Decompressing a GZip Stream with Zlib

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Introduction

This C++ program reads a file named input.txt.gz into memory, then uses the zlib library to decompress and print the data. After uncompression, the lyrics to Rick Astley's Never Gonna Give You Up will be printed.

Some things to note:

The input.txt.gz file was generated using 7-zip by compressing input.txt (this file is also included in the download).
The zipped file supplied contains a Code::Blocks project file and was written and tested under Windows using MinGW only.
The necessary zlib files are included. They were obtained from the official zlib website. zdll.lib was renamed to libzdll.a in order to make it work with MinGW.

Download

You may download the source code and Code::Blocks project files here.

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Source

Here is the source code listing for the program.

/*
This C++ program uses zlib to read then decompress a gzipped file in memory.
Author: Andrew Lim Chong Liang
http://windrealm.org
*/
#include <cstdio>
#include <string>
#include <cstring>
#include <cstdlib>
#include "zlib.h"
#include "zconf.h"
using namespace std;

bool gzipInflate( const std::string& compressedBytes, std::string& uncompressedBytes ) {
  if ( compressedBytes.size() == 0 ) {
    uncompressedBytes = compressedBytes ;
    return true ;
  }

  uncompressedBytes.clear() ;

  unsigned full_length = compressedBytes.size() ;
  unsigned half_length = compressedBytes.size() / 2;

  unsigned uncompLength = full_length ;
  char* uncomp = (char*) calloc( sizeof(char), uncompLength );

  z_stream strm;
  strm.next_in = (Bytef *) compressedBytes.c_str();
  strm.avail_in = compressedBytes.size() ;
  strm.total_out = 0;
  strm.zalloc = Z_NULL;
  strm.zfree = Z_NULL;

  bool done = false ;

  if (inflateInit2(&strm, (16+MAX_WBITS)) != Z_OK) {
    free( uncomp );
    return false;
  }

  while (!done) {
    // If our output buffer is too small
    if (strm.total_out >= uncompLength ) {
      // Increase size of output buffer
      char* uncomp2 = (char*) calloc( sizeof(char), uncompLength + half_length );
      memcpy( uncomp2, uncomp, uncompLength );
      uncompLength += half_length ;
      free( uncomp );
      uncomp = uncomp2 ;
    }

    strm.next_out = (Bytef *) (uncomp + strm.total_out);
    strm.avail_out = uncompLength - strm.total_out;

    // Inflate another chunk.
    int err = inflate (&strm, Z_SYNC_FLUSH);
    if (err == Z_STREAM_END) done = true;
    else if (err != Z_OK)  {
      break;
    }
  }

  if (inflateEnd (&strm) != Z_OK) {
    free( uncomp );
    return false;
  }

  for ( size_t i=0; i<strm.total_out; ++i ) {
    uncompressedBytes += uncomp[ i ];
  }
  free( uncomp );
  return true ;
}

/* Reads a file into memory. */
bool loadBinaryFile( const std::string& filename, std::string& contents ) {
  // Open the gzip file in binary mode
  FILE* f = fopen( filename.c_str(), "rb" );
  if ( f == NULL )
    return false ;

  // Clear existing bytes in output vector
  contents.clear();

  // Read all the bytes in the file
  int c = fgetc( f );
  while ( c != EOF ) {
    contents +=  (char) c ;
    c = fgetc( f );
  }
  fclose (f);

  return true ;
}

int main() {
  // Read the gzip file data into memory
  std::string fileData ;
  if ( !loadBinaryFile( "input.txt.gz", fileData ) ) {
    printf( "Error loading input file." );
    return 0 ;
  }

  // Uncompress the file data
  std::string data ;
  if ( !gzipInflate( fileData, data ) ) {
    printf( "Error decompressing file." );
    return 0 ;
  }

  // Print the data
  printf( "Data: \"" );
  for ( size_t i=0; i<data.size(); ++i ) {
    printf( "%c", data[i] );
  }
  printf ( "\"\n" );
  return 0;
}

Decompressing a GZip Stream with Zlib

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Introduction

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Source

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