# credentials

## 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_.
    
           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 background
           jobs 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(3),
           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 group 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.  Before Linux 2.6.4, a process can be a
              member of up to 32 supplementary groups; since Linux 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).
    
           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),
              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)).
    
       **Modifying process user and group IDs**
           Subject to rules described in the relevant manual pages, a process
           can use the following APIs to modify its user and group IDs:
    
           setuid(2) (setgid(2))
                  Modify the process's real (and possibly effective and
                  saved-set) user (group) IDs.
    
           seteuid(2) (setegid(2))
                  Modify the process's effective user (group) ID.
    
           setfsuid(2) (setfsgid(2))
                  Modify the process's filesystem user (group) ID.
    
           setreuid(2) (setregid(2))
                  Modify the process's real and effective (and possibly
                  saved-set) user (group) IDs.
    
           setresuid(2) (setresgid(2))
                  Modify the process's real, effective, and saved-set user
                  (group) IDs.
    
           setgroups(2)
                  Modify the process's supplementary group list.
    
           Any changes to a process's effective user (group) ID are
           automatically carried over to the process's filesystem user
           (group) ID.  Changes to a process's effective user or group ID can
           also affect the process "dumpable" attribute, as described in
           prctl(2).
    
           Changes to process user and group IDs can affect the capabilities
           of the process, as described in capabilities(7).
    


## STANDARDS



           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 extension.
    


## NOTES



           Various fields in the _/proc/_ pid _/status_ file show the process
           credentials described above.  See proc(5) for further information.
    
           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 nptl(7) for further details.
    


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