Write int to binary buffer (Android) and read with C++

Question

I want to write an integer in a binary file with Java (Android) and then read it with a C++ code. My code in Java is:

byte [] mybuffer = ByteBuffer.allocateDirect(4).putInt(1000).array;
out.write(mybuffer, 0, 4); // out is FileOutputStream

The reader in C++

std::ifstream fileToRead;
fileToRead.open("myFile", std::ios::binary);
if (!fileToRead.is_open()){
    std::cout << "[ERROR] Can't open file" << std::endl;
    exit(-1);
}
int  * myInt = new int;
fileToRead.read((char*)&myInt[0], 4);
std::cout << " The integer is " << myInt[0] <<  std::endl;

But I get values which doesnt make sense.

Thanks

output Java:

buffer[0] = 0
buffer[1] = 0
buffer[2] = 3
buffer[3] = -24

output c++:

The integer is -402456576

you should add your weird outputs to your question :) (also there's no actual "question" in you question) — m02ph3u5
– m02ph3u5, Commented Aug 7, 2015 at 12:50
The actual reading-part of the C++ code looks legit, but it certainly got the feel of a Java coder. No need to dynamically allocate that myInt variable, the address-of operator & is enough: int myInt; fileToRead.read(reinterpret_cast<char*>(&myInt), sizeof(myInt)); — Some programmer dude
– Some programmer dude, Commented Aug 7, 2015 at 12:51
It'd be useful to see sample input and outputs, especially printed byte-by-byte. — shakurov
– shakurov, Commented Aug 7, 2015 at 12:52
In the Android part I get: mybuffer[0] = 0 mybuffer[1] = 0 mybuffer[2] = 3 mybuffer[3] = -24 and in the C++ part: myInt = -402456576 I removed the new of the int as well — Snoopyjackson
– Snoopyjackson, Commented Aug 7, 2015 at 13:00

score 4 · Accepted Answer · 2015-08-07 13:15:09Z

4

You may encounter a byte-order problem:

#include <cstdint>
#include <fstream>
#include <iostream>
// For ntohl with Linux (Windows has similar):
#include <arpa/inet.h>

int main()
{
    // You can use the constructor to open the file:
    std::ifstream fileToRead("myFile", std::ios::binary);
    // Just check the general state of the stream:
    if(!fileToRead){
        std::cout << "[ERROR] Can't open file" << std::endl;
        // Please do not use exit to terminate a program:
        return -1;
    }
    // No need to allocate an integer. Also be specific about the size:
    int32_t myInt;
    // There might be byte order issues, here (Java is big-endian):
    fileToRead.read((char*)&myInt, sizeof(int32_t));
    // To fix it convert the integer from network byte order to host byte order:
    myInt = ntohl(myInt);
    std::cout << " The integer is " << myInt <<  std::endl;
}

edited Aug 7, 2015 at 13:15

answered Aug 7, 2015 at 13:08

user2249683

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1 Comment

Snoopyjackson Over a year ago

Yes it works thanks ! Will this method works with a big array of float too ? Thanks

Joop Eggen · Accepted Answer · 2015-08-07 13:16:26Z

1

For good order, as java uses per default BIG_ENDIAN byte order:

byte[] mybuffer = ByteBuffer.allocateDirect(4)
                  .order(Order.LITTLE_ENDIAN).putInt(1000).array();

That is the order on Intel processor's memory architecture.

answered Aug 7, 2015 at 13:16

Joop Eggen

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