FileUnbufferedIO.h

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// # FileUnbufferedIO.h: Class for unbuffered IO on a file
// # Copyright (C) 2022
// # 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.
// #
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#ifndef CASA_FILEUNBUFFEREDIO_H
#define CASA_FILEUNBUFFEREDIO_H
 
// # Includes
#include <casacore/casa/aips.h>
#include <casacore/casa/IO/FiledesIO.h>
#include <casacore/casa/OS/RegularFile.h>
 
namespace casacore {  // # NAMESPACE CASACORE - BEGIN
 
// <summary>
// Class for unbuffered IO on a file.
// </summary>
 
// <use visibility=export>
 
// <reviewed reviewer="Friso Olnon" date="1996/11/06" tests="tByteIO" demos="">
// </reviewed>
 
// <prerequisite>
//    <li> <linkto class=ByteIO>ByteIO</linkto> class
//    <li> file descriptors
// </prerequisite>
 
// <synopsis>
// This class is a specialization of class
// <linkto class=ByteIO>FiledesIO</linkto> to make it easier to open a file.
// It reads/writes the data directly instead of using a buffer to reduce the
// number of IO-operations.
// It is meant for file access using large reads and writes as done by MultiFile.
//
// Optionally the file can be used with O_DIRECT to bypass the system's file cache
// for more predictable IO behaviour which can be of importance in
// real-time applications.
// </synopsis>
 
// <motivation>
// Make it possible to use the Casacore IO functionality on any file.
// In this way any device can be hooked to the IO framework.
// </motivation>
 
class FileUnbufferedIO : public FiledesIO {
 public:
  // The constructor opens or creates a file.
  // For option NewNoReplace it is checked if the file does not exist yet.
  // If useODirect=True and if supported by the OS, the file will be opened
  // with O_DIRECT which bypasses the kernel's file cache for more predictable
  // I/O behaviour. It requires the size and the alignment of the data read
  // or written to be a multiple of the the disk's logical block size.
  explicit FileUnbufferedIO(const RegularFile& fileName, ByteIO::OpenOption option,
                            Bool useODirect = False);
 
  // The destructor closes the file.
  ~FileUnbufferedIO() override;
 
  // Copy constructor and assignment cannot be used.
  FileUnbufferedIO(const FileUnbufferedIO&) = delete;
  FileUnbufferedIO& operator=(const FileUnbufferedIO&) = delete;
 
  // Reopen the file for read/write.
  // Nothing is done if already opened for read/write.
  // An exception is thrown if the file is readonly.
  void reopenRW() override;
 
 private:
  Bool itsUseODirect;
};
 
}  // namespace casacore
 
#endif