MMapfdIO.h

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// # MMapfdIO.h: Memory-mapped IO on a file descriptor
// #
// # Copyright (C) 2009
// # Associated Universities, Inc. Washington DC, USA.
// #
// # This library is free software; you can redistribute it and/or modify it
// # under the terms of the GNU Library General Public License as published by
// # the Free Software Foundation; either version 2 of the License, or (at your
// # option) any later version.
// #
// # This library is distributed in the hope that it will be useful, but WITHOUT
// # ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
// # FITNESS FOR A PARTICULAR PURPOSE.  See the GNU Library General Public
// # License for more details.
// #
// # You should have received a copy of the GNU Library General Public License
// # along with this library; if not, write to the Free Software Foundation,
// # Inc., 675 Massachusetts Ave, Cambridge, MA 02139, USA.
// #
// # Correspondence concerning AIPS++ should be addressed as follows:
// #        Internet email: casa-feedback@nrao.edu.
// #        Postal address: AIPS++ Project Office
// #                        National Radio Astronomy Observatory
// #                        520 Edgemont Road
// #                        Charlottesville, VA 22903-2475 USA
 
#ifndef CASA_MMAPFDIO_H
#define CASA_MMAPFDIO_H
 
// # Includes
#include <casacore/casa/aips.h>
#include <casacore/casa/IO/FiledesIO.h>
#include <casacore/casa/OS/RegularFile.h>
 
namespace casacore {
 
// <summary>
// Memory-mapped IO on a file.
// </summary>
 
// <synopsis>
// Memory-mapped IO lets the OS take care of caching file segments.
// This is particularly useful for the Tiled Storage Manager which keeps a
// cache of tiles. When using memory-mapped IO it does not need to do that
// anymore.
//
// On 32-bit systems its use is limited because for large files the 4 GB
// memory space is insufficient. However, for 64-bit systems the memory
// space is large enough to make use of it.
//
// In the general case there is direct access to the mapped file space.
// The read and write methods copies the data into/from a buffer.
// However, to avoid the copying it is possible to get a direct pointer
// to the mapped data. This should be used with care, because writing to
// it will cause a segmentation if the file is readonly. If the file is
// writable, writing into the mapped data segment means changing the file
// contents.
// </synopsis>
 
class MMapfdIO : public FiledesIO {
 public:
  // Default constructor.
  // A file can be memory-mapped using the map function.
  MMapfdIO();
 
  // Map the given file descriptor entirely into memory with read access.
  // The map has also write access if the file is opened for write.
  // The file name is only used in possible error messages.
  MMapfdIO(int fd, const String& fileName);
 
  // Destructor.
  // If needed, it will flush and unmap the file, but not close it.
  ~MMapfdIO();
 
  // Forbid copy constructor and assignment
  // <group>
  MMapfdIO(const MMapfdIO&) = delete;
  MMapfdIO& operator=(const MMapfdIO&) = delete;
  // </group>
 
  // Map the given file descriptor entirely into memory with read access.
  // The map has also write access if the file is opened for write.
  // An exception is thrown if a file descriptor was already attached.
  // The file name is only used in possible error messages.
  void map(int fd, const String& fileName);
 
  // Map or remap the entire file.
  // Remapping is needed if the file has grown elsewhere.
  void mapFile();
 
  // Flush changed mapped data to the file.
  // Nothing is done if the file is readonly.
  void flush();
 
  // Write the number of bytes from the seek position on.
  // The file will be extended and remapped if writing beyond end-of-file.
  // In that case possible pointers obtained using <src>getXXPointer</src>
  // are not valid anymore.
  virtual void write(Int64 size, const void* buf);
 
  // Read <src>size</src> bytes from the File. Returns the number of bytes
  // actually read. Will throw an exception (AipsError) if the requested
  // number of bytes could not be read unless throwException is set to
  // False. Will always throw an exception if the file is not readable or
  // the system call returns an undocumented value.
  virtual Int64 read(Int64 size, void* buf, Bool throwException = True);
 
  // Get a read or write pointer to the given position in the mapped file.
  // An exception is thrown if beyond end-of-file or it not writable.
  // These functions should be used with care. If the pointer is used to
  // access data beyond the file size, a segmentation fault will occur.
  // So it means that the write pointer can only be used to update the file,
  // not to extend it. The <src>seek</src> and <src>write</src> functions
  // should be used to extend a file.
  // <group>
  const void* getReadPointer(Int64 offset) const;
  void* getWritePointer(Int64 offset);
  // </group>
 
  // Get the file size.
  Int64 getFileSize() const { return itsFileSize; }
 
 protected:
  // Reset the position pointer to the given value. It returns the
  // new position.
  virtual Int64 doSeek(Int64 offset, ByteIO::SeekOption);
 
  // Unmap the file.
  void unmapFile();
 
 private:
  Int64 itsFileSize;  // # File size
  Int64 itsPosition;  // # Current seek position
  char* itsPtr;       // # Pointer to memory map
  Bool itsIsWritable;
};
 
}  // namespace casacore
 
#endif