Basic concepts and primitives


Internally, time is represent using the webrtc::Timestamp class. This represents time with a resolution of one microsecond, using a 64-bit integer, and provides converters to milliseconds or seconds as needed.

All timestamps need to be measured from the system monotonic time.

The epoch is not specified (because we can't always know if the system clock is correct), but whenever an absolute epoch is needed, the Unix time epoch (Jan 1, 1970 at 0:00 GMT) is used.

Conversion from/to other formats (for example milliseconds, NTP times, timestamp strings) should happen as close to the interface requiring that format as possible.

NOTE: There are parts of the codebase that don‘t use Timestamp, parts of the codebase that use the NTP epoch, and parts of the codebase that don’t use the monotonic clock. They need to be updated.


All execution happens on a TaskQueue instance. How a TaskQueue is implemented varies by platform, but they all have the webrtc::TaskQueueBase API.

This API offers primitives for posting tasks, with or without delay.

Some core parts use the rtc::Thread, which is a subclass of TaskQueueBase. This may contain a SocketServer for processing I/O, and is used for policing certain calling pattern between a few core threads (the NetworkThread cannot do Invoke on the Worker thread, for instance).

Reserved class suffixes

C++ classes with names ending in the suffixes “Factory”, “Builder” and “Manager” are supposed to behave in certain well known ways.

For a particular class name Foo, the following classes, if they exist, should behave as follows:

  • FooFactory: Has a Create function that creates a Foo object and returns the object or an owning reference to it (for instance std::unique_ptr or rtc::scoped_refptr). The Create function should NOT alter the factory state; ideally, it is marked const. Ownership of the returned object is only with the caller.

  • FooBuilder: Has a Build function that returns ownership of a Foo object (as above). The Builder can only be used once, and resources given to the Builder before the Build function is called are either released or owned by the Foo object. The Create function may be reference-qualified (declared as Foo Build() &&), which means it is invoked as std::move(builder).Build(), and C++ will ensure that it is not used again.

  • FooManager: Has a Create function that returns an rtc::scoped_refptr (if shared ownership) or a Foo* (if the Manager retains sole ownership). If Create() cannot fail, consider returning a Foo&. The Manager is responsible for keeping track of the object; if the Create function returns a Foo*, the Foo object is guaranteed to be destroyed when the FooManager is destroyed.

If a Manager class manages multiple classes of objects, the Create functions should be appropriately named (the FooAndBarManager would have CreateFoo() and CreateBar() functions), and the class will have a suitable name for the group of objects it is managing.

FooFactory is mainly useful for the case where preparation for producing Foo objects is complex. If Foo can be created with just an argument list, consider exposing its constructor instead; if Foo creation can fail, consider having a free function called CreateFoo instead of a factory.

Note that classes with these names exist that do not follow these conventions. When they are detected, they need to be marked with TODO statements and bugs filed on them to get them into a conformant state.

Synchronization primitives

PostTask and thread-guarded variables

The preferred method for synchronization is to post tasks between threads, and to let each thread take care of its own variables (lock-free programming). All variables in classes intended to be used with multiple threads should therefore be annotated with RTC_GUARDED_BY(thread).

For classes used with only one thread, the recommended pattern is to let them own a webrtc::SequenceChecker (conventionally named sequence_checker_) and let all variables be RTC_GUARDED_BY(sequence_checker_).

Member variables marked const do not need to be guarded, since they never change. (But note that they may point to objects that can change!)

When posting tasks with callbacks, it is the duty of the caller to check that the object one is calling back into still exists when the callback is made. A helper for this task is the webrtc::ScopedTaskSafety flag, which can automatically drop callbacks in this situation, and associated classes.

Synchronization primitives to be used when needed

When it is absolutely necessary to let one thread wait for another thread to do something, Thread::Invoke can be used. This function is DISCOURAGED, since it leads to performance issues, but is currently still widespread.

When it is absolutely necessary to access one variable from multiple threads, the webrtc::Mutex can be used. Such variables MUST be marked up with RTC_GUARDED_BY(mutex), to allow static analysis that lessens the chance of deadlocks or unintended consequences.

Synchronization primitives that are being removed

The following non-exhaustive list of synchronization primitives are in the (slow) process of being removed from the codebase.

  • sigslot. Use webrtc::CallbackList instead, or, when there's only one signal consumer, a single std::function.

  • AsyncInvoker.

  • RecursiveCriticalSection. Try to use webrtc::Mutex instead, and don't recurse.

Enum-To-String functions

If there is a need to convert an enum to a string representation, such as for enums exposed at the Javascript API interface, the recommended way is to write a function named AsString, declared “static constexpr” and returning an absl::string_view. The declaration should be right after the enum declaration, in the same scope; the implementation (which must be marked “inline”) should be at the end of the same header file.

If the enum is not defined within a class, the “static” keyword is not needed.