source: rtems-docs/posix_users/process_environment.rst @ d389819

Process Environment Manager
###########################

Introduction
============

The process environment manager is responsible for providing the
functions related to user and group Id management.

The directives provided by the process environment manager are:

- ``getpid`` - Get Process ID

- ``getppid`` - Get Parent Process ID

- ``getuid`` - Get User ID

- ``geteuid`` - Get Effective User ID

- ``getgid`` - Get Real Group ID

- ``getegid`` - Get Effective Group ID

- ``setuid`` - Set User ID

- ``setgid`` - Set Group ID

- ``getgroups`` - Get Supplementary Group IDs

- ``getlogin`` - Get User Name

- ``getlogin_r`` - Reentrant Get User Name

- ``getpgrp`` - Get Process Group ID

- ``setsid`` - Create Session and Set Process Group ID

- ``setpgid`` - Set Process Group ID for Job Control

- ``uname`` - Get System Name

- ``times`` - Get Process Times

- ``getenv`` - Get Environment Variables

- ``setenv`` - Set Environment Variables

- ``ctermid`` - Generate Terminal Pathname

- ``ttyname`` - Determine Terminal Device Name

- ``ttyname_r`` - Reentrant Determine Terminal Device Name

- ``isatty`` - Determine if File Descriptor is Terminal

- ``sysconf`` - Get Configurable System Variables

Background
==========

Users and Groups
----------------

RTEMS provides a single process, multi-threaded execution environment.
In this light, the notion of user and group is somewhat without meaning.
But RTEMS does provide services to provide a synthetic version of
user and group.  By default, a single user and group is associated
with the application.  Thus unless special actions are taken,
every thread in the application shares the same user and group Id.
The initial rationale for providing user and group Id functionality
in RTEMS was for the filesystem infrastructure to implement
file permission checks.  The effective user/group Id capability
has since been used to implement permissions checking by
the ``ftpd`` server.

In addition to the "real" user and group Ids, a process may
have an effective user/group Id.  This allows a process to
function using a more limited permission set for certain operations.

User and Group Names
--------------------

POSIX considers user and group Ids to be a unique integer that
may be associated with a name.  This is usually accomplished
via a file named ``/etc/passwd`` for user Id mapping and``/etc/groups`` for group Id mapping.  Again, although
RTEMS is effectively a single process and thus single user
system, it provides limited support for user and group
names.  When configured with an appropriate filesystem, RTEMS
will access the appropriate files to map user and group Ids
to names.

If these files do not exist, then RTEMS will synthesize
a minimal version so this family of services return without
error.  It is important to remember that a design goal of
the RTEMS POSIX services is to provide useable and
meaningful results even though a full process model
is not available.

Environment Variables
---------------------

POSIX allows for variables in the run-time environment.  These are
name/value pairs that make be dynamically set and obtained by
programs.  In a full POSIX environment with command line shell
and multiple processes,  environment variables may be set in
one process - such as the shell - and inherited by child
processes.  In RTEMS, there is only one process and thus
only one set of environment variables across all processes.

Operations
==========

Accessing User and Group Ids
----------------------------

The user Id associated with the current thread may be obtain
using the ``getuid()`` service.  Similarly, the group Id
may be obtained using the ``getgid()`` service.

Accessing Environment Variables
-------------------------------

The value associated with an environment variable may be
obtained using the ``getenv()`` service and set using
the ``putenv()`` service.

Directives
==========

This section details the process environment manager's directives.
A subsection is dedicated to each of this manager's directives
and describes the calling sequence, related constants, usage,
and status codes.

getpid - Get Process ID
-----------------------
.. index:: getpid
.. index:: get process id

**CALLING SEQUENCE:**

.. code:: c

    int getpid( void );

**STATUS CODES:**

The process Id is returned.

**DESCRIPTION:**

This service returns the process Id.

**NOTES:**

NONE

getppid - Get Parent Process ID
-------------------------------
.. index:: getppid
.. index:: get parent process id

**CALLING SEQUENCE:**

.. code:: c

    int getppid( void );

**STATUS CODES:**

The parent process Id is returned.

**DESCRIPTION:**

This service returns the parent process Id.

**NOTES:**

NONE

getuid - Get User ID
--------------------
.. index:: getuid
.. index:: get user id

**CALLING SEQUENCE:**

.. code:: c

    int getuid( void );

**STATUS CODES:**

The effective user Id is returned.

**DESCRIPTION:**

This service returns the effective user Id.

**NOTES:**

NONE

geteuid - Get Effective User ID
-------------------------------
.. index:: geteuid
.. index:: get effective user id

**CALLING SEQUENCE:**

.. code:: c

    int geteuid( void );

**STATUS CODES:**

The effective group Id is returned.

**DESCRIPTION:**

This service returns the effective group Id.

**NOTES:**

NONE

getgid - Get Real Group ID
--------------------------
.. index:: getgid
.. index:: get real group id

**CALLING SEQUENCE:**

.. code:: c

    int getgid( void );

**STATUS CODES:**

The group Id is returned.

**DESCRIPTION:**

This service returns the group Id.

**NOTES:**

NONE

getegid - Get Effective Group ID
--------------------------------
.. index:: getegid
.. index:: get effective group id

**CALLING SEQUENCE:**

.. code:: c

    int getegid( void );

**STATUS CODES:**

The effective group Id is returned.

**DESCRIPTION:**

This service returns the effective group Id.

**NOTES:**

NONE

setuid - Set User ID
--------------------
.. index:: setuid
.. index:: set user id

**CALLING SEQUENCE:**

.. code:: c

    int setuid(
    uid_t uid
    );

**STATUS CODES:**

This service returns 0.

**DESCRIPTION:**

This service sets the user Id to ``uid``.

**NOTES:**

NONE

setgid - Set Group ID
---------------------
.. index:: setgid
.. index:: set group id

**CALLING SEQUENCE:**

.. code:: c

    int setgid(
    gid_t  gid
    );

**STATUS CODES:**

This service returns 0.

**DESCRIPTION:**

This service sets the group Id to ``gid``.

**NOTES:**

NONE

getgroups - Get Supplementary Group IDs
---------------------------------------
.. index:: getgroups
.. index:: get supplementary group ids

**CALLING SEQUENCE:**

.. code:: c

    int getgroups(
    int    gidsetsize,
    gid_t  grouplist[]
    );

**STATUS CODES:**

NA

**DESCRIPTION:**

This service is not implemented as RTEMS has no notion of
supplemental groups.

**NOTES:**

If supported, this routine would only be allowed for
the super-user.

getlogin - Get User Name
------------------------
.. index:: getlogin
.. index:: get user name

**CALLING SEQUENCE:**

.. code:: c

    char \*getlogin( void );

**STATUS CODES:**

Returns a pointer to a string containing the name of the
current user.

**DESCRIPTION:**

This routine returns the name of the current user.

**NOTES:**

This routine is not reentrant and subsequent calls to``getlogin()`` will overwrite the same buffer.

getlogin_r - Reentrant Get User Name
------------------------------------
.. index:: getlogin_r
.. index:: reentrant get user name
.. index:: get user name, reentrant

**CALLING SEQUENCE:**

.. code:: c

    int getlogin_r(
    char   \*name,
    size_t  namesize
    );

**STATUS CODES:**

*EINVAL*
    The arguments were invalid.

**DESCRIPTION:**

This is a reentrant version of the ``getlogin()`` service.  The
caller specified their own buffer, ``name``, as well as the
length of this buffer, ``namesize``.

**NOTES:**

NONE

getpgrp - Get Process Group ID
------------------------------
.. index:: getpgrp
.. index:: get process group id

**CALLING SEQUENCE:**

.. code:: c

    pid_t getpgrp( void );

**STATUS CODES:**

The procress group Id is returned.

**DESCRIPTION:**

This service returns the current progress group Id.

**NOTES:**

This routine is implemented in a somewhat meaningful
way for RTEMS but is truly not functional.

setsid - Create Session and Set Process Group ID
------------------------------------------------
.. index:: setsid
.. index:: create session and set process group id

**CALLING SEQUENCE:**

.. code:: c

    pid_t setsid( void );

**STATUS CODES:**

*EPERM*
    The application does not have permission to create a process group.

**DESCRIPTION:**

This routine always returns ``EPERM`` as RTEMS has no way
to create new processes and thus no way to create a new process
group.

**NOTES:**

NONE

setpgid - Set Process Group ID for Job Control
----------------------------------------------
.. index:: setpgid
.. index:: set process group id for job control

**CALLING SEQUENCE:**

.. code:: c

    int setpgid(
    pid_t pid,
    pid_t pgid
    );

**STATUS CODES:**

*ENOSYS*
    The routine is not implemented.

**DESCRIPTION:**

This service is not implemented for RTEMS as process groups are not
supported.

**NOTES:**

NONE

uname - Get System Name
-----------------------
.. index:: uname
.. index:: get system name

**CALLING SEQUENCE:**

.. code:: c

    int uname(
    struct utsname \*name
    );

**STATUS CODES:**

*EPERM*
    The provided structure pointer is invalid.

**DESCRIPTION:**

This service returns system information to the caller.  It does this
by filling in the ``struct utsname`` format structure for the
caller.

**NOTES:**

The information provided includes the operating system (RTEMS in
all configurations), the node number, the release as the RTEMS
version, and the CPU family and model.  The CPU model name
will indicate the multilib executive variant being used.

times - Get process times
-------------------------
.. index:: times
.. index:: get process times

**CALLING SEQUENCE:**

.. code:: c

    #include <sys/time.h>
    clock_t times(
    struct tms \*ptms
    );

**STATUS CODES:**

This routine returns the number of clock ticks that have elapsed
since the system was initialized (e.g. the application was
started).

**DESCRIPTION:**

``times`` stores the current process times in ``ptms``.  The
format of ``struct tms`` is as defined in``<sys/times.h>``.  RTEMS fills in the field ``tms_utime``
with the number of ticks that the calling thread has executed
and the field ``tms_stime`` with the number of clock ticks
since system boot (also returned).  All other fields in the``ptms`` are left zero.

**NOTES:**

RTEMS has no way to distinguish between user and system time
so this routine returns the most meaningful information
possible.

getenv - Get Environment Variables
----------------------------------
.. index:: getenv
.. index:: get environment variables

**CALLING SEQUENCE:**

.. code:: c

    char \*getenv(
    const char \*name
    );

**STATUS CODES:**

*NULL*
    when no match

*pointer to value*
    when successful

**DESCRIPTION:**

This service searches the set of environment variables for
a string that matches the specified ``name``.  If found,
it returns the associated value.

**NOTES:**

The environment list consists of name value pairs that
are of the form *name = value*.

setenv - Set Environment Variables
----------------------------------
.. index:: setenv
.. index:: set environment variables

**CALLING SEQUENCE:**

.. code:: c

    int setenv(
    const char \*name,
    const char \*value,
    int overwrite
    );

**STATUS CODES:**

Returns 0 if successful and -1 otherwise.

**DESCRIPTION:**

This service adds the variable ``name`` to the environment with``value``.  If ``name`` is not already exist, then it is
created.  If ``name`` exists and ``overwrite`` is zero, then
the previous value is not overwritten.

**NOTES:**

NONE

ctermid - Generate Terminal Pathname
------------------------------------
.. index:: ctermid
.. index:: generate terminal pathname

**CALLING SEQUENCE:**

.. code:: c

    char \*ctermid(
    char \*s
    );

**STATUS CODES:**

Returns a pointer to a string indicating the pathname for the controlling
terminal.

**DESCRIPTION:**

This service returns the name of the terminal device associated with
this process.  If ``s`` is NULL, then a pointer to a static buffer
is returned.  Otherwise, ``s`` is assumed to have a buffer of
sufficient size to contain the name of the controlling terminal.

**NOTES:**

By default on RTEMS systems, the controlling terminal is ``/dev/console``.
Again this implementation is of limited meaning, but it provides
true and useful results which should be sufficient to ease porting
applications from a full POSIX implementation to the reduced
profile supported by RTEMS.

ttyname - Determine Terminal Device Name
----------------------------------------
.. index:: ttyname
.. index:: determine terminal device name

**CALLING SEQUENCE:**

.. code:: c

    char \*ttyname(
    int fd
    );

**STATUS CODES:**

Pointer to a string containing the terminal device name or
NULL is returned on any error.

**DESCRIPTION:**

This service returns a pointer to the pathname of the terminal
device that is open on the file descriptor ``fd``.  If``fd`` is not a valid descriptor for a terminal device,
then NULL is returned.

**NOTES:**

This routine uses a static buffer.

ttyname_r - Reentrant Determine Terminal Device Name
----------------------------------------------------
.. index:: ttyname_r
.. index:: reentrant determine terminal device name

**CALLING SEQUENCE:**

.. code:: c

    int ttyname_r(
    int   fd,
    char \*name,
    int   namesize
    );

**STATUS CODES:**

This routine returns -1 and sets ``errno`` as follows:

*EBADF*
    If not a valid descriptor for a terminal device.

*EINVAL*
    If ``name`` is NULL or ``namesize`` are insufficient.

**DESCRIPTION:**

This service the pathname of the terminal device that is open
on the file descriptor ``fd``.

**NOTES:**

NONE

isatty - Determine if File Descriptor is Terminal
-------------------------------------------------
.. index:: isatty
.. index:: determine if file descriptor is terminal

**CALLING SEQUENCE:**

.. code:: c

    int isatty(
    int fd
    );

**STATUS CODES:**

Returns 1 if ``fd`` is a terminal device and 0 otherwise.

**DESCRIPTION:**

This service returns 1 if ``fd`` is an open file descriptor
connected to a terminal and 0 otherwise.

**NOTES:**

sysconf - Get Configurable System Variables
-------------------------------------------
.. index:: sysconf
.. index:: get configurable system variables

**CALLING SEQUENCE:**

.. code:: c

    long sysconf(
    int name
    );

**STATUS CODES:**

The value returned is the actual value of the system resource.
If the requested configuration name is a feature flag, then
1 is returned if the available and 0 if it is not.  On any
other error condition, -1 is returned.

**DESCRIPTION:**

This service is the mechanism by which an application determines
values for system limits or options at runtime.

**NOTES:**

Much of the information that may be obtained via ``sysconf``
has equivalent macros in ``<unistd.h``.  However, those
macros reflect conservative limits which may have been altered
by application configuration.

.. COMMENT: COPYRIGHT (c) 1988-2002.

.. COMMENT: On-Line Applications Research Corporation (OAR).

.. COMMENT: All rights reserved.