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Contributing to mongo-c-driver

Thanks for considering contributing to the mongo-c-driver!

This document intends to be a short guide to helping you contribute to the codebase. It expects a familiarity with the C programming language and writing portable software. Whenever in doubt, feel free to ask others that have contributed or look at the existing body of code.


The mongo-c-driver has a few guidelines that help direct the process.


mongo-c-driver is portable software. It needs to run on a multitude of operating systems and architectures.

  • Linux (RHEL 5 and newer)
  • FreeBSD (10 and newer)
  • Windows (Vista a newer)
  • Solaris x86_64/SPARC (11 and newer)
  • SmartOS (Solaris based)
  • Possibly more if users show an interest.
  • ARM/SPARC/x86/x86_64


Some of the mongo-c-driver users embed the library statically in their products. Therefore, the driver needs to be liberally licensed (as opposed to the authors usual preference of LGPL-2+). Therefore, all contributions must also be under this license. As a policy, we have chosen Apache 2.0 as the license for the project.

Coding Style

We try not to be pedantic with taking contributions that are not properly formatted, but we will likely perform a followup commit that cleans things up. The basics are, in vim:

: set ts=3 sw=3 et

3 space tabs, insert spaces instead of tabs.

Place a space between the function name and the parameter as such:

static void
my_func (Param *p)

my_func (p);

Not all of the code does this today, but it should be cleaned up at some point.

Just look at the code around for more pedantic styling choices.

Enum, Struct, Variable Naming

The naming conventions for mongo-c-driver should feel very object oriented. In fact, mongo-c-driver is OOP. Those that have used the GLib library will feel right at home, as the author has spent many years contributing to that project as well.

Structs are suffixed in _t, and underscores.

typedef struct _my_struct_t my_struct_t;

struct _my_struct_t
   int foo;

Function names should be prefixed by the type name, without the _t.

int my_struct_get_foo (my_struct_t *my);

Enums are also named with the _t suffix.

typedef enum
   MY_FLAGS_A = 1,
   MY_FLAGS_B = 1 << 1,
   MY_FLAGS_C = 1 << 2,
} my_flags_t;

Adding a new error code or domain

When adding a new error code or domain, you must do the following. This is most applicable if you are adding a new symbol with a bson_error_t as a parameter, and the existing codes or domains are inappropriate.

  • Add the domain to mongoc_error_domain_t in src/mongoc/mongoc-error.h
  • Add the code to mongoc_error_code_t in src/mongoc/mongoc-error.h
  • Add documentation for the domain or code to the table in doc/mongoc_errors.page

Adding a new symbol

This should be done rarely but there are several things that you need to do when adding a new symbol.

  • Add the symbol to src/libmongoc.symbols
  • Add the symbol to build/autotools/versions.ldscript
  • Add the symbol to all the .def files in build/cmake/.
  • Add documentation for the new symbol in doc/mongoc_your_new_symbol_name.page


We strive to document all symbols. See doc/ for documentation examples. If you add a new function, add a new .txt file describing the function so that we can generate man pages and HTML for it.


To run the entire test suite, including authentication tests, start mongod with auth enabled:

$ mongod --auth

In another terminal, use the mongo shell to create a user:

$ mongo --eval "db.createUser({user: 'admin', pwd: 'pass', roles: ['root']})" admin

To authenticate against MongoDB 3.0+ requires SCRAM-SHA-1, which in turn requires a driver built with OpenSSL:

$ ./configure --enable-ssl`

Set the user and password environment variables, then build and run the tests:

$ export MONGOC_TEST_USER=admin
$ make test

Additional environment variables:

  • MONGOC_TEST_HOST: default localhost, the host running MongoDB.
  • MONGOC_TEST_PORT: default 27017, MongoDB's listening port.
  • MONGOC_TEST_URI: override both host and port with a full connection string, like "mongodb://server1,server2".
  • MONGOC_TEST_SERVER_VERBOSE: set to on for wire protocol logging from tests that use mock_server_t.
  • MONGOC_TEST_MONITORING_VERBOSE: set to on for verbose output from Application Performance Monitoring tests.

If you start mongod with SSL, set these variables to configure how make test connects to it:

  • MONGOC_TEST_SSL: set to on to connect to the server with SSL.
  • MONGOC_TEST_SSL_PEM_FILE: path to a client PEM file.
  • MONGOC_TEST_SSL_PEM_PWD: the PEM file's password.
  • MONGOC_TEST_SSL_CA_FILE: path to a certificate authority file.
  • MONGOC_TEST_SSL_CA_DIR: path to a certificate authority directory.
  • MONGOC_TEST_SSL_CRL_FILE: path to a certificate revocation list.
  • MONGOC_TEST_SSL_WEAK_CERT_VALIDATION: set to on to relax the client's validation of the server's certificate.

The SASL / GSSAPI / Kerberos tests are skipped by default. To run them, set up a separate mongod with Kerberos and set its host and Kerberos principal name as environment variables:


URI-escape the username, for example write "user@realm" as "user%40realm". The user must be authorized to query test.collection.

The SASL / GSSAPI / Kerberos tests are skipped by default. To run them, set up a separate mongod with Kerberos and set its host and Kerberos principal name as environment variables:


URI-escape the username, for example write "user@realm" as "user%40realm". The user must be authorized to query test.collection.

MongoDB 3.2 adds support for readConcern, but does not enable support for read concern majority by default. mongod must be launched using --enableMajorityReadConcern. The test framework does not (and can't) automatically discover if this option was provided to MongoDB, so an additional variable must be set to enable these tests:


Set this environment variable to on if MongoDB has enabled majority read concern.

Some tests require Internet access, e.g. to check the error message when failing to open a MongoDB connection to example.com. Skip them with:


For quick checks during development, disable long-running tests:


All tests should pass before submitting a patch.

Configuring the test runner

The test runner can be configured by declaring the TEST_ARGS environment variable. The following options can be provided:

    -h, --help    Show this help menu.
    -f, --no-fork Do not spawn a process per test (abort on first error).
    -l NAME       Run test by name, e.g. "/Client/command" or "/Client/*".
    -v            Be verbose with logs.
    -s, --silent  Suppress all output.
    -F FILENAME   Write test results (JSON) to FILENAME.
    -d            Print debug output (useful if a test hangs).
    -t, --trace   Enable mongoc tracing (useful to debug tests).

TEST_ARGS is set to "--no-fork" by default, meaning that the suite aborts on the first test failure. Use "--fork" to continue after failures.

To run just a specific portion of the test suite use the -l option like so:

$ make test TEST_ARGS="-l /server_selection/*"

The full list of tests is shown in the help.

Debugging failed tests

The easiest way to debug a failed tests is to use the debug make target:

$ make debug TEST_ARGS="-l /WriteConcern/bson_omits_defaults"

This will build all dependencies and leave you in a debugger ready to run the test.