zero_copy_stream.h

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// Protocol Buffers - Google's data interchange format
// Copyright 2008 Google Inc. All rights reserved.
// https://developers.google.com/protocol-buffers/
//
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// Author: [email protected] (Kenton Varda)
// Based on original Protocol Buffers design by
// Sanjay Ghemawat, Jeff Dean, and others.
//
// This file contains the ZeroCopyInputStream and ZeroCopyOutputStream
// interfaces, which represent abstract I/O streams to and from which
// protocol buffers can be read and written. For a few simple
// implementations of these interfaces, see zero_copy_stream_impl.h.
//
// These interfaces are different from classic I/O streams in that they
// try to minimize the amount of data copying that needs to be done.
// To accomplish this, responsibility for allocating buffers is moved to
// the stream object, rather than being the responsibility of the caller.
// So, the stream can return a buffer which actually points directly into
// the final data structure where the bytes are to be stored, and the caller
// can interact directly with that buffer, eliminating an intermediate copy
// operation.
//
// As an example, consider the common case in which you are reading bytes
// from an array that is already in memory (or perhaps an mmap()ed file).
// With classic I/O streams, you would do something like:
// char buffer[BUFFER_SIZE];
// input->Read(buffer, BUFFER_SIZE);
// DoSomething(buffer, BUFFER_SIZE);
// Then, the stream basically just calls memcpy() to copy the data from
// the array into your buffer. With a ZeroCopyInputStream, you would do
// this instead:
// const void* buffer;
// int size;
// input->Next(&buffer, &size);
// DoSomething(buffer, size);
// Here, no copy is performed. The input stream returns a pointer directly
// into the backing array, and the caller ends up reading directly from it.
//
// If you want to be able to read the old-fashion way, you can create
// a CodedInputStream or CodedOutputStream wrapping these objects and use
// their ReadRaw()/WriteRaw() methods. These will, of course, add a copy
// step, but Coded*Stream will handle buffering so at least it will be
// reasonably efficient.
//
// ZeroCopyInputStream example:
// // Read in a file and print its contents to stdout.
// int fd = open("myfile", O_RDONLY);
// ZeroCopyInputStream* input = new FileInputStream(fd);
//
// const void* buffer;
// int size;
// while (input->Next(&buffer, &size)) {
// cout.write(buffer, size);
// }
//
// delete input;
// close(fd);
//
// ZeroCopyOutputStream example:
// // Copy the contents of "infile" to "outfile", using plain read() for
// // "infile" but a ZeroCopyOutputStream for "outfile".
// int infd = open("infile", O_RDONLY);
// int outfd = open("outfile", O_WRONLY);
// ZeroCopyOutputStream* output = new FileOutputStream(outfd);
//
// void* buffer;
// int size;
// while (output->Next(&buffer, &size)) {
// int bytes = read(infd, buffer, size);
// if (bytes < size) {
// // Reached EOF.
// output->BackUp(size - bytes);
// break;
// }
// }
//
// delete output;
// close(infd);
// close(outfd);

#ifndef GOOGLE_PROTOBUF_IO_ZERO_COPY_STREAM_H__
#define GOOGLE_PROTOBUF_IO_ZERO_COPY_STREAM_H__

#include <string>
#include <google/protobuf/stubs/common.h>

namespace google {

namespace protobuf {
namespace io {

// Defined in this file.
class ZeroCopyInputStream;
class ZeroCopyOutputStream;

// Abstract interface similar to an input stream but designed to minimize
// copying.
class LIBPROTOBUF_EXPORT ZeroCopyInputStream {
 public:
 inline ZeroCopyInputStream() {}
 virtual ~ZeroCopyInputStream();

 // Obtains a chunk of data from the stream.
 //
 // Preconditions:
 // * "size" and "data" are not NULL.
 //
 // Postconditions:
 // * If the returned value is false, there is no more data to return or
 // an error occurred. All errors are permanent.
 // * Otherwise, "size" points to the actual number of bytes read and "data"
 // points to a pointer to a buffer containing these bytes.
 // * Ownership of this buffer remains with the stream, and the buffer
 // remains valid only until some other method of the stream is called
 // or the stream is destroyed.
 // * It is legal for the returned buffer to have zero size, as long
 // as repeatedly calling Next() eventually yields a buffer with non-zero
 // size.
 virtual bool Next(const void** data, int* size) = 0;

 // Backs up a number of bytes, so that the next call to Next() returns
 // data again that was already returned by the last call to Next(). This
 // is useful when writing procedures that are only supposed to read up
 // to a certain point in the input, then return. If Next() returns a
 // buffer that goes beyond what you wanted to read, you can use BackUp()
 // to return to the point where you intended to finish.
 //
 // Preconditions:
 // * The last method called must have been Next().
 // * count must be less than or equal to the size of the last buffer
 // returned by Next().
 //
 // Postconditions:
 // * The last "count" bytes of the last buffer returned by Next() will be
 // pushed back into the stream. Subsequent calls to Next() will return
 // the same data again before producing new data.
 virtual void BackUp(int count) = 0;

 // Skips a number of bytes. Returns false if the end of the stream is
 // reached or some input error occurred. In the end-of-stream case, the
 // stream is advanced to the end of the stream (so ByteCount() will return
 // the total size of the stream).
 virtual bool Skip(int count) = 0;

 // Returns the total number of bytes read since this object was created.
 virtual int64 ByteCount() const = 0;


 private:
 GOOGLE_DISALLOW_EVIL_CONSTRUCTORS(ZeroCopyInputStream);
};

// Abstract interface similar to an output stream but designed to minimize
// copying.
class LIBPROTOBUF_EXPORT ZeroCopyOutputStream {
 public:
 inline ZeroCopyOutputStream() {}
 virtual ~ZeroCopyOutputStream();

 // Obtains a buffer into which data can be written. Any data written
 // into this buffer will eventually (maybe instantly, maybe later on)
 // be written to the output.
 //
 // Preconditions:
 // * "size" and "data" are not NULL.
 //
 // Postconditions:
 // * If the returned value is false, an error occurred. All errors are
 // permanent.
 // * Otherwise, "size" points to the actual number of bytes in the buffer
 // and "data" points to the buffer.
 // * Ownership of this buffer remains with the stream, and the buffer
 // remains valid only until some other method of the stream is called
 // or the stream is destroyed.
 // * Any data which the caller stores in this buffer will eventually be
 // written to the output (unless BackUp() is called).
 // * It is legal for the returned buffer to have zero size, as long
 // as repeatedly calling Next() eventually yields a buffer with non-zero
 // size.
 virtual bool Next(void** data, int* size) = 0;

 // Backs up a number of bytes, so that the end of the last buffer returned
 // by Next() is not actually written. This is needed when you finish
 // writing all the data you want to write, but the last buffer was bigger
 // than you needed. You don't want to write a bunch of garbage after the
 // end of your data, so you use BackUp() to back up.
 //
 // Preconditions:
 // * The last method called must have been Next().
 // * count must be less than or equal to the size of the last buffer
 // returned by Next().
 // * The caller must not have written anything to the last "count" bytes
 // of that buffer.
 //
 // Postconditions:
 // * The last "count" bytes of the last buffer returned by Next() will be
 // ignored.
 virtual void BackUp(int count) = 0;

 // Returns the total number of bytes written since this object was created.
 virtual int64 ByteCount() const = 0;

 // Write a given chunk of data to the output. Some output streams may
 // implement this in a way that avoids copying. Check AllowsAliasing() before
 // calling WriteAliasedRaw(). It will GOOGLE_CHECK fail if WriteAliasedRaw() is
 // called on a stream that does not allow aliasing.
 //
 // NOTE: It is caller's responsibility to ensure that the chunk of memory
 // remains live until all of the data has been consumed from the stream.
 virtual bool WriteAliasedRaw(const void* data, int size);
 virtual bool AllowsAliasing() const { return false; }


 private:
 GOOGLE_DISALLOW_EVIL_CONSTRUCTORS(ZeroCopyOutputStream);
};

} // namespace io
} // namespace protobuf

} // namespace google
#endif // GOOGLE_PROTOBUF_IO_ZERO_COPY_STREAM_H__