How to Implement Custom Input/Output Streams in C for Decompressing Data?

How to Implement Custom Input/Output Streams in C

Introduction

This discussion centers around understanding the proper implementation of custom input/output streams in C . A hypothetical scenario involving reading an image from a compressed custom stream using the extraction operator illustrates the concept.

Custom Input Stream Design

Instead of extending the istream class, the recommended approach in C involves deriving from the std::streambuf class and overriding the underflow() operation for reading. For writing, both the overflow() and sync() operations should be overridden.

The core elements of this design include:

• Creating a filtering stream buffer that takes an existing stream buffer as an argument.
• Implementing the underflow() method for decompressing data into a buffer for reading.
• Implementing the overflow() and sync() methods for writing and flushing compressed data.

Example Code

Below is a simplified example that demonstrates the implementation of a stream buffer for image decompression:

<code class="cpp">class decompressbuf : public std::streambuf {
    std::streambuf* sbuf_;
    char*           buffer_;
public:
    decompressbuf(std::streambuf* sbuf)
        : sbuf_(sbuf), buffer_(new char[1024]) {}
    ~decompressbuf() { delete[] this->buffer_; }
    int underflow() {
        if (this->gptr() == this->egptr()) {
            // Decompress data into buffer_, obtaining its own input from
            // this->sbuf_; if necessary resize buffer
            // the next statement assumes &quot;size&quot; characters were produced (if
            // no more characters are available, size == 0.
            this->setg(this->buffer_, this->buffer_, this->buffer_ + size);
        }
        return this->gptr() == this->egptr()
             ? std::char_traits<char>::eof()
             : std::char_traits<char>::to_int_type(*this->gptr());
    }
};</code>

Using the Custom Stream Buffer

Once the stream buffer is created, it can be used to initialize an std::istream object:

<code class="cpp">std::ifstream fin("some.file");
decompressbuf   sbuf(fin.rdbuf());
std::istream  in(&sbuf);</code>

Conclusion

This custom stream buffer approach allows for seamless integration of data decompression into the standard C I/O system, enabling efficient reading of compressed data.

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