Environment Variables

The following table describes (most of) the environment variables used by the netcdf-c library. There are some not listed that are only used for specialized debugging.

Name	Description
ALLUSERSPROFILE	This is more-or-less the Windows equivalent of "HOME"
AWS_ACCESS_KEY_ID	Used by the aws access libraries; overrides ${HOME}/.aws/config entries.
AWS_CONFIG_FILE	Used by the aws access libraries; specifies absolute path to the config file.
AWS_DEFAULT_REGION	Used by the aws access libraries; specifies default S3 region.
AWS_PROFILE	Used by the aws access libraries; specifies default AWS profile.
AWS_REGION	Used by the aws access libraries; specifies specific region to use.
AWS_SECRET_ACCESS_KEY	Used by the aws access libraries; overrides ${HOME}/.aws/config entries.
CURLOPT_VERBOSE	Causes libcurl to produce verbose output.
HOME	The user's home directory.
MSYS2_PREFIX	If platform is MSYS2, then specify the root prefix.
NC_DEFAULT_CREATE_PERMS	For NCZarr, specify the default creation permissions for a file.
NC_DEFAULT_DIR_PERMS	For NCZarr, specify the default creation permissions for a directory.
NCLOGGING	Specify the log level: one of "OFF","ERR","WARN","NOTE","DEBUG".
NCPATHDEBUG	Causes path manager to output debugging information.
NCRCENV_HOME	Overrides ${HOME} as the location of the .rc file.
NCRCENV_IGNORE	Causes any .rc files to be ignored.
NCRCENV_RC	The absolute path to use for the .rc file.
NCTRACING	Specify the level of tracing detail.
NCZARRFORMAT	Force use of a specific Zarr format version: 2 or 3.
NETCDF_LOG_LEVEL	Specify the log level for HDF5 logging (separate from e.g. NCLOGGING).
TEMP	For Windows platform, specifies the location of a directory to store temporary files.
USERPROFILE	For Windows platform, overrides ${HOME}.

Resource Control (.rc) Files

In addition to using environment variables, the netcdf-c library supports run-time configuration of the library using the so-called ".rc" file mechanism. This means that as part of its initialization, the netcdf-c library will search for and process a set of files where these files contain entries specifying (key,value) pairs. These pairs are compiled into a single internal database that can be queried by other parts of the netcdf-c library.

Locating The .rc Files

For historical reasons, multiple .rc files are allowed.

Search Order

The netcdf-c library searches for, and loads from, the following files, in this order:

$HOME/.ncrc
$HOME/.dodsrc
$CWD/.ncrc
$CWD/.dodsrc

$HOME is the user's home directory and $CWD is the current working directory. Entries in later files override any of the earlier files

It is strongly suggested that you pick a uniform location and a uniform name and use them always. Otherwise you may observe unexpected results when the netcdf-c library loads an rc file you did not expect.

RC File Format

The rc file format is a series of lines of the general form:

[<URL>]<key>=<value>

where the bracket-enclosed URL is optional. Note that the brackets are part of the line.

URL Constrained RC File Entries

Each line of the rc file can begin with a URL enclosed in square brackets. The parts of the URL that are used for choosing an entry are the host, the port, and the URL path. Note that the host+port is the only part used when searching for libcurl related entries. This is because libcurl's authorization grain is not any finer than host+port level. The URL path may be used for non-curl related entries. Also note that the protocol is ignored.

Here are some examples.

    [https://remotetest.unidata.ucar.edu/thredds]HTTP.VERBOSE=1
or
    [https://fake.ucar.edu:9090]HTTP.VERBOSE=0

For selection purposes, the host+port+path is used when the path argument for nc_open() or nc_create() takes the form of a URL. If the url request from, say, the netcdf_open method has a host,port, and path matching one of the prefixes in the rc file, then the corresponding entry will be used, otherwise ignored. This means that an entry with a matching host+port+path will take precedence over an entry without a host+port+path.

For example, passing this URL to nc_open

http://remotetest.unidata.ucar.edu/thredds/dodsC/testdata/testData.nc

will have HTTP.VERBOSE set to 1 because its host and path match the example above.

Similarly, using this path

http://fake.ucar.edu:9090/dts/test.01

will have HTTP.VERBOSE set to 0 because its host+port matches the example above.

Programmatic Access to .rc File

It is possible for client programs to query and modify the internal .rc database through the following API.

char* nc_rc_get(const char* key); Get the value corresponding to key or return NULL if not defined. The caller must free the resulting value.
int nc_rc_set(const char* key, const char* value); Set/overwrite the value corresponding to the specified key.

Note that this API does not (currently) support URL prefixed keys, so the client will need to take this into account.

Defined .rc file keys

There a a number of keys used by the netcdf-c library. Most of them are authorization-related. The file "auth.md" describes these keys.

Other keys are as follows:

libdap4/d4curlfunctions.c and oc2/ocinternal.c
- HTTP.READ.BUFFERSIZE – set the read buffer size for DAP2/4 connection
- HTTP.KEEPALIVE – turn on keep-alive for DAP2/4 connection
libdispatch/ds3util.c
- AWS.PROFILE – alternate way to specify the default AWS profile
- AWS.REGION – alternate way to specify the default AWS region
libnczarr/zinternal.c
- ZARR.DIMENSION_SEPARATOR – alternate way to specify the Zarr dimension separator character
oc2/occurlfunctions.c
- HTTP.NETRC – alternate way to specify the path of the .netrc file

History

Author: Dennis Heimbigner
Email: dmh at ucar dot edu
Initial Version: 01/09/2023
Last Revised: 07/30/2024

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