Moving compressed strings between C++ and PHP with zlib

Question

I'm working on a project where a Windows web server running PHP is communicating over a very slow connection with a back end Linux server running an application written in C++. Because the connection between the two machines is so slow, I'd like to compress the traffic moving between them.

I've gotten to where I can compress a string, save it to a file, read the file, and uncompress the string in C++ using Zlib, and likewise in PHP. However, if I try to compress a string in one language and decompress it in the other (as will be happening in the real world), I get errors griping that the compressed data is corrupted. I've also noticed that the same string compressed in C++ results in a different file than in PHP, which leads me to believe that Zlib is using a different compression algorithm on each language.

I'm using default settings on both sides. The C++ I'm using to do the compression and decompression is

compress((Bytef*)compressed, (uLongf*)&compressedLength, (Bytef*)uncompressed, (uLong)uncomressedLength);
uncompress((Bytef*)uncompressed, (uLongf*)&uncomressedLength, (Bytef*)compressed, (uLong)compressedLength);

while the PHP code is

$compressed = gzcompress($uncompressed);
$uncompressed = gzuncompress($compressed);

Why are these resulting in different compressed strings? Is that what's causing the problems with decompression? What should I be doing to get this to work? Also, I'm not committed to Zlib. Zlib's what my initial research uncovered, but if there's a better way to do this, I'm all ears.

Edit: Actually, after doing a little more testing, it appears that C++ was working with my initial test case, but not universally. I tried it with the input "hellohellohello", and on decompression, it reported a Z_DATA_ERROR and decompressed it to just "hello". I guess that means I'm doing something wrong on the C++ side, which may explain why PHP is unhappy decompressing C++ compressed strings.

Edit 2: I tried out the zpipe.c sample program, and it correctly uncompresses strings compressed by PHP and produces compressed strings PHP can uncompress. Clearly, the problem(s) exist in my C++ code. Either my usage of compress and uncompress is incorrect, or I'm reading and writing the file incorrectly. Neither the compress or decompress programs interact correctly with zpipe.

Update: I've now gotten to where I can compress a string using PHP and read it with either PHP or C++, and I can compress a string with C++ and read it with C++, but attempting to read it with PHP results in PHP Warning: gzuncompress(): data error. What could be different that would cause this combination of working/not working scenarios?

That's not the C++ you're using for the (de)compression, that's just the function declarations. — Ignacio Vazquez-Abrams
They're library calls. compressed, compressedLength, etc. are the actual names of the variables I'm using, and since Zlib uses its own types rather than standard types, I'm casting them when I do the call. — Warren Pena

Steve Townsend Steve Townsend · Accepted Answer · 2010-10-28T19:00:46

Zlib's default compression level is 6 - you could try passing that as the second param on gzcompress for PHP.

string gzcompress ( string $data [, int $level = -1 ] )

From the ZLIB manual:

The compression level must be Z_DEFAULT_COMPRESSION, or between 0 and 9: 1 gives best speed, 9 gives best compression, 0 gives no compression at all (the input data is simply copied a block at a time). Z_DEFAULT_COMPRESSION requests a default compromise between speed and compression (currently equivalent to level 6)

Moving compressed strings between C++ and PHP with zlib

1 Answers