Provided by: manpages_3.54-1ubuntu1_all bug


       credentials - process identifiers


   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

       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
          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
          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

       *  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).


       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 filesystem user and group IDs
       are a Linux extension.


       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.


       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)


       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