CODEWITHSUNDEEP
c++filestreamendianness
BeBel42
BeBel42

Reputation: 133

C++ Is endianness affecting fstream's read and write functions?

I am learning how to write to/read from binary files, and I want my files to be readable for any machine that supports C++ 17. So I had an horrifying thought: what if read() and write() are affected by the endianness of the machine?

For exemple: I run this code on a big endian machine,

#include <iostream>
#include <fstream>
#include <cstdint>

int main() {

    std::ofstream Output("Hey.txt", std::ofstream::out | std::ofstream::binary);

    if (Output.is_open()) {
        int16_t i = 42;
        Output.write((const char*)&i, 2);
    }

}

Then I pass Hey.txt to a small endian machine, then run this code on the machine,

#include <iostream>
#include <fstream>
#include <cstdint>

int main() {

std::ifstream Input("Hey.txt", std::ifstream::in | std::ifstream::binary);

    if (Input.is_open()) {
        int16_t i;
        Input.read((char*)&i, 2);
        std::cout << i << std::endl;
    }

}

Will the program print 42? Or is fstream handling the endian difference? If not, is there a way to prevent endian issues?

Thanks for your attention.

Upvotes: 5

Views: 2153

Answers (1)

David Schwartz
David Schwartz

Reputation: 182753

Two simple rules:

  1. If you're going to write binary data to a file, document the file format.

  2. When you write binary data to a file, make sure the bytes you are writing match the requirements of the file format.

If the file format says that there's a 16-bit integer in big-endian format, and you write code that reads or writes that, everything will work fine.

Don't use a cast like this (char*)&i because then the bytes will hold whatever format the native system happens to use for 16-bit integers and that is not even guaranteed (by the C++ standard) to be stable across runs of the same code on the same system.

Upvotes: 4

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