google::protobuf::io::GzipOutputStream does not write anything if the file handle is closed at the end

Question

The following code writes to file as expected

  int ofd = open(filename.c_str(),  O_WRONLY | O_CREAT | O_TRUNC, 0777);
  google::protobuf::io::FileOutputStream outp(ofd);
  google::protobuf::io::GzipOutputStream fout(&outp);

  MyMessage msg;
  ConstructMessage(&msg);
  CHECK(google::protobuf::util::SerializeDelimitedToZeroCopyStream(msg, &fout));
  fout.Close();
  // close(ofd);

However if I uncomment the last line // close(ofd);, I get empty file. Why is that?

Also if I skip using the Gzip wrapper, the last line causes no problem. Does this look like a bug?

score 2 · Accepted Answer · answered Feb 27 '22 at 15:44

You should close things in the opposite order of opening:

int ofd = open(filename.c_str(),  O_WRONLY | O_CREAT | O_TRUNC, 0777);
google::protobuf::io::FileOutputStream outp(ofd);
google::protobuf::io::GzipOutputStream fout(&outp);

...

fout.Close();
outp.Close();
close(ofd);

With the missing outp.Close();, some data may remain buffered in it. The destructor will eventually flush it out, but at that point the ofd is already closed so there is nothing to write to.

google::protobuf::io::GzipOutputStream does not write anything if the file handle is closed at the end

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