- MongoDB C Driver 1.9.3-dev »

# Tutorial

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

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

Contents

- Tutorial
  - Installing
  - Starting MongoDB
  - Include and link libmongoc in your C program
  - Use libmongoc in a Microsoft Visual Studio Project
  - Making a Connection
  - Creating BSON Documents
  - Basic CRUD Operations
  - Executing Commands
  - Threading
  - Next Steps

## Installing

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

## 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
    >

## Include and link libmongoc in your C program

### Include mongoc.h

All libmongoc’s functions and types are available in one header file. Simply include `mongoc.h`:

    #include <mongoc.h>

#### CMake

The libmongoc installation includes a CMake config-file package, so you can use CMake’s find_package command to find libmongoc’s header and library paths and link to libmongoc:

CMakeLists.txt

    # Specify the minimum version you require.
    find_package (libmongoc-1.0 1.7 REQUIRED)

    message ("--   mongoc found version \"${MONGOC_VERSION}\"")
    message ("--   mongoc include path \"${MONGOC_INCLUDE_DIRS}\"")
    message ("--   mongoc libraries \"${MONGOC_LIBRARIES}\"")

    # The "hello_mongoc.c" sample program is shared among four tests.
    add_executable (hello_mongoc ../../hello_mongoc.c)
    target_include_directories (hello_mongoc PRIVATE "${MONGOC_INCLUDE_DIRS}")
    target_link_libraries (hello_mongoc PRIVATE "${MONGOC_LIBRARIES}")
    target_compile_definitions (hello_mongoc PRIVATE "${MONGOC_DEFINITIONS}")

By default, libmongoc is dynamically linked. You can use libmongoc as a static library instead: Use the included `libmongoc-static-1.0` config-file package:

    # Specify the minimum version you require.
    find_package (libmongoc-static-1.0 1.7 REQUIRED)

    message ("--   mongoc found version \"${MONGOC_STATIC_VERSION}\"")
    message ("--   mongoc include path \"${MONGOC_STATIC_INCLUDE_DIRS}\"")
    message ("--   mongoc libraries \"${MONGOC_STATIC_LIBRARIES}\"")

    # The "hello_mongoc.c" sample program is shared among four tests.
    add_executable (hello_mongoc ../../hello_mongoc.c)
    target_include_directories (hello_mongoc PRIVATE "${MONGOC_STATIC_INCLUDE_DIRS}")
    target_link_libraries (hello_mongoc PRIVATE "${MONGOC_STATIC_LIBRARIES}")
    target_compile_definitions (hello_mongoc PRIVATE "${MONGOC_STATIC_DEFINITIONS}")

#### pkg-config

If you’re not using CMake, use pkg-config on the command line to set header and library paths:

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

Or to statically link to libmongoc:

    gcc -o hello_mongoc hello_mongoc.c $(pkg-config --libs --cflags libmongoc-static-1.0)

#### Specifying header and include paths manually

If you aren’t using CMake or pkg-config, paths and libraries can be managed manually.

    $ gcc -o hello_mongoc hello_mongoc.c \
        -I/usr/local/include/libbson-1.0 -I/usr/local/include/libmongoc-1.0 \
        -lmongoc-1.0 -lbson-1.0
    $ ./hello_mongoc
    { "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 hello_mongoc.c
    C:\> hello_mongoc
    { "ok" : 1.000000 }

## Use libmongoc in a Microsoft Visual Studio Project

See the libmongoc and Visual Studio guide.

## Making a Connection

Access MongoDB with a mongoc_client_t. It transparently connects to standalone servers, replica sets and sharded clusters on demand. To perform operations on a database or collection, create a mongoc_database_t or mongoc_collection_t struct from the mongoc_client_t.

At the start of an application, call mongoc_init() before any other libmongoc functions. At the end, call the appropriate destroy function for each collection, database, or client handle, in reverse order from how they were constructed. Call mongoc_cleanup() before exiting.

The example below establishes a connection to a standalone server on `localhost`, registers the client application as “connect-example,” 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.

hello_mongoc.c

    #include <mongoc.h>

    int
    main (int argc, char *argv[])
    {
       const char *uri_str = "mongodb://localhost:27017";
       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 ();

       /*
        * Optionally get MongoDB URI from command line
        */
       if (argc > 1) {
          uri_str = argv[1];
       }

       /*
        * Create a new client instance
        */
       client = mongoc_client_new (uri_str);

       /*
        * Register the application name so we can track it in the profile logs
        * on the server. This can also be done from the URI (see other examples).
        */
       mongoc_client_set_appname (client, "connect-example");

       /*
        * 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_one (collection, insert, NULL, 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;
    }

## 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.

For example, to create a document like this:

    {
       born : ISODate("1906-12-09"),
       died : ISODate("1992-01-01"),
       name : {
          first : "Grace",
          last : "Hopper"
       },
       languages : [ "MATH-MATIC", "FLOW-MATIC", "COBOL" ],
       degrees: [ { degree: "BA", school: "Vassar" }, { degree: "PhD", school: "Yale" } ]
    }

Use the following code:

    #include <bson.h>

    int
    main (int   argc,
          char *argv[])
    {
       struct tm   born = { 0 };
       struct tm   died = { 0 };
       const char *lang_names[] = {"MATH-MATIC", "FLOW-MATIC", "COBOL"};
       const char *schools[] = {"Vassar", "Yale"};
       const char *degrees[] = {"BA", "PhD"};
       uint32_t    i;
       char        buf[16];
       const       char *key;
       size_t      keylen;
       bson_t     *document;
       bson_t      child;
       bson_t      child2;
       char       *str;

       document = bson_new ();

       /*
        * Append { "born" : ISODate("1906-12-09") } to the document.
        * Passing -1 for the length argument tells libbson to calculate the string length.
        */
       born.tm_year = 6;  /* years are 1900-based */
       born.tm_mon = 11;  /* months are 0-based */
       born.tm_mday = 9;
       bson_append_date_time (document, "born", -1, mktime (&born) * 1000);

       /*
        * Append { "died" : ISODate("1992-01-01") } to the document.
        */
       died.tm_year = 92;
       died.tm_mon = 0;
       died.tm_mday = 1;

       /*
        * For convenience, this macro passes length -1 by default.
        */
       BSON_APPEND_DATE_TIME (document, "died", mktime (&died) * 1000);

       /*
        * Append a subdocument.
        */
       BSON_APPEND_DOCUMENT_BEGIN (document, "name", &child);
       BSON_APPEND_UTF8 (&child, "first", "Grace");
       BSON_APPEND_UTF8 (&child, "last", "Hopper");
       bson_append_document_end (document, &child);

       /*
        * Append array of strings. Generate keys "0", "1", "2".
        */
       BSON_APPEND_ARRAY_BEGIN (document, "languages", &child);
       for (i = 0; i < sizeof lang_names / sizeof (char *); ++i) {
          keylen = bson_uint32_to_string (i, &key, buf, sizeof buf);
          bson_append_utf8 (&child, key, (int) keylen, lang_names[i], -1);
       }
       bson_append_array_end (document, &child);

       /*
        * Array of subdocuments:
        *    degrees: [ { degree: "BA", school: "Vassar" }, ... ]
        */
       BSON_APPEND_ARRAY_BEGIN (document, "degrees", &child);
       for (i = 0; i < sizeof degrees / sizeof (char *); ++i) {
          keylen = bson_uint32_to_string (i, &key, buf, sizeof buf);
          bson_append_document_begin (&child, key, (int) keylen, &child2);
          BSON_APPEND_UTF8 (&child2, "degree", degrees[i]);
          BSON_APPEND_UTF8 (&child2, "school", schools[i]);
          bson_append_document_end (&child, &child2);
       }
       bson_append_array_end (document, &child);

       /*
        * Print the document as a JSON string.
        */
       str = bson_as_canonical_extended_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 <bson.h>

    int
    main (int   argc,
          char *argv[])
    {
       struct tm born = { 0 };
       struct tm died = { 0 };
       bson_t   *document;
       char     *str;

       born.tm_year = 6;
       born.tm_mon = 11;
       born.tm_mday = 9;

       died.tm_year = 92;
       died.tm_mon = 0;
       died.tm_mday = 1;

       document = BCON_NEW (
          "born", BCON_DATE_TIME (mktime (&born) * 1000),
          "died", BCON_DATE_TIME (mktime (&died) * 1000),
          "name", "{",
          "first", BCON_UTF8 ("Grace"),
          "last", BCON_UTF8 ("Hopper"),
          "}",
          "languages", "[",
          BCON_UTF8 ("MATH-MATIC"),
          BCON_UTF8 ("FLOW-MATIC"),
          BCON_UTF8 ("COBOL"),
          "]",
          "degrees", "[",
          "{", "degree", BCON_UTF8 ("BA"), "school", BCON_UTF8 ("Vassar"), "}",
          "{", "degree", BCON_UTF8 ("PhD"), "school", BCON_UTF8 ("Yale"), "}",
          "]");

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

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

    #include <bson.h>

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

       const char *json = "{\"name\": {\"first\":\"Grace\", \"last\":\"Hopper\"}}";
       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_canonical_extended_json (bson, NULL);
       printf ("%s\n", string);
       bson_free (string);

       return 0;
    }

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

## 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_one() 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.

    #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/?appname=insert-example");
        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_one (
               collection, doc, NULL, 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_with_opts(). 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.

Use 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”.

    #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/?appname=find-example");
       collection = mongoc_client_get_collection (client, "mydb", "mycoll");
       query = bson_new ();
       cursor = mongoc_collection_find_with_opts (collection, query, NULL, NULL);

       while (mongoc_cursor_next (cursor, &doc)) {
          str = bson_as_canonical_extended_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"}`.

    #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/?appname=find-specific-example");
       collection = mongoc_client_get_collection (client, "mydb", "mycoll");
       query = bson_new ();
       BSON_APPEND_UTF8 (query, "hello", "world");

       cursor = mongoc_collection_find_with_opts (collection, query, NULL, NULL);

       while (mongoc_cursor_next (cursor, &doc)) {
          str = bson_as_canonical_extended_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_one() 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.

    #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/?appname=update-example");
       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_one (collection, 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_one (
              collection, query, update, NULL, 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_delete_one() to delete a document.

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

    #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/?appname=delete-example");
       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_one (collection, 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_delete_one (
              collection, doc, NULL, 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”.

    #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/?appname=count-example");
       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

## 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”.

    #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/?appname=executing-example");
       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_canonical_extended_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 }

## 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.

example-pool.c

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

    /* ./example-pool [CONNECTION_STRING] */

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

    static pthread_mutex_t mutex;
    static bool in_shutdown = false;

    static void *
    worker (void *data)
    {
       mongoc_client_pool_t *pool = data;
       mongoc_client_t *client;
       bson_t ping = BSON_INITIALIZER;
       bson_error_t error;
       bool r;

       BSON_APPEND_INT32 (&ping, "ping", 1);

       while (true) {
          client = mongoc_client_pool_pop (pool);
          /* Do something with client. If you are writing an HTTP server, you
           * probably only want to hold onto the client for the portion of the
           * request performing database queries.
           */
          r = mongoc_client_command_simple (
             client, "admin", &ping, NULL, NULL, &error);

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

          mongoc_client_pool_push (pool, client);

          pthread_mutex_lock (&mutex);
          if (in_shutdown || !r) {
             pthread_mutex_unlock (&mutex);
             break;
          }

          pthread_mutex_unlock (&mutex);
       }

       bson_destroy (&ping);
       return NULL;
    }

    int
    main (int argc, char *argv[])
    {
       const char *uristr = "mongodb://127.0.0.1/?appname=pool-example";
       mongoc_uri_t *uri;
       mongoc_client_pool_t *pool;
       pthread_t threads[10];
       unsigned i;
       void *ret;

       pthread_mutex_init (&mutex, NULL);
       mongoc_init ();

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

       uri = mongoc_uri_new (uristr);
       if (!uri) {
          fprintf (stderr, "Failed to parse URI: \"%s\".\n", uristr);
          return EXIT_FAILURE;
       }

       pool = mongoc_client_pool_new (uri);
       mongoc_client_pool_set_error_api (pool, 2);

       for (i = 0; i < 10; i++) {
          pthread_create (&threads[i], NULL, worker, pool);
       }

       sleep (10);
       pthread_mutex_lock (&mutex);
       in_shutdown = true;
       pthread_mutex_unlock (&mutex);

       for (i = 0; i < 10; i++) {
          pthread_join (threads[i], &ret);
       }

       mongoc_client_pool_destroy (pool);
       mongoc_uri_destroy (uri);

       mongoc_cleanup ();

       return 0;
    }

## 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.

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