Decompress with gz* functions succeeded but failed with inflate* functions using zlib

问题

I have a gzip file sample, with standard gzip header 1F 8B 08 00 ..., when I inflate it with inflate* functions in zlib, the output is only 11 bytes(in fact the output should be about 4KB), but when I decompress it with gz* functions, it produce the correct output, the code:

using gz* (this can produce correct output):

#define CHUNK 10240
int gz_decompress(const char *path) {
    gzFile f = gzopen(path, "rb");
    if(!f)
        return -1;

    unsigned char result[CHUNK];
    int bytes_read = gzread(f, result, CHUNK);
    if(bytes_read < CHUNK)
        if(!gzeof(f))
            return -2;
    gzclose (f);
    return 0;
}

using inflate* (output is only 11 bytes):

#define CHUNK 10240
int inf(FILE *source)
{
    int ret;
    unsigned have;
    z_stream strm;
    unsigned char in[CHUNK];
    unsigned char out[CHUNK];

    /* allocate inflate state */
    strm.zalloc = Z_NULL;
    strm.zfree = Z_NULL;
    strm.opaque = Z_NULL;
    strm.avail_in = 0;
    strm.next_in = Z_NULL;
    // ret = inflateInit(&strm);
    ret = inflateInit2(&strm, 16 + 15);
    if (ret != Z_OK)
        return ret;

    /* decompress until deflate stream ends or end of file */
    do {
        strm.avail_in = fread(in, 1, CHUNK, source);
        if (strm.avail_in == 0)
            break;
        strm.next_in = in;

        /* run inflate() on input until output buffer not full */
        do {
            strm.avail_out = CHUNK;
            strm.next_out = out;
            ret = inflate(&strm, Z_NO_FLUSH);
            assert(ret != Z_STREAM_ERROR);  /* state not clobbered */
            switch (ret) {
            case Z_NEED_DICT:
                ret = Z_DATA_ERROR;     /* and fall through */
            case Z_DATA_ERROR:
            case Z_MEM_ERROR:
                (void)inflateEnd(&strm);
                return ret;
            }
            have = CHUNK - strm.avail_out;
        } while (strm.avail_out == 0);

        /* done when inflate() says it's done */
    } while (ret != Z_STREAM_END);

    /* clean up and return */
    (void)inflateEnd(&strm);
    return ret == Z_STREAM_END ? Z_OK : Z_DATA_ERROR;
}

In fact, the second snippet comes from the official zlib usage example zpipe.c, I changed only the invocation of inflateInit(&strm); to inflateInit2(&strm, 16 + 15); according to this zlib gzip discussion, but now I have no idea why it fails, anyone could help?

回答1:

Per your comment, inf() is returning Z_OK, which means that the gzip stream is being successfully decompressed and verified.

What do you mean "output is only 11 bytes"? Your inf() function can produce no output at all.

来源：https://stackoverflow.com/questions/17872152/decompress-with-gz-functions-succeeded-but-failed-with-inflate-functions-using

标签

gzip

zlib

compression

inflate