Magic RingBuffer for Windows, Linux and macOS

MagicRingBuffer.h

#pragma once

//
// Magic Ring Buffer
// https://gist.github.com/mmozeiko/3b09a340f3c53e5eaed699a1aea95250
//
// Sets up memory mapping so the same memory block follows itself in virtual address space:
//
//   [abcd...xyz][abc...xyz]
//
// This allows reading or writing memory to this ringbuffer without worries of accessing
// memory past the end of buffer. Normally you would need to split such reads or writes
// into two read/write operations. But because memory is duplicated in virtual address
// space, the buffer reads/writes can always happen on sequential bytes in one operation.
//
// Typical usage:
//
//   MagicRG Buffer;
//   MRB_Init(&Buffer, 4096); // buffer will have space for at least 4096 bytes
//
//   size_t WriteAvailable = MRB_GetFree(&Buffer);   // how many bytes available to write
//   void* WritePtr = MRB_WritePtr(&Buffer);         // get pointer to write to
//   ... write up to WriteAvailable bytes to WritePtr
//   MRB_WriteEnd(&Buffer, WriteSize);               // finish writing, now reader is allowed
//                                                   // to read WriteSize amount of bytes
//
//   size_t ReadAvailable = MRB_GetUsed(&Buffer);    // how many bytes available to read
//   void* ReadPtr = MRB_ReadPtr(&Buffer);           // get pointer to read from
//   ... read up to ReadAvailable bytes from ReadPtr
//   MRB_ReadEnd(&Buffer, ReadSize);                 // finish reading, now writer is allowed
//                                                   // to write extra ReadSize amount of bytes
//
// Basically writer is allowed to write up to "free" amount of bytes.
// And reader is allowed to read up to "used" amount of bytes.
// It's up to you to check how many free or used bytes are in buffer before reading and writing.
//

#include <stddef.h>
#include <stdbool.h>

//
// interface
//

typedef struct {
	void* Data;
	size_t Size;
	size_t Read;
	size_t Write;
} MagicRB;

// allocation & freeing, size will be rounded up to multiple of page size
static inline void MRB_Init(MagicRB* RingBuffer, size_t Size);
static inline void MRB_Done(MagicRB* RingBuffer);

// size queries:
// used = how many bytes are available for reading
// free = how many bytes are available for writing
static inline size_t MRB_GetUsed(const MagicRB* RingBuffer);
static inline size_t MRB_GetFree(const MagicRB* RingBuffer);

// check if buffer is empty or full
static inline bool MRB_IsEmpty(const MagicRB* RingBuffer);
static inline bool MRB_IsFull(const MagicRB* RingBuffer);

// reading, must not read more than "used" amount of bytes
static inline void* MRB_ReadPtr(MagicRB* RingBuffer);
static inline void  MRB_ReadEnd(MagicRB* RingBuffer, size_t ReadSize);

// writing, must not write more than "free" amount of bytes
static inline void* MRB_WritePtr(MagicRB* RingBuffer);
static inline void  MRB_WriteEnd(MagicRB* RingBuffer, size_t WriteSize);

//
// implementation
//

#if defined(_WIN32)
#	include <windows.h>
#	pragma comment (lib, "onecore")
#elif defined(__linux__)
#	include <sys/syscall.h>
#	include <sys/mman.h>
#	include <unistd.h>
#	include <fcntl.h>
#elif defined(__APPLE__)
#	include <mach/mach.h>
#	include <mach/mach_vm.h>
#	include <mach/vm_map.h>
#	include <mach/vm_page_size.h>
#else
#	error target not supported
#endif

#if !defined(MRB_ASSERT)
#	if defined(_MSC_VER)
#		if !defined(NDEBUG)
#			include <intrin.h>
#			define MRB_ASSERT(cond) do { if (!(cond)) __debugbreak(); } while (0)
#		else
#			define MRB_ASSERT(cond) do { (void)sizeof(cond); } while (0)
#		endif
#	else
#		include <assert.h>
#		define MRB_ASSERT(cond) assert(cond)
#	endif
#endif

void MRB_Init(MagicRB* RingBuffer, size_t Size)
{
	MRB_ASSERT(Size != 0 && Size <= ~((size_t)0) >> 2);

#if defined(_WIN32)

	SYSTEM_INFO SystemInfo;
	GetSystemInfo(&SystemInfo);

	const size_t PageSize = SystemInfo.dwPageSize;
	Size = (Size + PageSize - 1) & ~(PageSize - 1);

	char* Placeholder1 = (char*)VirtualAlloc2(NULL, NULL, 2 * Size, MEM_RESERVE | MEM_RESERVE_PLACEHOLDER, PAGE_NOACCESS, NULL, 0);
	char* Placeholder2 = (char*)Placeholder1 + Size;
	MRB_ASSERT(Placeholder1 && "failed to reserve placeholder");

	BOOL Ok = VirtualFree(Placeholder1, Size, MEM_RELEASE | MEM_PRESERVE_PLACEHOLDER);
	MRB_ASSERT(Ok && "failed to split reservation into two placeholders");

	HANDLE Section = CreateFileMappingW(INVALID_HANDLE_VALUE, NULL, PAGE_READWRITE, (DWORD)(Size >> 32), (DWORD)Size, NULL);
	MRB_ASSERT(Section && "failed to create mapping");

	void* View1 = MapViewOfFile3(Section, NULL, Placeholder1, 0, Size, MEM_REPLACE_PLACEHOLDER, PAGE_READWRITE, NULL, 0);
	MRB_ASSERT(View1 && "failed to map first half of mapping");

	void* View2 = MapViewOfFile3(Section, NULL, Placeholder2, 0, Size, MEM_REPLACE_PLACEHOLDER, PAGE_READWRITE, NULL, 0);
	MRB_ASSERT(View2 && "failed to map second half of mapping");

	CloseHandle(Section);
	VirtualFree(Placeholder1, 0, MEM_RELEASE);
	VirtualFree(Placeholder2, 0, MEM_RELEASE);

	RingBuffer->Data = View1;

#elif defined(__linux__)

	const size_t PageSize = sysconf(_SC_PAGESIZE);
	Size = (Size + PageSize - 1) & ~(PageSize - 1);

	int MemFd = syscall(__NR_memfd_create, "MagicRingBuffer", FD_CLOEXEC);
	MRB_ASSERT(MemFd > 0 && "failed to create memfd");

	int Ok = ftruncate(MemFd, Size);
	MRB_ASSERT(Ok == 0 && "failed to set memfd size");

	char* Base = (char*)mmap(NULL, 2 * Size, PROT_NONE, MAP_ANONYMOUS | MAP_PRIVATE, -1, 0);
	MRB_ASSERT(Base != MAP_FAILED && "faioled to create memory mapping");

	void* Mapped1 = mmap(Base + 0 * Size, Size, PROT_READ | PROT_WRITE, MAP_FIXED | MAP_SHARED, MemFd, 0);
	MRB_ASSERT(Mapped1 != MAP_FAILED && "failed to map first half of mapping");

	void* Mapped2 = mmap(Base + 1 * Size, Size, PROT_READ | PROT_WRITE, MAP_FIXED | MAP_SHARED, MemFd, 0);
	MRB_ASSERT(Mapped2 != MAP_FAILED && "failed to map second half of mapping");

	close(MemFd);

	RingBuffer->Data = Base;

#elif defined(__APPLE__)

	Size = mach_vm_round_page(Size);

	mach_port_t Task = mach_task_self();

	mach_vm_address_t Address;
	int AllocateOk = mach_vm_allocate(Task, &Address, 2 * Size, VM_FLAGS_ANYWHERE);
	MRB_ASSERT(AllocateOk == 0 && "failed to allocate memory");

	int Mapping1 = mach_vm_allocate(Task, &Address, Size, VM_FLAGS_FIXED | VM_FLAGS_OVERWRITE);
	MRB_ASSERT(Mapping1 == 0 && "failed to map first half of mapping");

	const vm_prot_t PageProtection = VM_PROT_READ | VM_PROT_WRITE;

	mach_port_t MemoryPort;
	mach_vm_size_t MemorySize = Size;
	int PortOk = mach_make_memory_entry_64(Task, &MemorySize, Address, PageProtection, &MemoryPort, MACH_PORT_NULL);
	MRB_ASSERT(PortOk == 0 && "failed to create mach port");

	mach_vm_address_t Address2 = Address + Size;
	int Mapping2 = mach_vm_map(Task, &Address2, Size, 0, VM_FLAGS_FIXED | VM_FLAGS_OVERWRITE, MemoryPort, 0, FALSE, PageProtection, PageProtection, VM_INHERIT_NONE);
	MRB_ASSERT(Mapping2 == 0 && "failed to map second half of mapping");

	mach_port_deallocate(Task, MemoryPort);

	RingBuffer->Data = (void*)Address;

#endif

	RingBuffer->Size = Size;
	RingBuffer->Read = 0;
	RingBuffer->Write = 0;
}

void MRB_Done(MagicRB* RingBuffer)
{
#if defined(_WIN32)
	UnmapViewOfFileEx((char*)RingBuffer->Data, 0);
	UnmapViewOfFileEx((char*)RingBuffer->Data + RingBuffer->Size, 0);
#elif defined(__linux__)
	munmap(RingBuffer->Data, 2 * RingBuffer->Size);
#elif defined(__APPLE__)
	mach_vm_deallocate(mach_task_self(), (mach_vm_address_t)RingBuffer->Data, 2 * RingBuffer->Size);
#endif
}

size_t MRB_GetUsed(const MagicRB* RingBuffer)
{
	return RingBuffer->Write - RingBuffer->Read;
}

size_t MRB_GetFree(const MagicRB* RingBuffer)
{
	return RingBuffer->Size - MRB_GetUsed(RingBuffer);
}

bool MRB_IsEmpty(const MagicRB* RingBuffer)
{
	return MRB_GetUsed(RingBuffer) == 0;
}

bool MRB_IsFull(const MagicRB* RingBuffer)
{
	return MRB_GetFree(RingBuffer) == 0;
}

void* MRB_ReadPtr(MagicRB* RingBuffer)
{
	MRB_ASSERT(MRB_GetUsed(RingBuffer) != 0);
	size_t Offset = RingBuffer->Read & (RingBuffer->Size - 1);
	return (char*)RingBuffer->Data + Offset;
}

void MRB_ReadEnd(MagicRB* RingBuffer, size_t ReadSize)
{
	MRB_ASSERT(ReadSize <= MRB_GetUsed(RingBuffer));
	RingBuffer->Read += ReadSize;
}

void* MRB_WritePtr(MagicRB* RingBuffer)
{
	MRB_ASSERT(MRB_GetFree(RingBuffer) != 0);
	size_t Offset = RingBuffer->Write & (RingBuffer->Size - 1);
	return (char*)RingBuffer->Data + Offset;
}

void MRB_WriteEnd(MagicRB* RingBuffer, size_t WriteSize)
{
	MRB_ASSERT(WriteSize <= MRB_GetFree(RingBuffer));
	RingBuffer->Write += WriteSize;
}