREDENTIALS(7)

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