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mmapper.cpp
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mmapper.cpp
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// MMapper -- Cross-platform (Win/Posix) mmap interface.
// Author: Oliver "kfsone" Smith 2012, 2018 <oliver@kfs.org>
// Redistribution and re-use fully permitted contingent on inclusion of these 3 lines in copied- or derived- works.
#include "mmapper_platform.h"
#include <stdexcept>
#include <cstring>
#include "mmapper.h"
#include "filehandle.h"
#include "internal_includes.h"
namespace KFS
{
//////////////////////////////////////////////////////////////////////
// Populate filename with the combination of file and directory name.
static filename_str_t
_populateFilename(filename_str_t lhs_, filename_str_t rhs_)
{
filename_str_t into{ lhs_ };
if (!into.empty() && into.back() != c_PathSeparator)
into += c_PathSeparator;
into += rhs_;
return into;
}
//////////////////////////////////////////////////////////////////////
// Constructor (POSIX + Windows versions combined)
// Note: POSIX doesn't support wchar_t filenames.
MMappedFile::MMappedFile(filename_str_t filename_, filename_str_t dirname_) MMAPPER_MAYBE_NOEXCEPT
{
bool mapped = mapFile(filename_, dirname_);
// If MMAPPER_NO_THROW is defined, don't throw.
#ifndef MMAPPER_NO_THROW
if (!mapped)
{
#if IO_MAP_TYPE == _IMT_WINDOWS
throw std::runtime_error(strerror(GetLastError()));
#else
throw std::runtime_error(strerror(errno));
#endif
}
#endif
}
//////////////////////////////////////////////////////////////////////
// Destructor.
MMappedFile::~MMappedFile() noexcept
{
unmapFile();
}
//////////////////////////////////////////////////////////////////////
// Attempt to open a file. Returns false on error.
bool
MMappedFile::mapFile(filename_str_t filename_, filename_str_t dirname_) MMAPPER_MAYBE_NOEXCEPT
{
// Release any file we currently have open.
unmapFile();
// New filename.
m_filename = _populateFilename(dirname_, filename_);
FileHandle fh{ m_filename };
if (!fh.isValid())
return false;
auto size = fh.uncachedFileSize();
if (!size)
{
#ifndef MMAPPER_NO_THROW
throw std::runtime_error("trying to map zero-sized file");
#endif
return false;
}
// Ask the OS to provide an in-memory view of the data; which is
// basically saying "load this file into buffers like you would,
// but then give us direct access to the buffer memory".
#if MMAPPER_API == MMAPPER_WIN32
// Windows implementation.
FileHandle mapFh{ CreateFileMapping(fh, NULL, PAGE_READONLY, 0, 0, NULL) };
if (!mapFh.isValid())
{
#ifndef MMAPPER_NO_THROW
throw std::runtime_error("Failed to create file mapping");
#endif
return false;
}
// We don't need the original 'fh' now, so we can go ahead and close it,
// the MapView call can then potentially re-use it and avoid some
// allocations.
fh.close();
LPVOID const ptr = MapViewOfFileEx(mapFh, FILE_MAP_READ, 0, 0, 0, NULL);
constexpr LPVOID MapFailure = nullptr;
#else
// POSIX implementation.
constexpr int flags =
MAP_FILE // Compatibility flag, not really required.
| MAP_SHARED // Share buffers with any other mmappers of this file.
;
// We ask the OS to give us a byte more than the file requires so that
// we can be sure we have a null-byte after the real data.
void* const ptr = mmap(NULL, size + 1, PROT_READ, flags, fh, 0);
static const void* MapFailure = MAP_FAILED;
#endif
if (ptr == MapFailure)
{
#ifndef MMAPPER_NO_THROW
throw std::runtime_error("Mapping failed");
#endif
return false;
}
// Both methods return a pointer to the beginning of the OS'es internal
// buffers. There may not be any data there, and the first access may
// result in a page fault (the OS has to actually fetch data, akin to the
// first call of read()).
m_basePtr = static_cast<const void*>(ptr);
// For convenience, pre-calculate where the end of the data is.
m_endPtr = begin() + size;
// All the file handles we have open at this point are now safe to close.
return true;
}
//////////////////////////////////////////////////////////////////////
// Close and unmap the mapping.
bool MMappedFile::unmapFile() MMAPPER_MAYBE_NOEXCEPT
{
if (!isMapped())
{
#ifndef MMAPPER_NO_THROW
throw std::logic_error("File is already unmapped.");
#endif
return false;
}
#if MMAPPER_API == MMAPPER_WIN32
UnmapViewOfFile(m_basePtr);
#else
// We asked for an extra byte when we mmap()d, so we have to
// include it when we unmap.
munmap(const_cast<void*>(m_basePtr), size() + 1);
#endif
m_filename.clear();
m_basePtr = nullptr;
m_endPtr = nullptr;
return true;
}
}