// Protocol Buffers - Google's data interchange format
// Copyright 2008 Google Inc.
// http://code.google.com/p/protobuf/
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
// http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.
// Author: kenton@google.com (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);
// ZeroCopyInputStream* 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;
private:
GOOGLE_DISALLOW_EVIL_CONSTRUCTORS(ZeroCopyOutputStream);
};
} // namespace io
} // namespace protobuf
} // namespace google
#endif // GOOGLE_PROTOBUF_IO_ZERO_COPY_STREAM_H__
