# Tutorial

This guide offers a brief introduction to the MongoDB C Driver.

For more information on the C API, please refer to the API Documentation.

## 0. Installing

For detailed instructions on installing the MongoDB C Driver on a particular platform, please see the installation guide.

## 1. Starting MongoDB

To run the examples in this tutorial, MongoDB must be installed and running on localhost on the default port, 27017. To check if it is up and running, connect to it with the MongoDB shell.

    $ mongo --host localhost --port 27017
    MongoDB shell version: 3.0.6
    connecting to: localhost:27017/test
    >

## 2. Making a Connection

The C Driver provides a convenient way to access MongoDB -- regardless of cluster configuration -- via a mongoc_client_t. It transparently connects to standalone servers, replica sets and sharded clusters on demand. Once a connection has been made, handles to databases and collections can be obtained via the structs mongoc_database_t and mongoc_collection_t, respectively. MongoDB operations can then be performed through these handles.

At the start of an application, call mongoc_init() before any other libmongoc functions and call mongoc_cleanup() before exiting. When creating handles to clients, databases and servers, call the appropriate destroy functions when finished.

The example below establishes a connection to a standalone server on localhost and performs a simple command. More information about database operations can be found in the CRUD Operations and Executing Commands sections. Examples of connecting to replica sets and sharded clusters can be found on the Advanced Connections page.

### connect.c

    #include <bson.h>
    #include <bcon.h>
    #include <mongoc.h>

    int
    main (int   argc,
          char *argv[])
    {
       mongoc_client_t      *client;
       mongoc_database_t    *database;
       mongoc_collection_t  *collection;
       bson_t               *command,
                             reply,
                            *insert;
       bson_error_t          error;
       char                 *str;
       bool                  retval;

       /*
        * Required to initialize libmongoc's internals
        */
       mongoc_init ();

       /*
        * Create a new client instance
        */
       client = mongoc_client_new ("mongodb://localhost:27017");

       /*
        * Get a handle on the database "db_name" and collection "coll_name"
        */
       database = mongoc_client_get_database (client, "db_name");
       collection = mongoc_client_get_collection (client, "db_name", "coll_name");

       /*
        * Do work. This example pings the database, prints the result as JSON and
        * performs an insert
        */
       command = BCON_NEW ("ping", BCON_INT32 (1));

       retval = mongoc_client_command_simple (client, "admin", command, NULL, &reply, &error);

       if (!retval) {
          fprintf (stderr, "%s\n", error.message);
          return EXIT_FAILURE;
       }

       str = bson_as_json (&reply, NULL);
       printf ("%s\n", str);

       insert = BCON_NEW ("hello", BCON_UTF8 ("world"));

       if (!mongoc_collection_insert (collection, MONGOC_INSERT_NONE, insert, NULL, &error)) {
          fprintf (stderr, "%s\n", error.message);
       }

       bson_destroy (insert);
       bson_destroy (&reply);
       bson_destroy (command);
       bson_free (str);

       /*
        * Release our handles and clean up libmongoc
        */
       mongoc_collection_destroy (collection);
       mongoc_database_destroy (database);
       mongoc_client_destroy (client);
       mongoc_cleanup ();

       return 0;
    }

On a UNIX-like system, the code can be compiled and run like so:

    $ gcc -o connect connect.c $(pkg-config --cflags --libs libmongoc-1.0)
    $ ./connect
    { "ok" : 1.000000 }

Alternatively, if pkg-config is not available, paths and libraries can be managed manually.

    $ gcc -o connect connect.c -I/usr/local/include -lmongoc-1.0 -lbson-1.0
    $ ./connect
    { "ok" : 1.000000 }

For Windows users, the code can be compiled and run with the following commands. (This assumes that the MongoDB C Driver has been installed to C:\mongo-c-driver; change the include directory as needed.)

    C:\> cl.exe /IC:\mongo-c-driver\include\libbson-1.0 /IC:\mongo-c-driver\include\libmongoc-1.0 connect.c
    C:\> connect
    { "ok" : 1.000000 }

## 3. Creating BSON Documents

Documents are stored in MongoDB's data format, BSON. The C driver uses libbson to create BSON documents. There are several ways to construct them: appending key-value pairs, using BCON, or parsing JSON.

### Appending BSON

A BSON document, represented as a bson_t in code, can be constructed one field at a time using libbson's append functions.

    #include <bson.h>

    int
    main (int   argc,
          char *argv[])
    {
       bson_t *document;
       bson_t  child;
       char   *str;

       document = bson_new ();

       /*
        * Append {"hello" : "world"} to the document.
        * Passing -1 for the length argument tells libbson to calculate the string length.
        */
       bson_append_utf8 (document, "hello", -1, "world", -1);

       /*
        * For convenience, this macro is equivalent.
        */
       BSON_APPEND_UTF8 (document, "hello", "world");

       /*
        * Begin a subdocument.
        */
       BSON_APPEND_DOCUMENT_BEGIN (document, "subdoc", &child);
       BSON_APPEND_UTF8 (&child, "subkey", "value");
       bson_append_document_end (document, &child);

       /*
        * Print the document as a JSON string.
        */
       str = bson_as_json (document, NULL);
       printf ("%s\n", str);
       bson_free (str);

       /*
        * Clean up allocated bson documents.
        */
       bson_destroy (document);
       return 0;
    }

See the libbson documentation for all of the types that can be appended to a bson_t.

### Using BCON

BSON C Object Notation, BCON for short, is an alternative way of constructing BSON documents in a manner closer to the intended format. It has less type-safety than BSON's append functions but results in less code.

    #include <bcon.h>
    #include <bson.h>
    #include <stdio.h>

    int
    main (int   argc,
          char *argv[])
    {
       bson_t *doc;
       char *str;

       doc = BCON_NEW ("name", BCON_UTF8 ("Babe Ruth"),
                       "statistics", "{",
                          "batting_average", BCON_DOUBLE (.342),
                          "hits", BCON_INT32 (2873),
                          "home_runs", BCON_INT32 (714),
                          "rbi", BCON_INT32 (2213),
                       "}",
                       "nicknames", "[",
                          BCON_UTF8 ("the Sultan of Swat"),
                          BCON_UTF8 ("the Bambino"),
                       "]");

       str = bson_as_json (doc, NULL);
       printf ("%s\n", str);
       bson_free (str);

       bson_destroy (doc);

       return 0;
    }

Notice that BCON can create arrays, subdocuments and arbitrary fields.

### Creating BSON from JSON

For single documents, BSON can be created from JSON strings via bson_new_from_json.

To initialize BSON from a sequence of JSON documents, use bson_json_reader_t.

    #include <bson.h>

    int
    main (int   argc,
          char *argv[])
    {
       bson_error_t error;
       bson_t      *bson;
       char        *string;

       const char *json = "{\"hello\": \"world\"}";
       bson = bson_new_from_json ((const uint8_t *)json, -1, &error);

       if (!bson) {
          fprintf (stderr, "%s\n", error.message);
          return EXIT_FAILURE;
       }

       string = bson_as_json (bson, NULL);
       printf ("%s\n", string);
       bson_free (string);

       return 0;
    }

## 4. Basic CRUD Operations

This section demonstrates the basics of using the C Driver to interact with MongoDB.

### Inserting a Document

To insert documents into a collection, first obtain a handle to a mongoc_collection_t via a mongoc_client_t. Then, use mongoc_collection_insert() to add BSON documents to the collection. This example inserts into the database "mydb" and collection "mycoll".

When finished, ensure that allocated structures are freed by using their respective destroy functions.

#### insert.c

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

    int
    main (int   argc,
          char *argv[])
    {
        mongoc_client_t *client;
        mongoc_collection_t *collection;
        bson_error_t error;
        bson_oid_t oid;
        bson_t *doc;

        mongoc_init ();

        client = mongoc_client_new ("mongodb://localhost:27017/");
        collection = mongoc_client_get_collection (client, "mydb", "mycoll");

        doc = bson_new ();
        bson_oid_init (&oid, NULL);
        BSON_APPEND_OID (doc, "_id", &oid);
        BSON_APPEND_UTF8 (doc, "hello", "world");

        if (!mongoc_collection_insert (collection, MONGOC_INSERT_NONE, doc, NULL, &error)) {
            fprintf (stderr, "%s\n", error.message);
        }

        bson_destroy (doc);
        mongoc_collection_destroy (collection);
        mongoc_client_destroy (client);
        mongoc_cleanup ();

        return 0;
    }

Compile the code and run it:

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

On Windows:

    C:\> cl.exe /IC:\mongo-c-driver\include\libbson-1.0 /IC:\mongo-c-driver\include\libmongoc-1.0 insert.c
    C:\> insert

To verify that the insert succeeded, connect with the MongoDB shell.

    $ mongo
    MongoDB shell version: 3.0.6
    connecting to: test
    > use mydb
    switched to db mydb
    > db.mycoll.find()
    { "_id" : ObjectId("55ef43766cb5f36a3bae6ee4"), "hello" : "world" }
    >

### Finding a Document

To query a MongoDB collection with the C driver, use the function mongoc_collection_find(). This returns a cursor to the matching documents. The following examples iterate through the result cursors and print the matches to stdout as JSON strings.

Note that mongoc_collection_find uses a document as a query specifier; for example,

    { "color" : "red" }

will match any document with a field named "color" with value "red". An empty document {} can be used to match all documents.

This first example uses an empty query specifier to find all documents in the database "mydb" and collection "mycoll".

#### find.c

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

    int
    main (int   argc,
         char *argv[])
    {
      mongoc_client_t *client;
      mongoc_collection_t *collection;
      mongoc_cursor_t *cursor;
      const bson_t *doc;
      bson_t *query;
      char *str;

      mongoc_init ();

      client = mongoc_client_new ("mongodb://localhost:27017/");
      collection = mongoc_client_get_collection (client, "mydb", "mycoll");
      query = bson_new ();
      cursor = mongoc_collection_find (collection, MONGOC_QUERY_NONE, 0, 0, 0, query, NULL, NULL);

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

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

      return 0;
    }

Compile the code and run it:

    $ gcc -o find find.c $(pkg-config --cflags --libs libmongoc-1.0)
    $ ./find
    { "_id" : { "$oid" : "55ef43766cb5f36a3bae6ee4" }, "hello" : "world" }

On Windows:

    C:\> cl.exe /IC:\mongo-c-driver\include\libbson-1.0 /IC:\mongo-c-driver\include\libmongoc-1.0 find.c
    C:\> find
    { "_id" : { "$oid" : "55ef43766cb5f36a3bae6ee4" }, "hello" : "world" }

To look for a specific document, add a specifier to query. This example adds a call to BSON_APPEND_UTF8() to look for all documents matching {"hello" : "world"}.

#### find-specific.c

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

      int
      main (int   argc,
            char *argv[])
      {
          mongoc_client_t *client;
          mongoc_collection_t *collection;
          mongoc_cursor_t *cursor;
          const bson_t *doc;
          bson_t *query;
          char *str;

          mongoc_init ();

          client = mongoc_client_new ("mongodb://localhost:27017/");
          collection = mongoc_client_get_collection (client, "mydb", "mycoll");
          query = bson_new ();
          BSON_APPEND_UTF8 (query, "hello", "world");

          cursor = mongoc_collection_find (collection, MONGOC_QUERY_NONE, 0, 0, 0, query, NULL, NULL);

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

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

          return 0;
      }

    $ gcc -o find-specific find-specific.c $(pkg-config --cflags --libs libmongoc-1.0)
    $ ./find-specific
    { "_id" : { "$oid" : "55ef43766cb5f36a3bae6ee4" }, "hello" : "world" }

    C:\> cl.exe /IC:\mongo-c-driver\include\libbson-1.0 /IC:\mongo-c-driver\include\libmongoc-1.0 find-specific.c
    C:\> find-specific
    { "_id" : { "$oid" : "55ef43766cb5f36a3bae6ee4" }, "hello" : "world" }

### Updating a Document

This code snippet gives an example of using mongoc_collection_update() to update the fields of a document.

Using the "mydb" database, the following example inserts an example document into the "mycoll" collection. Then, using its _id field, the document is updated with different values and a new field.

#### update.c

    #include <bcon.h>
    #include <bson.h>
    #include <mongoc.h>
    #include <stdio.h>

    int
    main (int   argc,
          char *argv[])
    {
        mongoc_collection_t *collection;
        mongoc_client_t *client;
        bson_error_t error;
        bson_oid_t oid;
        bson_t *doc = NULL;
        bson_t *update = NULL;
        bson_t *query = NULL;

        mongoc_init ();

        client = mongoc_client_new ("mongodb://localhost:27017/");
        collection = mongoc_client_get_collection (client, "mydb", "mycoll");

        bson_oid_init (&oid, NULL);
        doc = BCON_NEW ("_id", BCON_OID (&oid),
                        "key", BCON_UTF8 ("old_value"));

        if (!mongoc_collection_insert (collection, MONGOC_INSERT_NONE, doc, NULL, &error)) {
            fprintf (stderr, "%s\n", error.message);
            goto fail;
        }

        query = BCON_NEW ("_id", BCON_OID (&oid));
        update = BCON_NEW ("$set", "{",
                               "key", BCON_UTF8 ("new_value"),
                               "updated", BCON_BOOL (true),
                           "}");

        if (!mongoc_collection_update (collection, MONGOC_UPDATE_NONE, query, update, NULL, &error)) {
            fprintf (stderr, "%s\n", error.message);
            goto fail;
        }

    fail:
        if (doc)
            bson_destroy (doc);
        if (query)
            bson_destroy (query);
        if (update)
            bson_destroy (update);

        mongoc_collection_destroy (collection);
        mongoc_client_destroy (client);
        mongoc_cleanup ();

        return 0;
    }

Compile the code and run it:

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

On Windows:

    C:\> cl.exe /IC:\mongo-c-driver\include\libbson-1.0 /IC:\mongo-c-driver\include\libmongoc-1.0 update.c
    C:\> update
    { "_id" : { "$oid" : "55ef43766cb5f36a3bae6ee4" }, "hello" : "world" }

To verify that the update succeeded, connect with the MongoDB shell.

    $ mongo
    MongoDB shell version: 3.0.6
    connecting to: test
    > use mydb
    switched to db mydb
    > db.mycoll.find({"updated" : true})
    { "_id" : ObjectId("55ef549236fe322f9490e17b"), "updated" : true, "key" : "new_value" }
    >

### Deleting a Document

This example illustrates the use of mongoc_collection_remove() to delete documents.

The following code inserts a sample document into the database "mydb" and collection "mycoll". Then, it deletes all documents matching {"hello" : "world"}.

#### delete.c

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

    int
    main (int   argc,
          char *argv[])
    {
        mongoc_client_t *client;
        mongoc_collection_t *collection;
        bson_error_t error;
        bson_oid_t oid;
        bson_t *doc;

        mongoc_init ();

        client = mongoc_client_new ("mongodb://localhost:27017/");
        collection = mongoc_client_get_collection (client, "test", "test");

        doc = bson_new ();
        bson_oid_init (&oid, NULL);
        BSON_APPEND_OID (doc, "_id", &oid);
        BSON_APPEND_UTF8 (doc, "hello", "world");

        if (!mongoc_collection_insert (collection, MONGOC_INSERT_NONE, doc, NULL, &error)) {
            fprintf (stderr, "Insert failed: %s\n", error.message);
        }

        bson_destroy (doc);

        doc = bson_new ();
        BSON_APPEND_OID (doc, "_id", &oid);

        if (!mongoc_collection_remove (collection, MONGOC_REMOVE_SINGLE_REMOVE, doc, NULL, &error)) {
            fprintf (stderr, "Delete failed: %s\n", error.message);
        }

        bson_destroy (doc);
        mongoc_collection_destroy (collection);
        mongoc_client_destroy (client);
        mongoc_cleanup ();

        return 0;
    }

Compile the code and run it:

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

On Windows:

    C:\> cl.exe /IC:\mongo-c-driver\include\libbson-1.0 /IC:\mongo-c-driver\include\libmongoc-1.0 delete.c
    C:\> delete

Use the MongoDB shell to prove that the documents have been removed successfully.

    $ mongo
    MongoDB shell version: 3.0.6
    connecting to: test
    > use mydb
    switched to db mydb
    > db.mycoll.count({"hello" : "world"})
    0
    >

### Counting Documents

Counting the number of documents in a MongoDB collection is similar to performing a find operation. This example counts the number of documents matching {"hello" : "world"} in the database "mydb" and collection "mycoll".

#### count.c

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

    int
    main (int   argc,
          char *argv[])
    {
       mongoc_client_t *client;
       mongoc_collection_t *collection;
       bson_error_t error;
       bson_t *doc;
       int64_t count;

       mongoc_init ();

       client = mongoc_client_new ("mongodb://localhost:27017/");
       collection = mongoc_client_get_collection (client, "mydb", "mycoll");
       doc = bson_new_from_json ((const uint8_t *)"{\"hello\" : \"world\"}", -1, &error);

       count = mongoc_collection_count (collection, MONGOC_QUERY_NONE, doc, 0, 0, NULL, &error);

       if (count < 0) {
          fprintf (stderr, "%s\n", error.message);
       } else {
          printf ("%" PRId64 "\n", count);
       }

       bson_destroy (doc);
       mongoc_collection_destroy (collection);
       mongoc_client_destroy (client);
       mongoc_cleanup ();

       return 0;
    }

Compile the code and run it:

    $ gcc -o count count.c $(pkg-config --cflags --libs libmongoc-1.0)
    $ ./count
    1

On Windows:

    C:\> cl.exe /IC:\mongo-c-driver\include\libbson-1.0 /IC:\mongo-c-driver\include\libmongoc-1.0 count.c
    C:\> count
    1

## 5. Executing Commands

The driver provides helper functions for executing MongoDB commands on client, database and collection structures. These functions return cursors; the _simple variants return booleans indicating success or failure.

This example executes the collStats command against the collection "mycoll" in database "mydb".

### executing.c

    #include <bson.h>
    #include <bcon.h>
    #include <mongoc.h>
    #include <stdio.h>

    int
    main (int   argc,
          char *argv[])
    {
        mongoc_client_t *client;
        mongoc_collection_t *collection;
        bson_error_t error;
        bson_t *command;
        bson_t reply;
        char *str;

        mongoc_init ();

        client = mongoc_client_new ("mongodb://localhost:27017/");
        collection = mongoc_client_get_collection (client, "mydb", "mycoll");

        command = BCON_NEW ("collStats", BCON_UTF8 ("mycoll"));
        if (mongoc_collection_command_simple (collection, command, NULL, &reply, &error)) {
            str = bson_as_json (&reply, NULL);
            printf ("%s\n", str);
            bson_free (str);
        } else {
            fprintf (stderr, "Failed to run command: %s\n", error.message);
        }

        bson_destroy (command);
        bson_destroy (&reply);
        mongoc_collection_destroy (collection);
        mongoc_client_destroy (client);
        mongoc_cleanup ();

        return 0;
    }

Compile the code and run it:

    $ gcc -o executing executing.c $(pkg-config --cflags --libs libmongoc-1.0)
    $ ./executing
    { "ns" : "mydb.mycoll", "count" : 1, "size" : 48, "avgObjSize" : 48, "numExtents" : 1, "storageSize" : 8192,
    "lastExtentSize" : 8192.000000, "paddingFactor" : 1.000000, "userFlags" : 1, "capped" : false, "nindexes" : 1,
    "indexDetails" : {  }, "totalIndexSize" : 8176, "indexSizes" : { "_id_" : 8176 }, "ok" : 1.000000 }

On Windows:

    C:\> cl.exe /IC:\mongo-c-driver\include\libbson-1.0 /IC:\mongo-c-driver\include\libmongoc-1.0 executing.c
    C:\> executing
    { "ns" : "mydb.mycoll", "count" : 1, "size" : 48, "avgObjSize" : 48, "numExtents" : 1, "storageSize" : 8192,
    "lastExtentSize" : 8192.000000, "paddingFactor" : 1.000000, "userFlags" : 1, "capped" : false, "nindexes" : 1,
    "indexDetails" : {  }, "totalIndexSize" : 8176, "indexSizes" : { "_id_" : 8176 }, "ok" : 1.000000 }

## 6. Threading

The MongoDB C Driver is thread-unaware in the vast majority of its operations. This means it is up to the programmer to guarantee thread-safety.

However, mongoc_client_pool_t is thread-safe and is used to fetch a mongoc_client_t in a thread-safe manner. After retrieving a client from the pool, the client structure should be considered owned by the calling thread. When the thread is finished, the client should be placed back into the pool.

    #include <mongoc.h>
    #include <pthread.h>

    #define N_THREADS 10

    static void *
    worker (void *data) {
       mongoc_client_pool_t *pool = data;
       mongoc_client_t      *client;

       client = mongoc_client_pool_pop (pool);

       /* Do something... */

       mongoc_client_pool_push (pool, client);

       return NULL;
    }

    int
    main (int   argc,
          char *argv[])
    {
       mongoc_client_pool_t *pool;
       mongoc_uri_t         *uri;
       pthread_t             threads[N_THREADS];

       mongoc_init ();

       uri = mongoc_uri_new ("mongodb://localhost/");
       pool = mongoc_client_pool_new (uri);

       for (i = 0; i < N_THREADS; i++) {
          pthread_create (&threads[i], NULL, worker, pool);
       }

       for (i = 0; i < N_THREADS; i++) {
          pthread_join (threads[i], NULL);
       }

       mongoc_client_pool_destroy (pool);
       mongoc_uri_destroy (uri);
       mongoc_cleanup ();

       return 0;
    }

## 7. Next Steps

To find information on advanced topics, browse the rest of the C driver guide or the official MongoDB documentation.

For help with common issues, consult the Troubleshooting page. To report a bug or request a new feature, follow these instructions.