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

Question

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?

Answer 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.