Question

I have a gzip file that is in memory, and I would like to uncompress it using zlib, version 1.1.3. Uncompress() is returning -3, Z_DATA_ERROR, indicating the source data is corrupt.

I know that my in memory buffer is correct – if I write the buffer out to a file, it is the same as my source gzip file.

The gzip file format indicates that there is a 10 byte header, optional headers, the data, and a footer. Is it possible to determine where the data starts, and strip that portion out? I performed a search on this topic, and a couple people have suggested using inflateInit2(). However, in my version of zlib, that function is oddly commented out. Is there any other options?

Answer 1

Is it possible to determine where the data starts, and strip that portion out?

Gzip has the following magic number:

static const unsigned char gzipMagicBytes[] = { 0x1f, 0x8b, 0x08, 0x00 };

You can read through a file stream and look for these bytes:

static const int testElemSize = sizeof(unsigned char);
static const int testElemCount = sizeof(gzipMagicBytes);

const char *fn = "foo.bar";
FILE *fp = fopen(fn, "rbR");
char testMagicBuffer[testElemCount] = {0};
unsigned long long testMagicOffset = 0ULL;

if (fp != NULL) {
    do {
        if (memcmp(testMagicBuffer, gzipMagicBytes, sizeof(gzipMagicBytes)) == 0) {
            /* we found gzip magic bytes, do stuff here... */
            fprintf(stdout, "gzip stream found at byte offset: %llu\n", testMagicOffset);
            break;
        }
        testMagicOffset += testElemSize * testElemCount;
        fseek(fp, testMagicOffset - testElemCount + 1, SEEK_SET);
        testMagicOffset -= testElemCount + 1;
    } while (fread(testMagicBuffer, testElemSize, testElemCount, fp));
}

fclose(fp);

Once you have the offset, you could do copy and paste operations, or overwrite other bytes, etc.