I want to uncompress a gz file (e.g. "MyFile.gz") and saving its content in a specified path using zlib in C++.
In other words, if the path is "C:\StoredData\", I would like to write a C++ function that creates the file "C:\StoredData\MyFile"
How can I write it?
I do not understand what is the problem here.
Once the data is uncompressed, you can write it anywhere you want using the File Stream.
If you are facing any issues, you can edit your post.
Sorry to post as answer, as i can't add comments.
Read the documentation in zlib.h. The gz* functions will do what you want on the uncompression side. Then you just write the uncompressed data wherever you like.
Related
I have a Parquet file and I don't want to read the whole file into memory. I want to read the metadata and then read the rest of the file on demand. That is, for example, I want to read the second page of the first column in the third-row group. How would I do that using Apache Parquet cpp library? I have the offset of the part that I want to read from the metadata and can read it directly from the disk. Is there any way to pass that buffer to Apache Parquet library to uncompress, decode and iterate through the values? How about the same thing for column chunk or row groups? Basically, I want to read the file partially and then pass it to the parquet APIs to process it as opposes to give the file handler to the API and let it go through the file. Is it possible?
Behind the scences this is what the Apache Parquet C++ library actually does. When you pass in a file handle, it will only read the parts it needs to. As it requires the file footer (the main metadata) to know where to find the segments of data, this will always be read. The data segments will only be read once you request them.
No need to write special code for this, the library already has it built-in. Thus, if you want to know in fine detail on how this is working, you only need to read the source of the library: https://github.com/apache/arrow/tree/master/cpp/src/parquet
Is there a way to add JPEG comments ("COM" markers) to an existing JPEG image file using libjpeg?
It is certainly possible to do so by first decompressing an existing image to an in-memory buffer and then compressing the raw image again with jpeg_write_marker( ... JPEG_COM ... ) to add comments, and saving to disk. Unless there is a need to decompress first, doing this seems to be an overkill.
There's a tool called wrjpgcom, it's part of libjpeg. I think it does what you want. Perhaps you could look through its source to find out how it's done.
You can use jpeg_write_marker() during the writing of the output file to write the comment after setting it up. Then, use jpeg_read_coefficients() and jpeg_write_coefficients() (in place of the ordinary jpeg_read_scanlines() and jpeg_write_scanlines()) to read and write the raw, compressed data without actually decompressing and recompressing it.
See the section "Really raw data: DCT coefficients" in the libjpeg documentation. Be sure to read all the caveats mentioned there.
Someone may use JPEG Comment Editor created by Mwisoft. It automate adding / editing JPEG comments using Windows, instead of right-click JPEG file and click properties to manually add comments.
I'd like to read the contents of a .csv file from a website, into a c++ program. Specifically, it is financial data of the form from google finance.
http://www.google.com/finance/historical?cid=22144&startdate=Nov+1%2C+2011&enddate=Nov+14%2C+2011
(If you append "&output=csv" to the above link it will download the data as a csv file)
I know that I can use something like libcurl to download the file and then read it in from there, but I wanted to read it directly into the program without having to write it to a file first.
Can I get some suggestions on the best way to do this? I was thinking boost.asio but I have no experience with it (or network programming in general).
If you are trying to download it from a web resource you will need to implement at least some part of the HTTP protocol. libcurl will do this for you.
You don't need to save it as a file. This example will show you how to download and store it in a memory buffer.
Greetings all,
I am currently a rising Sophomore (CS major), and this summer, I'm trying to teach myself C++ (my school codes mainly in Java).
I have read many guides on C++ and gotten to the part with ofstream, saving and editing .txt files.
Now, I am interested in simply importing an image (jpeg, bitmap, not really important) and renaming the aforementioned image.
I have googled, asked around but to no avail.
Is this process possible without the download of external libraries (I dled CImg)?
Any hints or tips on how to expedite my goal would be much appreciated
Renaming an image is typically about the same as renaming any other file.
If you want to do more than that, you can also change the data in the Title field of the IPTC metadata. This does not require JPEG decoding, or anything like that -- you need to know the file format well enough to be able to find the IPTC metadata, and study the IPTC format well enough to find the Title field, but that's about all. Exactly how you'll get to the IPTC metadata will vary -- navigating a TIFF (for one example) takes a fair amount of code all by itself.
When you say "renaming the aforementioned image," do you mean changing metadata in the image file, or just changing the file name? If you are referring to metadata, then you need to either understand the file format or use a library that understands the file format. It's going to be different for each type of image file. If you basically just want to copy a file, you can either stream the contents from one file stream to another, or use a file system API.
std::ifstream infs("input.txt", std::ios::binary);
std::ofstream outfs("output.txt", std::ios::binary);
outfs << insfs.rdbuf();
An example of a file system API is CopyFile on Win32.
It's possible without libraries - you just need the image specs and 'C', the question is why?
Targa or bmp are probably the easiest, it's just a header and the image data as a binary block of values.
Gif, jpeg and png are more complex - the data is compressed
I'm searching for a way to extract a file in c++ by using the boost::iostreams classes.
There is an example in the boost documentation. But it outputs the content of the compressed file to std::cout.
I'm looking for a way to extract it to a file structure.
Does anybody know how to do that?
Thanks!
Boost.IOStreams does not support compressed archives, just single compressed files. If you want to extract a .zip or .tar file to a directory tree, you'll need to use a different library.
The example in the documentation shows how to decompress the file and push the result to another stream.
If you want the output to be directed to an in-memory array instead, you can use a stream of type boost::iostreams::stream<boost::iostreams::array_source>instead.
That is basically a stream-wrapper around an array.
I'm not sure what you mean when you say you want the output in "a file structure" though.
Looks to me like the call to boost::iostreams::copy takes an ostream as the second parameter. Have you tried creating an ofstream with your output file name and using that?
You probably don't want that library. You might want to look around for some others.
E.g. zziplib