mongoc_cursor_t

Client-side cursor abtraction

Synopsis

typedef struct _mongoc_cursor_t mongoc_cursor_t;

mongoc_cursor_t provides access to a MongoDB query cursor. It wraps up the wire protocol negotation required to initiate a query and retreive an unknown number of documents. Cursors are lazy, meaning that no network traffic occurs until the first call to mongoc_cursor_next(). At that point we can:

Determine which host we've connected to with mongoc_cursor_get_host().
Retreive more records with repeated calls to mongoc_cursor_next().
Test for more records with mongoc_cursor_more()
Clone a query to repeat execution at a later point with mongoc_cursor_clone().
Test for errors with mongoc_cursor_error().

Thread Safety

mongoc_cursor_t is NOT thread safe. It may only be used from the thread it was created from.

Example

Query MongoDB and iterate results

/* gcc example.c -o example $(pkg-config --cflags --libs libmongoc-1.0) */

/* ./example-client [CONNECTION_STRING [COLLECTION_NAME]] */

#include <mongoc.h>
#include <stdio.h>
#include <stdlib.h>

int
main (int   argc,
      char *argv[])
{
   mongoc_client_t *client;
   mongoc_collection_t *collection;
   mongoc_cursor_t *cursor;
   bson_error_t error;
   const bson_t *doc;
   const char *uristr = "mongodb://127.0.0.1/";
   const char *collection_name = "test";
   bson_t query;
   char *str;

   mongoc_init ();

   if (argc > 1) {
      uristr = argv [1];
   }

   if (argc > 2) {
      collection_name = argv [2];
   }

   client = mongoc_client_new (uristr);

   if (!client) {
      fprintf (stderr, "Failed to parse URI.\n");
      return EXIT_FAILURE;
   }

   bson_init (&query);

#if 0
   bson_append_utf8 (&query, "hello", -1, "world", -1);
#endif

   collection = mongoc_client_get_collection (client, "test", collection_name);
   cursor = mongoc_collection_find (collection,
                                    MONGOC_QUERY_NONE,
                                    0,
                                    0,
                                    0,
                                    &query,
                                    NULL,  /* Fields, NULL for all. */
                                    NULL); /* Read Prefs, NULL for default */

   while (mongoc_cursor_next (cursor, &doc)) {
      str = bson_as_json (doc, NULL);
      fprintf (stdout, "%s\n", str);
      bson_free (str);
   }

   if (mongoc_cursor_error (cursor, &error)) {
      fprintf (stderr, "Cursor Failure: %s\n", error.message);
      return EXIT_FAILURE;
   }

   bson_destroy (&query);
   mongoc_cursor_destroy (cursor);
   mongoc_collection_destroy (collection);
   mongoc_client_destroy (client);

   mongoc_cleanup ();

   return EXIT_SUCCESS;
}