.. configoptions:

================================================================================
Configuration options
================================================================================

This page discussed runtime configuration options for GDAL, and is distinct from
options to the build-time configure script. Runtime configuration options apply
on all platforms, and are evaluated at runtime. They can be set programmatically,
by commandline switches or in the environment by the user.

Configuration options are normally used to alter the default behavior of GDAL/OGR
drivers and in some cases the GDAL/OGR core. They are essentially global
variables the user can set.

How to set configuration options ?
----------------------------------

One example of a config option is the GDAL_CACHEMAX option. It controls the size
of the GDAL block cache, in megabytes. It can be set in the environment on Unix
(bash/bourne) shell like this:

::

    export GDAL_CACHEMAX=64

In a DOS/Windows command shell it is done like this:

::

    set GDAL_CACHEMAX=64

It can also be set on the commandline for most GDAL and OGR utilities with the
``--config`` switch, though in a few cases these switches are not evaluated in
time to affect behavior.

::

    gdal_translate --config GDAL_CACHEMAX 64 in.tif out.tif

In C/C++ configuration switches can be set programmatically with
:cpp:func:`CPLSetConfigOption`:

.. code-block:: c

    #include "cpl_conv.h"
    ...
        CPLSetConfigOption( "GDAL_CACHEMAX", "64" );

Normally a configuration option applies to all threads active in a program, but
they can be limited to only the current thread with
:cpp:func:`CPLSetThreadLocalConfigOption`

.. code-block:: c

    CPLSetThreadLocalConfigOption( "GTIFF_DIRECT_IO", "YES" );

For boolean options, the values YES, TRUE or ON can be used to turn the option on;
NO, FALSE or OFF to turn it off.

List of configuration options and where they apply
--------------------------------------------------

.. note::
    This list is known to be incomplete. It depends on proper annotation of configuration
    options where they are mentionned elsewhere in the documentation.
    If you want to help to extend it, use the ``:decl_configoption:`NAME```
    syntax in places where a configuration option is mentionned.


.. include:: configoptions_index_generated.rst
