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SerenityOS Patterns

Introduction

Over time numerous reoccurring patterns have emerged from or were adopted by the serenity code base. This document aims to track and describe them, so they can be propagated further and the code base can be kept consistent.

TRY(...) Error Handling

The TRY(..) macro is used for error propagation in the serenity code base. The goal being to reduce the amount of boiler plate error code required to properly handle and propagate errors throughout the code base.

Any code surrounded by TRY(..) will attempt to be executed, and any error will immediately be returned from the function. If no error occurs then the result of the contents of the TRY will be the result of the macro's execution.

Examples:

Example from LibGfx:

#include <AK/Try.h>

... snip ...

ErrorOr<NonnullRefPtr<Bitmap>> Bitmap::create_shareable(BitmapFormat format, IntSize size, int scale_factor)
{
    if (size_would_overflow(format, size, scale_factor))
        return Error::from_string_literal("Gfx::Bitmap::create_shareable size overflow");

    auto const pitch = minimum_pitch(size.width() * scale_factor, format);
    auto const data_size = size_in_bytes(pitch, size.height() * scale_factor);

    auto buffer = TRY(Core::AnonymousBuffer::create_with_size(round_up_to_power_of_two(data_size, PAGE_SIZE)));
    auto bitmap = TRY(Bitmap::create_with_anonymous_buffer(format, buffer, size, scale_factor, {}));
    return bitmap;
}

Example from the Kernel:

#include <AK/Try.h>

... snip ...

ErrorOr<Region*> AddressSpace::allocate_region(VirtualRange const& range, StringView name, int prot, AllocationStrategy strategy)
{
    VERIFY(range.is_valid());
    OwnPtr<KString> region_name;
    if (!name.is_null())
        region_name = TRY(KString::try_create(name));
    auto vmobject = TRY(AnonymousVMObject::try_create_with_size(range.size(), strategy));
    auto region = TRY(Region::try_create_user_accessible(range, move(vmobject), 0, move(region_name), prot_to_region_access_flags(prot), Region::Cacheable::Yes, false));
    TRY(region->map(page_directory()));
    return add_region(move(region));
}

Note: Our TRY(...) macro functions similarly to the ? operator in rust.

MUST(...) Error Handling

The MUST(...) macro is similar to TRY(...) except the macro enforces that the code run inside the macro must succeed, otherwise we assert.

Note that MUST(...) should not be used as a replacement for TRY(...) in cases where error propagation is not (currently) possible. Instead, the release_value_but_fixme_should_propagate_errors() method of ErrorOr<> should be used to retrieve the value and to mark the location for future improvement. MUST(...) is reserved for cases where we determine through other circumstances that it should not be possible for the code inside the macro to fail or if a failure is serious enough that the program needs to crash.

Example:

#include <AK/Vector.h>

... snip ...

ErrorOr<void> insert_one_to_onehundred(Vector<int>& vector)
{
    TRY(vector.try_ensure_capacity(vector.size() + 100));

    for (int i = 1; i <= 100; i++) {
        // We previously made sure that we allocated enough space, so the append operation shouldn't ever fail.
        MUST(vector.try_append(i));
    }

    return {};
}

Fallible Constructors

The usual C++ constructors are incompatible with SerenityOS's method of handling errors, as potential errors are passed using the ErrorOr return type. As a replacement, classes that require fallible operations during their construction define a static function that is fallible instead.

This fallible function (which should usually be named create) will handle any errors while preparing arguments for the internal constructor and run any required fallible operations after the object has been initialized. The resulting object is then returned as ErrorOr<T> or ErrorOr<NonnullOwnPtr<T>>.

Example:

class Decompressor {
public:
    static ErrorOr<NonnullOwnPtr<Decompressor>> create(NonnullOwnPtr<Core::Stream::Stream> stream)
    {
        auto buffer = TRY(CircularBuffer::create_empty(32 * KiB));
        auto decompressor = TRY(adopt_nonnull_own_or_enomem(new (nothrow) Decompressor(move(stream), move(buffer))));
        TRY(decompressor->initialize_settings_from_header());
        return decompressor;
    }

... snip ...

private:
    Decompressor(NonnullOwnPtr<Core::Stream::Stream> stream, CircularBuffer buffer)
        : m_stream(move(stream))
        , m_buffer(move(buffer))
    {
    }

    CircularBuffer m_buffer;
    NonnullOwnPtr<Core::Stream::Stream> m_stream;
}

The serenity_main(..) program entry point

Serenity has moved to a pattern where executables do not expose a normal C main function. A serenity_main(..) is exposed instead. The main reasoning is that the Main::Arguments struct can provide arguments in a more idiomatic way that fits with the serenity API surface area. The ErrorOr likewise allows the program to propagate errors seamlessly with the TRY(...) macro, avoiding a significant amount of clunky C style error handling.

These executables are then linked with the LibMain library, which will link in the normal C int main(int, char**) function which will call into the programs serenity_main(..) on program startup.

The creation of the pattern was documented in the following video: OS hacking: A better main() for SerenityOS C++ programs

Examples:

A function main(..) would normally look something like:

int main(int argc, char** argv)
{
   return 0;
}

Instead, serenity_main(..) is defined like this:

#include <LibMain/Main.h>

ErrorOr<int> serenity_main(Main::Arguments arguments)
{
    return 0; 
}

Intrusive Lists

Intrusive lists are common in the Kernel and in some specific cases are used in the SerenityOS userland. A data structure is said to be "intrusive" when each element holds the metadata that tracks the element's membership in the data structure. In the case of a list, this means that every element in an intrusive linked list has a node embedded inside it. The main advantage of intrusive data structures is you don't need to worry about handling out of memory (OOM) on insertion into the data structure. This means error handling code is much simpler than say, using a Vector in environments that need to be durable to OOM.

The common pattern for declaring an intrusive list is to add the storage for the intrusive list node as a private member. A public type alias is then used to expose the list type to anyone who might need to create it. Here is an example from the Region class in the Kernel:

class Region final
    : public Weakable<Region> {

public:

... snip ...

private:
    bool m_syscall_region : 1 { false };

    IntrusiveListNode<Region> m_memory_manager_list_node;
    IntrusiveListNode<Region> m_vmobject_list_node;

public:
    using ListInMemoryManager = IntrusiveList<&Region::m_memory_manager_list_node>;
    using ListInVMObject = IntrusiveList<&Region::m_vmobject_list_node>;
};

You can then use the list by referencing the public type alias like so:

class MemoryManager {

... snip ...

    Region::ListInMemoryManager m_kernel_regions;
    Vector<UsedMemoryRange> m_used_memory_ranges;
    Vector<PhysicalMemoryRange> m_physical_memory_ranges;
    Vector<ContiguousReservedMemoryRange> m_reserved_memory_ranges;
};

Static Assertions of the size of a type

It's a universal pattern to use static_assert to validate the size of a type matches the author's expectations. Unfortunately when these assertions fail they don't give you the values that actually caused the failure. This forces one to go investigate by printing out the size, or checking it in a debugger, etc.

For this reason AK::AssertSize was added. It exploits the fact that the compiler will emit template argument values for compiler errors to provide debugging information. Instead of getting no information you'll get the actual type sizes in your compiler error output.

Example Usage:

#include <AK/StdLibExtras.h>

struct Empty { };

static_assert(AssertSize<Empty, 1>());

String View Literals

AK::StringView support for operator""sv which is a special string literal operator that was added as of C++17 to enable std::string_view literals.

[[nodiscard]] ALWAYS_INLINE constexpr AK::StringView operator""sv(const char* cstring, size_t length)
{
    return AK::StringView(cstring, length);
}

This allows AK::StringView to be constructed from string literals with no runtime cost to find the string length, and the data the AK::StringView points to will reside in the data section of the binary.

Example Usage:

#include <AK/String.h>
#include <AK/StringView.h>
#include <LibTest/TestCase.h>

TEST_CASE(string_view_literal_operator)
{
    StringView literal_view = "foo"sv;
    String test_string = "foo";

    EXPECT_EQ(literal_view.length(), test_string.length());
    EXPECT_EQ(literal_view, test_string);
}

Source Location

C++20 added std::source_location, which lets you capture the callers FILE / LINE / FUNCTION etc as a default argument to functions. See: https://en.cppreference.com/w/cpp/utility/source_location

AK::SourceLocation is the implementation of this feature in SerenityOS. It's become the idiomatic way to capture the location when adding extra debugging instrumentation, without resorting to littering the code with preprocessor macros.

To use it, you can add the AK::SourceLocation as a default argument to any function, using AK::SourceLocation::current() to initialize the default argument.

Example Usage:

#include <AK/SourceLocation.h>
#include <AK/StringView.h>

static StringView example_fn(const SourceLocation& loc = SourceLocation::current())
{
    return loc.function_name();
}

int main(int, char**)
{
    return example_fn().length();
}

If you only want to only capture AK::SourceLocation data with a certain debug macro enabled, avoid adding #ifdef's to all functions which have the AK::SourceLocation argument. Since SourceLocation is just a simple struct, you can just declare an empty class which can be optimized away by the compiler, and alias both to the same name.

Example Usage:

#if LOCK_DEBUG
#    include <AK/SourceLocation.h>
#endif

#if LOCK_DEBUG
using LockLocation = SourceLocation;
#else
struct LockLocation {
    static constexpr LockLocation current() { return {}; }

private:
    constexpr LockLocation() = default;
};
#endif

type[] vs. Array<type> vs. Vector<type> vs. FixedArray<type>

There are four "contiguous list" / array-like types, including C-style arrays themselves. They share a lot of their API, but their use cases are all slightly different, mostly relating to how they allocate their data.

Note that Span<type> differs from all of these types in that it provides a view on data owned by somebody else. The four types mentioned above all own their data, but they can provide Span's which view all or part of their data. For APIs that aren't specific to the kind of list and don't need to handle resizing in any way, Span is a good choice.

  • C-style arrays are generally discouraged (and this also holds for pointer+size-style arrays when passing them around). They are only used for the implementation of other collections or in specific circumstances.
  • Array is a thin wrapper around C-style arrays similar to std::array, where the template arguments include the size of the array. It allocates its data inline, just as arrays do, and never does any dynamic allocations.
  • Vector is similar to std::vector and represents a dynamic resizable array. For most basic use cases of lists, this is the go-to collection. It has an optional inline capacity (the second template argument) which will allocate inline as the name suggests, but this is not always used. If the contents outgrow the inline capacity, Vector will automatically switch to the standard out-of-line storage. This is allocated on the heap, and the space is automatically resized and moved when more (or less) space is needed.
  • FixedArray is essentially a runtime-sized Array. It can't resize like Vector, but it's ideal for circumstances where the size is not known at compile time but doesn't need to change once the collection is initialized. FixedArray guarantees to not allocate or deallocate except for its constructor and destructor.