Failed to unarchive file which compressed by c++ zlib.h

Question

Compress string (i tried to compress file but it didnt help) and save compress data to file:

#include <iostream>
#include <fstream>
#include <zlib.h>

int main() {
    char a[50] = "Hello, world!";
    char b[50];

    uLong ucompSize = strlen(a)+1;
    uLong compSize = compressBound(ucompSize);

    compress((Bytef *)b, &compSize, (Bytef *)a, ucompSize);

    std::ofstream f;
    f.open("res.txt.gz", std::ios::binary);
    f.write(b, compSize);

    // Assertion
    char c[50];
    uncompress((Bytef *)c, &ucompSize, (Bytef *)b, compSize);;
    assert(std::string(c) == std::string(a));
}

Next Im trying to unarchive it by gzip util and error appears:

$ gzip -d res.txt.gz 

gzip: res.txt.gz: not in gzip format

Where is the compression error?

Your minimal example appears to have several typos. Also you have to write a binary file and not a text file. And `b` is not a null terminated string. — drescherjm, Apr 19 '22 at 12:45
@drescherjm I added `std::ios::binary` to opening file but it still doesn't work — Vlad, Apr 19 '22 at 14:27
You need to fix `f << data;` you cant use `<<` you need to use `f.write()` and make sure that you use `compSize` as the size to write. — drescherjm, Apr 19 '22 at 14:28
Does this answer your question? [Writing binary data to fstream in c++](https://stackoverflow.com/questions/49492259/writing-binary-data-to-fstream-in-c) — drescherjm, Apr 19 '22 at 14:29
@drescherjm not completely. I need to understand how write char array — Vlad, Apr 19 '22 at 14:34
`std::ios::app` is wrong in that if you had a corrupt archive from previous tests it will append to that and still be corrupt. — drescherjm, Apr 19 '22 at 14:51

score 1 · Answer 1 · answered Apr 20 '22 at 01:46

1

It says it's not in gzip format because it's not in gzip format. compress() compresses to the zlib format. If you want zlib to produce gzip streams, you either need to use deflate() to do it in memory, or the gz* functions if you're writing to a file.

answered Apr 20 '22 at 01:46

Mark Adler

101,978
13
118
158

Failed to unarchive file which compressed by c++ zlib.h

1 Answers1