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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.

   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  file
          system  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)).

       *  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
          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 file system  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),
       etc.)  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), unix(7)

COLOPHON

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       project, and information about reporting bugs, can be found at  http://man7.org/linux/man-
       pages/.