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?

Answer 1

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)