Premature EOF error with List of List of struct implementation under VS2017/x64

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Feb 15, 2021, 2:56:44 PMFeb 15
to Cap'n Proto

I have been learning this wonderful software.
So I have been looking for an opportunity to use capnproto for my project,
and now I have it but I need to use it with MSVC 2017(recently updated to latest version 15.9.33), x64.
I only need serialization and no need for RPC or async.
This is my very first time posting a message to this community, please let me know if I'm not doing right here.

I have a schema whose basic construct is List of List of struct ( x,y points in 4d expressed by two level lists). Second level will have a few hundreds of entries and first level will have lots of entries.
And I am now stuck with a issue. If I put many entries in either of lists (more than a few hundreds) and write that to a file, then when I read the message, I get exception like this:
Any help would be very appreciated.

printSparseDir4D: std::exception: kj\io.c++:53: failed: expected n >= minBytes; Premature EOF
stack: 7ff672faac46 7ff672fa3fd6 7ff672fa2de5 7ff672fa1308 7ff672fb15d3 7ffc1aa97c23 7ffc1b18d720

Now I found that writing 4 entries in first level, two entries in second level (two x,y points each in 4 frames), I get the same error.

I'm putting my schema and my read/write code below. I'm using these codes with capnproto0.8.0. But actually this schema is a little striped version than the one I want to use. When I used that a little more complex schema, I faced similar issue with capnproto 0.7.0 and 0.8.0. 

I built capnproto following the steps in the website, had to skip heavy-tests project and kj-async (0.8.0), hoping the build was ok. 

Is there anything missing or done wrong? 
It would be very much appreciated for any help.
Thank you.

sparse4d.capnp :

# unique file ID, generated by `capnp id`

struct Sparse4DHeader {
    tSize @0 :Int32;
    zSize @1 :Int32;
    ySize @2 :Int32;
    xSize @3 :Int32;

struct Dir4D {
    header @0 :Sparse4DHeader;
    planes @1 :List(SparsePlane);

    struct SparsePlane {
        t @0 :UInt16;
        z @1 :UInt16;
        points @2 :List(Point);

struct Point {
    x @0 :UInt16;
    y @1 :UInt16;
    intensity @2 :Float32;

And read/write code:

 * Copyright (c) 2021 ASTOM R&D
 * Author: Yojiro Nobukuni
//#include "pch.h"

#include "../sparse4d.capnp.h"
#include <capnp/message.h>
#include <capnp/serialize-packed.h>
#include <iostream>
#include <ios>

#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
//#include <unistd.h>

  * g++ writer.cpp photodeco_kernel.capnp.c++ -o writer -lcapnp -lkj
  * ./writer > test.message

 * Write PhotodecoKernel message to fd
 * @param int fd fd to read message in
void writeSparse4D(int fd, unsigned int nofPlanes, unsigned int nofPoints, bool packed)
::capnp::MallocMessageBuilder message;

auto dir4d = message.initRoot<Dir4D>();
auto header = dir4d.initHeader();

auto planes = dir4d.initPlanes(nofPlanes);
for (auto plane : planes) {
auto points = plane.initPoints(nofPoints);
#if 0
                // This one or the other one in #else block, same behavior.
for (unsigned int i = 0; i < nofPoints; i++) {
for (auto p : points) {

try {
if (packed)
writePackedMessageToFd(fd, message);
writeMessageToFd(fd, message);
catch (std::exception& e) {
std::cout << "writeSparse4D: std::exception: " << e.what() << std::endl;

 * Print PhotodecoKernerl kernel in text
 * @param PhotodecoKernel::Reader kernel
void printDir4D(Dir4D::Reader dir4d, bool printPoints)
auto header = dir4d.getHeader();
std::cout << "  sizes[T,Z,Y,X] = "
<< header.getTSize() << ", " << header.getZSize() << ", "  << header.getYSize() << ", " << header.getXSize() << ", "
<< std::endl;
//std::cout << "  nofPoints = " << header.getNofPoints() << std::endl;

if (!printPoints) return;

size_t nofPoints = 0;
auto planes = dir4d.getPlanes();
for (auto plane : planes) {
const auto t = plane.getT();
const auto z = plane.getZ();
std::cout << "plane(t,z): " << t << ", " << z << std::endl;
for (auto p : plane.getPoints()) {
std::cout << t << ", " << z << ", " << p.getY() << ", " << p.getX() << ", " << p.getIntensity() << std::endl;

std::cout << "Total nof KernelPoints = " << nofPoints << std::endl;

 * Read PhotodecoKernerl message from fd and print it in text
 * @param int fd
 * @param bool packed
void printSparseDir4D(int fd, bool printPoints, bool packed)
std::cout << "printSparseDir4D" << std::endl;

// readerOptions for reading message.
capnp::ReaderOptions readerOptions;
readerOptions.traversalLimitInWords = 1000 * 1000 * 1000 * readerOptions.traversalLimitInWords;

::capnp::MessageReader *message;
try {
if (packed)
message = new ::capnp::PackedFdMessageReader(fd, readerOptions);
message = new ::capnp::StreamFdMessageReader(fd, readerOptions);

auto dir4d = message->getRoot<Dir4D>();
printDir4D(dir4d, printPoints);
catch (std::exception& e) {
std::cout << "printSparseDir4D: std::exception: " << e.what() << std::endl;

delete message;

Feb 16, 2021, 2:23:38 AMFeb 16
to Cap'n Proto

So, trying to understand what is happening.

using capnp.exe (32bit) in capnproto-tools-win32-0.8.0, message can be read correctly.
capnp.exe decode sparse4d.capnp Dir4D < myoutput.bin
By looking at hex dump of the message file, it looks like a proper message format.

So, writing new message is okay but reading it is not working properly.
Using capnproto-0.8.0 built with VS2017/x64 , and create a new message, save it to a file, and then reading it by my program fails.
Since all happens when I do message.getRoot<Dir4D>(),  this "Premature EOF" situation happens inside capnp library.

Is there any way to fix this? or to narrow down what is wrong?

Thank you.

2021年2月16日火曜日 4:56:44 UTC+9

Feb 16, 2021, 8:15:22 AMFeb 16
to Cap'n Proto

I could do a little more things.

Interestingly, capnp.exe built under x64 can do decode message.
I bet it does different processing than MessageReader's do and simply follow message format structure.

I made a test code using MapViewOfFile  and FlatArrayMessageReader,.
It can read messages that I couldnt by using StreamFdMessageReader.
But easily fails with a little large data, say 32 entries in first level List and 1 entry in the second level List,
getting following exception:

Exception: capnp\serialize.c++:51: failed: expected array.size() >= offset + segmentSize; Message ends prematurely in first segment.
stack: 7ff6a2af4b35 7ff6a2af3577 7ff6a2af14b6 7ff6a2b02323 7ffc1aa97c23 7ffc1b18d720

So I think I'm stuck. Any advice on how to proceed will be greatly appreciated.

Thank you in advance.

2021年2月16日火曜日 16:23:38 UTC+9

Kenton Varda

Feb 19, 2021, 11:55:08 AMFeb 19
to, Cap'n Proto

Sorry for the slow reply.

Just going to throw out a guess here, but seeing that you're using windows, did you make sure to use O_BINARY when opening the file descriptor? If you didn't, then the C library will silently corrupt your data by converting line endings.

If that's not it, then could you provide a complete self-contained program (with main() function and all parameters filled in) that demonstrates the problem?


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Feb 21, 2021, 6:09:41 AMFeb 21
to Cap'n Proto
Hi  Kenton,

Glad to see your reply.

Yes I am aware of open mode. I do, for writing,
return _sopen_s(pfd, fpath.c_str(), (_O_BINARY | _O_CREAT | _O_RDWR | _O_TRUNC), _SH_DENYNO, (_S_IREAD | _S_IWRITE));
and for reading,
return _sopen_s(pfd, fpath.c_str(), (_O_BINARY | _O_RDWR), (_SH_DENYNO), (_S_IREAD | _S_IWRITE));

Using _O_WRONLY for writing, _O_RDONLY for reading did not change the situation.
For shared flag,  I have tried SH_DENYNO or SH_DENYRW but made no difference, as I had expected.

I can provide my code. I will send it to you when its ready, have to clean it a little I guess.

Thank you very much for the message.

2021年2月20日土曜日 1:55:08 UTC+9

Feb 21, 2021, 10:07:45 AMFeb 21
to Cap'n Proto
Hi Kenton

I prepared my source for sending to you.
Can I send it to your email address? or reply to this group with it as attachment?

Thank you.

2021年2月21日日曜日 20:09:41 UTC+9

Kenton Varda

Feb 21, 2021, 12:05:32 PMFeb 21
to, Cap'n Proto
Hi Yojiro,

It would be best if you could reduce the problem to a minimal self-contained test case. If your program is multiple files then it is probably too big for us to look at. All I really need is for you to add a main() function to the code you sent before, so I can see exactly the code that demonstrates the problem.


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