CODEWITHSUNDEEP
c++gzipzlib
Raghavendra P
Raghavendra P

Reputation: 23

Writing a compressed buffer to a gzip compatible file

I would like to compress a bunch of data in a buffer and write to a file such that it is gzip compatible. The reason for doing this is that I have multiple threads that can be compressing their own data in parallel and require a lock only when writing to the common output file.

I have some dummy code below based on the zlib.h docs for writing a gz compatible, but I get a gzip: test.gz: unexpected end of file when I try to decompress the output. Can anyone tell me what might be going wrong ?

Thank you

#include <cassert>
#include <fstream>
#include <string.h>
#include <zlib.h>


int main()
{

    char compress_in[50] = "I waaaaaaaant tooooooo beeeee compressssssed";
    char compress_out[100];

    z_stream bufstream;
    bufstream.zalloc = Z_NULL;
    bufstream.zfree = Z_NULL;
    bufstream.opaque = Z_NULL;

    bufstream.avail_in = ( uInt )strlen(compress_in) + 1;
    bufstream.next_in = ( Bytef * ) compress_in;
    bufstream.avail_out = ( uInt )sizeof( compress_out );
    bufstream.next_out = ( Bytef * ) compress_out;

    int res = deflateInit2( &bufstream, Z_DEFAULT_COMPRESSION, Z_DEFLATED, 15 + 16, 8, Z_DEFAULT_STRATEGY );
    assert ( res == Z_OK );

    res = deflate( &bufstream, Z_FINISH );
    assert( res == Z_STREAM_END );

    deflateEnd( &bufstream );

    std::ofstream outfile( "test.gz", std::ios::binary | std::ios::out );
    outfile.write( compress_out, strlen( compress_out ) + 1 );
    outfile.close();

    return 0;
}

Upvotes: 0

Views: 426

Answers (1)

Mark Adler
Mark Adler

Reputation: 112547

The length of the compressed data written to the output buffer is the space you provided for the output buffer minus the space remaining in the output buffer. So, sizeof( compress_out ) - bufstream.avail_out. Do:

outfile.write( compress_out, sizeof( compress_out ) - bufstream.avail_out );

Upvotes: 2

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