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 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 getp-
       pid(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 pre-
       served 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 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 correspond-
	  ing  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 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 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 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), euidac-
       cess(3), initgroups(3), tcgetpgrp(3), tcsetpgrp(3), capabilities(7), path_resolution(7), unix(7)

COLOPHON

       This page is part of release 3.55 of the Linux man-pages project.  A description of the project, and information about reporting bugs,  can
       be found at http://www.kernel.org/doc/man-pages/.

Linux								    2008-06-03							    CREDENTIALS(7)