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Asked: 2026-06-08T10:25:24+00:00

I want to test the compression and decompression functions: compress () uncompresss ()provides by

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I want to test the compression and decompression functions: compress () uncompresss ()provides by the ZLIB library ; wrote the following code to open a file that already exists, read in a while () loop insidetake the contents of the file already exists, the compression portion write to a single file, the uncompress part written to another file, the code shown below, the size of the file that already exists (originalFile) about 78K , the first time to enter while() loop compression with decompression of the return value is 0, so that the first entry is successful, but the second and the next a few times to enter, return values ​​are -5 (according to official documents, buffered output size is not large to contain the content), why ? Where was wrong? pre-thank you very much!

enter code here

#include <string>
#include <time.h>
#include <stdio.h>
#include <iostream>
#include <string.h>
#include "zlib.h"
int main()
{
    unsigned long int fileLength;
    unsigned long int readLength;
    unsigned long int compressBufLength;
    unsigned long int uncompressLength;
    unsigned long int offset;

    unsigned char *readBuf = new unsigned char[512];//the readbuf of the exist file content
    unsigned char *compressBuf = new unsigned char[512];//the compress buffer   
    unsigned char *uncompressBuf = new unsigned char[512];//the uncompress  content buffer
    FILE *originalFile = fopen("/lgw150/temp/src/lg4/original.lg4","a+");//the exist file
    FILE *compressedFile = fopen("/lgw150/temp/src/lg4/compressed.lg4","a+");//compressfile
    FILE *uncompressFile = fopen("/lgw150/temp/src/lg4/uncompressed.lg4","a+");//

    fseek(originalFile,0,2);
    fileLength = ftell(originalFile);
    offset = 0;//
       while(offset <fileLength)//
    {


        printf("offset=%lu;fileLength=%lu\n",offset,fileLength);
        memset(readBuf,0,512);
        memset(compressBuf,0,512);
        memset(uncompressBuf,0,512);
        fseek(originalFile,offset,0);//
        readLength = fread(readBuf,sizeof(char),512,originalFile);
        offset += readLength;//
        int compressValue = compress(compressBuf,&compressBufLength,readBuf,readLength);
        int fwriteValue = fwrite(compressBuf,sizeof(char),compressBufLength,compressedFile);//
        printf("compressValue = %d;fwriteLength = %d;compressBufLength=%lu;readLength = %lu\n",compressValue,fwriteValue,compressBufLength,readLength);

        int uncompressValue = uncompress(uncompressBuf,&uncompressLength,compressBuf,compressBufLength);//
        int fwriteValue2= fwrite(uncompressBuf,sizeof(char),uncompressLength,uncompressFile);//
    }
    fseek(originalFile,0,0);
    fseek(compressedFile,0,0);
    fseek(uncompressFile,0,0);
    if(originalFile != NULL)
    {
        fclose(originalFile);
        originalFile = NULL;
    }

   if(compressedFile != NULL)
    {
        fclose(compressedFile);
        compressedFile = NULL;
    }
     if(uncompressFile != NULL)
    {
        fclose(uncompressFile);
        uncompressFile = NULL;
    }

    delete[] readBuf;
    delete[] compressBuf;
    delete[] uncompressBuf;
return 0;


}

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1 Answer

  1. Editorial Team

    First off, the reason you’re getting “buffered output size is not large enough to contain the content” is because the buffered output size is not large enough to contain the content. If you give incompressible data to compress it will expand the data. So 512 bytes is not large enough if the input is 512 bytes. Use the compressBound() function for the maximum expansion for sizing the compression output buffer.

    Second, compressing 512 bytes at a time is silly. You’re not giving the compression algorithm enough data to work with in order to get the mileage you should be getting from the compression. Your application of reading 512 byte chunks at a time should not be using compress() and uncompress(). You should be using deflate() and inflate(), which were written for this purpose — to feed chunks of data through the compression and decompression engines.

    You need to read zlib.h. All of it. You can also look at the example (after reading zlib.h).

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