Compress and then Decompress a string with zlib.

test.sh

# compile
$ g++ zlib-example.cpp -lz -o zlib-example

# run
$ ./zlib-example
Uncompressed size is: 36
Uncompressed string is: Hello Hello Hello Hello Hello Hello!

----------

Compressed size is: 13
Compressed string is: x��H���W��G*2

----------

Uncompressed size is: 36
Uncompressed string is: Hello Hello Hello Hello Hello Hello!

zlib-example.cpp

#include <stdio.h>
#include <string.h>  // for strlen
#include <assert.h>
#include "zlib.h"

// adapted from: http://stackoverflow.com/questions/7540259/deflate-and-inflate-zlib-h-in-c
int main(int argc, char* argv[])
{   
    // original string len = 36
    char a[50] = "Hello Hello Hello Hello Hello Hello!"; 

    // placeholder for the compressed (deflated) version of "a" 
    char b[50];

    // placeholder for the UNcompressed (inflated) version of "b"
    char c[50];
     

    printf("Uncompressed size is: %lu\n", strlen(a));
    printf("Uncompressed string is: %s\n", a);


    printf("\n----------\n\n");

    // STEP 1.
    // deflate a into b. (that is, compress a into b)
    
    // zlib struct
    z_stream defstream;
    defstream.zalloc = Z_NULL;
    defstream.zfree = Z_NULL;
    defstream.opaque = Z_NULL;
    // setup "a" as the input and "b" as the compressed output
    defstream.avail_in = (uInt)strlen(a)+1; // size of input, string + terminator
    defstream.next_in = (Bytef *)a; // input char array
    defstream.avail_out = (uInt)sizeof(b); // size of output
    defstream.next_out = (Bytef *)b; // output char array
    
    // the actual compression work.
    deflateInit(&defstream, Z_BEST_COMPRESSION);
    deflate(&defstream, Z_FINISH);
    deflateEnd(&defstream);
     
    // This is one way of getting the size of the output
    printf("Compressed size is: %lu\n", strlen(b));
    printf("Compressed string is: %s\n", b);
    

    printf("\n----------\n\n");


    // STEP 2.
    // inflate b into c
    // zlib struct
    z_stream infstream;
    infstream.zalloc = Z_NULL;
    infstream.zfree = Z_NULL;
    infstream.opaque = Z_NULL;
    // setup "b" as the input and "c" as the compressed output
    infstream.avail_in = (uInt)((char*)defstream.next_out - b); // size of input
    infstream.next_in = (Bytef *)b; // input char array
    infstream.avail_out = (uInt)sizeof(c); // size of output
    infstream.next_out = (Bytef *)c; // output char array
     
    // the actual DE-compression work.
    inflateInit(&infstream);
    inflate(&infstream, Z_NO_FLUSH);
    inflateEnd(&infstream);
     
    printf("Uncompressed size is: %lu\n", strlen(c));
    printf("Uncompressed string is: %s\n", c);
    

    // make sure uncompressed is exactly equal to original.
    assert(strcmp(a,c)==0);

    return 0;
}

I have modified your code to use random string by merging another example i found for random string generation; i found that with random string generated the compression doesn't work; meaning compressed and uncompressed length are same ; furthermore compressing compressed string always results in 3 less chars from tail . with default "hello" string as in original example it does compress and results are identical as you have shown.

do you see any reason, why this code should fail for random strings?

#include <stdio.h> 
#include <string.h>  // for strlen
#include <assert.h>
#include "zlib.h"
//#include <random.h>
#include <iostream>
#include <string>
#include <cstdlib>
#include <ctime>
// adapted from: http://stackoverflow.com/questions/7540259/deflate-and-inflate-zlib-h-in-c

static const char alphanum[] =
"0123456789"
"!@#$%^&*"
"ABCDEFGHIJKLMNOPQRSTUVWXYZ"
"abcdefghijklmnopqrstuvwxyz";

int stringLength = sizeof(alphanum) - 1;

int length = 50;

int main(int argc, char* argv[])
{
    // original string len = 36
    //char a[50] = "Hello Hello Hello Hello Hello Hello!";
    char a[length];
    // placeholder for the compressed (deflated) version of "a" 
    char b[length];

    // placeholder for the UNcompressed (inflated) version of "b"
    char c[length];


    srand(time(0));
//std::string a;
    for(unsigned int j = 0; j < 1; ++j)
    {
        //a = "";
        for(unsigned int i = 0; i < length ; ++i)
        {
        a[i]= alphanum[rand() % stringLength];

        }
                //char a[50]="Hello Hello Hello Hello Hello Hello!";
                printf("Uncompressed size is: %lu\n", strlen(a));
                printf("Uncompressed string is: %s\n", a);


                printf("\n----------\n\n");

                // STEP 1.
                // deflate a into b. (that is, compress a into b)

                // zlib struct
                z_stream defstream;
                defstream.zalloc = Z_NULL;
                defstream.zfree = Z_NULL;
                defstream.opaque = Z_NULL;
                // setup "a" as the input and "b" as the compressed output
                defstream.avail_in = (uInt)strlen(a)+1; // size of input, string + terminator
                defstream.next_in = (Bytef *)a; // input char array
                defstream.avail_out = (uInt)sizeof(b); // size of output
                defstream.next_out = (Bytef *)b; // output char array

                // the actual compression work.
                deflateInit(&defstream, Z_BEST_COMPRESSION);
                deflate(&defstream, Z_FINISH);
                deflateEnd(&defstream);
  // This is one way of getting the size of the output
                printf("Compressed size is: %lu\n", strlen(b));
                printf("Compressed string is: %s\n", b);


                printf("\n----------\n\n");
		// STEP 2.
    // inflate b into c
    // zlib struct
    z_stream infstream;
    infstream.zalloc = Z_NULL;
    infstream.zfree = Z_NULL;
    infstream.opaque = Z_NULL;
    // setup "b" as the input and "c" as the compressed output
    infstream.avail_in = (uInt)((char*)defstream.next_out - b); // size of input
    infstream.next_in = (Bytef *)b; // input char array
    infstream.avail_out = (uInt)sizeof(c); // size of output
    infstream.next_out = (Bytef *)c; // output char array
     
    // the actual DE-compression work.
    inflateInit(&infstream);
    inflate(&infstream, Z_NO_FLUSH);
    inflateEnd(&infstream);
     
    printf("Uncompressed size is: %lu\n", strlen(c));
    printf("Uncompressed string is: %s\n", c);
    

    // make sure uncompressed is exactly equal to original.
               
        }
        return 0;
}

and results are

 ./c.out 
Uncompressed size is: 50
Uncompressed string is: i5dY0LNq4^yilrkQqJYuW037$u^CDcw^hlzu^NwNou%Zx!8zfg

----------

Compressed size is: 50
706W)�svI.���*���/U��P��4M�4�+4���)�,�

----------

Uncompressed size is: 47
Uncompressed string is: i5dY0LNq4^yilrkQqJYuW037$u^CDcw^hlzu^NwNou%Zx!8

Your length of array should be 51.

does this run on windows ?

when i am running this code this in giving me this error
error C2275: 'z_stream' : illegal use of this type as an expression

be ware using strlen() with binary data

// defstream.avail_in = (uInt)strlen(a)+1;

How to execute these programs in windows

does this run on windows ?

no ig

How to execute these programs in windows

install windows subsystem for linux and run the code from the linux environment

Input File: file size: 52946 Aug 23 17:23 RGB_16bits_palette_sample_image.png
Output Comp File: file size: 52972 Aug 23 17:35 zlib_comp_16bits.j2k
Output DeComp File: file size: 52946 Aug 23 17:35 zlib_decomp_16bits.png

Why the output Compressed file size becoming greater then the ( 52972 > 52946) Input file size ?

It's normal.
recompressing data almost always increases the size.

Is this possible without the array length? like instead of [90] we could have []?

Yes. without array length
use const char* myvar;

How do I use this dynamically without knowing the output size, (when decompressing)

@anasouardini Did you find any answer?