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A C99 library for accessing the OpenCage geocoder
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README.md

c-ocgeo

This is a C99 library for accessing the OpenCage Geocoder API.

Building

We assume a POSIX compliant environment. We have tested it in MacOSX, OpenBSD, and Linux. It should work in Windows too although not tested yet. The sole prerequisite is the availability of libcurl that is used for making the HTTP requests. It additionally uses two very nice C libraries that are contained in source form in the code repository:

There is a (GNU) Makefile for building it as a static library (libocgeo.a) but you are free to copy the source files and use you own build process. You may need to adjust the Makefile if for example the header files and library file of libcurl is not installed in some "well-known" directory (usually /usr/local/include and /usr/local/lib respectively).

Usage

First of all it's important to check the best practices for using the OpenCage API, and in particular how to format forward geocoding queries.

You can see the example.c file for an example. The recommended use is as follows:

  • First of all, the user should have an API key by registering with the OpenCage Geocoder, otherwise the library can not be used (well, unless you use some testing API keys).

  • Initialization of an ocgeo_params struct that contains various (optional) parameters for the request:

    ocgeo_params_t params = ocgeo_default_params();
  • The user can then overwrite the default parameters if s/he so wishes, e.g.

    params.language = "hi"; // Hindi
    params.min_confidence = 5;
    params.countrycode = "in"; // India
  • Make a "forward" request providing the API key and your query:

    ocgeo_response_t response;
    const char* query = "Syena, Aswan Governorate, Egypt";
    ocgeo_forward(query, api_key, &params, &response);

    Or, make a "reverse" lookup request providing a latitude and longitude:

    double lat = 24.0875;
    double lon = 32.898889;
    ocgeo_reverse(lat, lon, api_key, &params, &response);
  • Check the returned code for possible errors:

    if (ocgeo_response_ok(&response)) {
      // Successful!
      // ...
    }
    else {
      printf("API returned error: %s\n", response.status.message);
    }
  • If successful, iterate over the results:

    printf("Got %d results:\n", response.total_results);
    ocgeo_result_t* result;
    foreach_ocgeo_result(result, &response) {
       printf("%s (type: %s, conf:%d)\n",
           result->formatted, result->type, result->confidence);
    }
  • Finally (do not forget this!) cleanup the response to free the memory used:

    ocgeo_response_cleanup(&response);

The basic API offered by this library is therefore the following functions:

/* Create a parameters "object" with default values.
 * The default values for all the fields are 0 or NULL (for the pointer fields)
 * and "invalid" latitude/longitude values for the ones (such as `proximity`)
 * that correspond to coordinates.
 */
ocgeo_params_t ocgeo_default_params(void);

/* Make a forward request i.e. find information about an address, place etc.
   You can supply NULL as `params` and the default values will be used
*/
ocgeo_response_t* ocgeo_forward(const char* query, const char* api_key, ocgeo_params_t* params, ocgeo_response_t* response);

/* Make a reverse request i.e. find what exists in the given latitude and longtitude.
   You can supply NULL as `params` and the default values will be used
*/
ocgeo_response_t* ocgeo_reverse(double lat, double lng, const char* api_key, ocgeo_params_t* params, ocgeo_response_t* response);

/* Free the memory used by the response of a forward or reverse call */
void ocgeo_response_cleanup(ocgeo_response_t* r);

Since some of the fields are optional and in order to be more "future-proof", there are a couple of additional functions to support a more generic API that allows to dynamically walk/traverse the JSON response:

/*
 * "Advanced" JSON traversing API!
 * This is useful for accessing the fields of the returned JSON document
 * in a generic way, since many of the fields may be missing or new fields
 * may be added in the future.
 * 
 * The caller provides a "path" string that contains a series of fields
 * separated by dots ('.') to access any internal field value. Positive
 * integer path segments are interpreted as indices in JSON arrays.
 *
 * Examples of paths:
 *  - "annotations.DMS.lat": get the "lat" value, in the "DMS" field of
 *               the "annotation" field in response
 *  - "annotations.currency.alternate_symbols.1": get the value at index 1 (2nd elem) 
 *                in the "alternate_symbols" field (which is a JSON array) of the
 *                "currency" annotation
 */
/* Return as string value. The caller should not alter the string, or free the returned
   pointer, as it points to internally managed memory. If successful (i.e. the field
   exists and has no 'null' value), `ok` will set to true */
const char* ocgeo_response_get_str(ocgeo_result_t* r, const char* path, bool* ok);
/* Return as int value. If successful (i.e. the field exists and has no 'null' value),
  `ok` will set to true */
int ocgeo_response_get_int(ocgeo_result_t* r, const char* path, bool* ok);
/* Return as double value. If successful (i.e. the field exists and has no 'null' value),
  `ok` will set to true */
double ocgeo_response_get_dbl(ocgeo_result_t* r, const char* path, bool* ok);

Again, please have a look at example.c and tests.c files for examples.

Design

  • A decimal latitude or longitude is represented as double This is to ensure that more precision is possible in specifying geographic coordinates.

  • The caller is responsible for the management of the memory. The design of the ocgeo_params_t parameters struct permits the declaration of corresponding variables in the stack or the heap. The library cannot shun the dynamic allocation for internal fields of the response (ocgeo_response_t) structure and thus the caller should always call ocgeo_response_cleanup after any request.

  • Some fields of the response (ocgeo_response_t) structure are optional. The caller should always check for NULL values in the pointers therein.

  • We try to parse the JSON response into "typed" C structs but since the OpenCageData Geocoder API is aggregating data from various sources that can frequently change their database there's high probability that the returned data structures are incomplete (e.g. new annotations maybe added in the future or new fields.). The path ("advanced") API is a means to cover these cases. Another option is to use the void* internal field of the ocgeo_result_t, which is actually a pointer to cJSON data, and the cJSON API to get whatever information is not available directly by this library.

  • The API is by design synchronous. Making it async is possible, using libcurl's multi interface, but:

    1. This would complicate the API
    2. It would make easier to exceed the request per sec limit of the user's plan, unless we provide an upper limit on the concurrent requests which will complicate the API even more :-)

    Of course it's an interesting exercise, please chime in if it's important for you :-)

Miscellaneous

  • The library sends requests using the following User Agent HTTP header :

    c-ocgeo/<version> (<curl-version>)
    

    where <version> comes from the Git commit tag or hash and <curl-version> is the version of the libcurl used.

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