Appendix D.2. Specifying Paths for NetCDF-C {#nc_paths_quickstart}
==============================

A key concept in netcdf-c is the notion of a "path".
A path specifies some dataset that is of interest to a user.
It is the primary argument to the *nc_open* and *nc_create*
functions in the NetCDF-C API, as defined by the file netcdf.h.
It is also the primary option for the NetCDF-C utilities:
*ncdump*, *nccopy*, and *ncgen*.
Hence understanding what kind of paths are acceptable is important
for using the NetCDF-C library.

## Classification of Paths {#nc_paths_kinds}

Basically, there are two kinds of paths:
1. File system paths, and
2. Uniform Resource Locator (URL) paths.

### File System Paths

The most common form of path accepted by the NetCDF-C library is a file system path.
Every user of some computer operating system is familiar with the idea of a file system path.

Each operating system has some special quirks when specifying file system paths.
Here are some example paths for various version of Linux/Unix.
* /
* /a/b/c/d

As a rule, Linux/Unix has a single root path typically indicated by "/",
and "/" is also used as the file separator.

For Windows, some example paths would be as follows:
* c:\\
* d:\\a\\b\\c

Windows has a notion of a drive ("d:") and each drive serves as the root
of its own file system. Windows uses "\\" as its file separator, although
many programs also accept "/".

## Uniform Resource Locator (URL) Paths

The NetCDF-C library can access datasets that reside on remote computers,
Hence NetCDF-C now also accepts URLs to specify those remote datasets.

The general form of a URL is as follows:
````
<protocol>://<user>:<pwd>@<host>:<port>/<path>?<query>#<fragment>
````
* \<protocol\> - specifies the format of the messages between the local and remote computers. Some examples used by NetCDF-C are *http*, *https*, *s3*, *dap*, *dap4*, or *file*.
* \<user:pwd\> - Pass authorization credentials to the remote computer.
* \<host\>:\<port\> - The specific remote computer to access, such as *thredds.ucar.edu:8080*.
* \<path\> - A specification of file-path-like string that indicates some specific resource on the remote computer.
* \<query\> - A sequence of (key=value) pairs, separated by "&", and providing special information to the remote computer.
* \<fragment\> - A sequence of (key=value) pairs, separated by "&", and providing special information to the local computer.

Most parts of a URL are optional. As a rule, the protocol is always required,
and either the host is required or, if the protocol is "file", then the path is required.
The query and fragment are optional.

### Examples of URL Paths for NetCDF-C
* https://thredds.ucar.edu/catalog

## Addendum A. Amazon S3 Specific URLS {#nc_paths_s3_urls}
A URL path is required for accessing datasets on the Amazon S3 storage cloud.
Unfortunately S3 URLs are complicated.
It has the following features:
* Protocol: _https_ or _s3_. The _s3_ scheme is equivalent to "https" plus setting various tags in the query and/or fragment part of the URL.
* Host: Amazon S3 defines three forms: _Virtual_, _Path_, and _S3_
  + _Virtual_: the host includes the bucket name as in __bucket.s3.&lt;region&gt;.amazonaws.com__ or __bucket.s3.amazonaws.com__
  + _Path_: the host does not include the bucket name, but rather the bucket name is the first segment of the path. For example __s3.&lt;region&gt;.amazonaws.com/bucket__ or __s3.amazonaws.com/bucket__
  + _S3_: the protocol is "s3:" and if the host is a single name, then it is interpreted as the bucket. The region is determined using an algorithm defined in the nczarr documentation.
  + _Other_: It is possible to use other non-Amazon cloud storage, but that is cloud library dependent.
* Query: currently not used.
* Fragment: the fragment is of the form _key=value&key=value&..._. Depending on the key, the _value_ part may be left out and some default value will be used. The exact set of possible keys is defined in the nczarr documentation.

## Addendum B. Known Fragment Keys {#nc_paths_frag_keys}

The fragment part of a URL is used to pass information deep into
the netcdf-c library to control its actions.
This appendix list known keys, although it may be somewhat out-of-date.

The current set of keys used in the netcdf-c library is as follows.
* _mode_ -- A special key that is used to provide single values for controlling the netcdf-c library. It consists of a comma separated sequence of values
primarily used to control the file format.
The *mode* key supports the following values
    - _dap2_ -- Specifies that the URL accesses a resource using the DAP2 protocol
    - _dap4_ -- Specifies that the URL accesses a resource using the DAP4 protocol
    - _netcdf-3_ -- Specifies that a file is a netcdf-classic file
    - _classic_ -- Alias for _netcdf-3_
    - _netcdf-4_ -- Specifies that a file is a netcdf-enhanced file
    - _enhanced_ -- Alias for _netcdf-4_
    - _udf0_ -- Specifies that the file format is defined by a User Defined format
    - _udf1_ -- Specifies that the file format is defined by a User Defined format
    - _nczarr_ -- Specifies that the file is in NCZarr format
    - _zarr_ -- Specifies that the file is in Zarr format
    - _xarray_ --Specifies that the file is in Zarr format and uses the XARRAY convention
    - _noxarray_ --Specifies that the file is in Zarr format and does not use the XARRAY convention
    - _s3_ --Specifies that the file is remote and is stored on the Amazon S3 cloud
    - _file_ --Specifies that the file is an NCZarr/Zarr file stored as a file tree
    - _zip_ --Specifies that the file is an NCZarr/Zarr file stored as a zip file
    - _bytes_ -- Specifies that the file is remote and is to be read using byte-range support
 in NCZarr format
* _dap2_ -- equivalent to "mode=dap2"
* _dap4_ -- equivalent to "mode=dap4"
* _bytes_ -- equivalent to "mode=bytes"
* _log_ -- turn on logging for the duration of the data request
* _show=fetch_ -- log curl fetch commands