Read only a part of a binary zip file

Question

I am looking for a way to read a part of a binary zip file (starting position and number of bytes to read). Currently I'm investigating this on Windows, but optimally it would be platform independent. For a normal binary file (unzipped), this can be achieved in the following way:

//Open the file
std::ifstream file (path, std::ios::in | std::ios::binary | std::ios::ate);

//Move to the position to start reading
file.seekg(64); 

//Read 128 bytes of the file
std::vector<unsigned char> mDataBuffer;
mDataBuffer.resize( 128 ) ;
file.read( (char*)( &mDataBuffer[0]), 128 ) ;

//Read as string
std::string s_data( mDataBuffer.begin(), mDataBuffer.end());

file.close()

This example is a slightly modified version of this one.

There are also many unzip packages available (e.g. zlib or minizip). Each covering functions to unzip a file. I could simply unzip my zipped file, save it on the disk and read it using the method above.

Unfortunately, I didn't find an example to read only a part of a binary zip file (if that is even possible), straight from the zipped file. Because my file is quite large, I don't want to unzip it completely onto the hard drive. Furthermore, the part that I want to read is quite small, so it would be a waste of cpu time to completely unzip the file. For the same reasons, I also don't want to decompress the complete file into my memory. I am looking for a genuine way to read only a part of a zipped file.

How could this be accomplished in c++?

A begin and end position for seek in the normal binary file (modified in question) — takje, Feb 01 '17 at 17:57
Are you supposed to extract a single file from the archive? Not that it matters, I'd still recommend you find a portable library to handle it for you. — Some programmer dude, Feb 01 '17 at 17:57
No, currently my archive contains multiple files, but I only want to read a few bytes from one of the files. — takje, Feb 01 '17 at 17:58
@takje Where should the begin/end positions of a particularly compressed files should be stored and retrieved from? The the `zip` format doesn't support that IIRC. Besides that your question is OT, because it asks for recommendation of a 3rd party resource. — πάντα ῥεῖ, Feb 01 '17 at 18:04
@πάντα ῥεῖ Modified my question to ask for a c++ solution instead. — takje, Feb 01 '17 at 20:37
@takje There's no standard c++ solution to tackle that problem, sorry. — πάντα ῥεῖ, Feb 01 '17 at 20:38

score 2 · Accepted Answer · edited May 23 '17 at 12:25

2

Apparently there is no general way to seek in zip files. This was according to:

A comment of @πάντα ῥεῖ.
A general thread on searching in zipped files here.
A similar question here (although the question itself is about Python).

edited May 23 '17 at 12:25

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answered Feb 01 '17 at 20:42

takje

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Read only a part of a binary zip file

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