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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 non-negative 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(2), 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 pro-
       cess  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 pro-
       cess 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  sete-
          gid(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  supple-
          mentary  group  IDs described below, are used to determine permissions for accessing files; see path_resolu-
          tion(7) for details.  Whenever a process's effective user (group) ID is changed, the kernel  also  automati-
          cally 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 effec-
          tive 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  get-
          groups(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_setsched-
          uler(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.  How-
       ever, 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),  get-
       sid(2),  kill(2), killpg(2), setegid(2), seteuid(2), setfsgid(2), setfsuid(2), setgid(2), setgroups(2), setres-
       gid(2), setresuid(2), setuid(2), waitpid(2), euidaccess(3), initgroups(3), tcgetpgrp(3), tcsetpgrp(3), capabil-
       ities(7), path_resolution(7), unix(7)

COLOPHON
       This  page  is part of release 3.22 of the Linux man-pages project.  A description of the project, and informa-
       tion about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.



Linux                             2008-06-03                    CREDENTIALS(7)