Read binary data using ifstream

Question

Probably, I don't know something about ifstream. I have binary file : 0604 0204 0a02.

std::ifstream input(input_filename, std::ios::binary);
unsigned char cmd;
unsigned char type;
while (!input.eof()) {
    input >> cmd;
    input >> type;
    std::cout << static_cast<int>(cmd) << " " << (int)type << "\n";
}

And result of this code:

6 4
2 4
2 0

What is going wrong?

What did you want instead? This is a very underspecified question. We cannot work out what you wanted, from code that by your own admission _does not do that thing_... — Asteroids With Wings, May 10 '20 at 14:59
This is obviously not your real code as you only perform input and output twice. Present an actual [mcve] not some made-up thing. — Asteroids With Wings, May 10 '20 at 15:06
Did you forget to [add error checking](https://coliru.stacked-crooked.com/a/2d31ab2f6efb3efb)? — Asteroids With Wings, May 10 '20 at 15:07
[`while !eof` is wrong](https://stackoverflow.com/q/5605125/4386278) — Asteroids With Wings, May 10 '20 at 15:08

score 2 · Accepted Answer · answered May 10 '20 at 15:12

operator >> is skipping white space. You need to call

while(input.read(&cmd, 1) && input.read(&type, 1)) {}

or

while(input >> std::noskipws >> cmd >> std::noskipws >> type) {}

Also please note the changed error checking (just casting the stream to bool).

score 0 · Answer 2 · answered May 10 '20 at 15:17

There are two problems here.

0x0A is an ASCII newline, and you're using formatted extraction. That's why your 10 is "skipped" and the 2 extracted instead.

Use .get(), not >>
You're using .eof(), which doesn't check for extraction errors, and checks only after you extract, so you're getting "extra" bad data past the end of your actual read (the 0 is nonsense). Don't use while (!stream.eof()) in a loop (usually).

Fixed code:

#include <fstream>
#include <iostream>

int main()
{
    std::ifstream input(input_filename, std::ios::binary);
    while (true)
    {
        const unsigned char cmd = input.get();
        const unsigned char type = input.get();

        if (!input)
            break;

        std::cout << static_cast<int>(cmd) << " " << (int)type << "\n";
    }
}

/*
g++ -std=c++17 -O2 -Wall -pedantic -pthread main.cpp && ./a.out
6 4
2 4
10 2
*/

Read binary data using ifstream

2 Answers2

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