uncompress error when using zlib

Question

It could be a silly question. When I tried to uncompress a compressed data in memory, got error. Here is the code.

#include <zlib.h>
#include <stdio.h>
#include <stdlib.h>

int readFile(char *fname, char buf[]) {
    FILE *fp = fopen(fname, "rb");
    if (fp == NULL) { printf("Failed to open %s\n", fname); exit(0);}
    int n = fread(buf, 1, 0x100000, fp);
    fclose(fp);
    return n;
}

char buf[2][0x10000];
int main(int argc, char *argv[]) {
    long n = readFile(argv[1], &buf[0][0]);
    unsigned int *pInt = (unsigned int*) (&buf[0][0]);
    printf("n=%d %08x\n", n, *pInt);
    long m = 0x10000;
    int rc = uncompress(&buf[1][0], &m, &buf[0][0], n);
    printf("rc = %d %s\n", rc, &buf[1][0]);
    return 0;
}

Got error:

./a.out te.html.gz
n=169 08088b1f
rc = -3 

te.html.gz is obtained from by running `gzip te.html'.

Thanks!

Answer 1

zlib format is not gzip format. The zlib uncompress function doesn't understand the gzip format.

You can generate some zlib format test data by writing a similar program that calls the compress function in zlib. Or you could use the openssl zlib command, if you have openssl installed.

Answer 2

A complete, working example on uncompressing gzipped data (thanks to link)

#include <zlib.h>
#include <stdio.h>
#include <stdlib.h>

int readFile(char *fname, char buf[]) {
    FILE *fp = fopen(fname, "rb");
    if (fp == NULL) { printf("Failed to open %s\n", fname); exit(0);}
    int n = fread(buf, 1, 0x100000, fp);
    fclose(fp);
    return n;
}
int inf(const char *src, int srcLen, const char *dst, int dstLen){
    z_stream strm;
    strm.zalloc=NULL;
    strm.zfree=NULL;
    strm.opaque=NULL;

    strm.avail_in = srcLen;
    strm.avail_out = dstLen;
    strm.next_in = (Bytef *)src;
    strm.next_out = (Bytef *)dst;

    int err=-1, ret=-1;
    err = inflateInit2(&strm, MAX_WBITS+16);
    if (err == Z_OK){
        err = inflate(&strm, Z_FINISH);
        if (err == Z_STREAM_END){
            ret = strm.total_out;
        }
        else{
            inflateEnd(&strm);
            return err;
        }
    }
    else{
        inflateEnd(&strm);
        return err;
    }
    inflateEnd(&strm);
    printf("%s\n", dst);
    return err;
}


char buf[2][0x10000];
int main(int argc, char *argv[]) {
    long n = readFile(argv[1], &buf[0][0]);
    unsigned int *pInt = (unsigned int*) (&buf[0][0]);
    printf("n=%d %08x\n", n, *pInt);
    long m = 0x10000;
    int rc = inf(&buf[0][0], n, &buf[1][0], m);
    printf("rc = %d %s\n", rc, &buf[1][0]);
    return 0;
}