How can I detect if a binary file was written in big-endian or little-endian format if I don't know which one was used?

Question

Ryan McCombe · Accepted Answer

This is a common challenge when working with binary files from unknown sources. The most reliable approach is to include a known "magic number" at the start of your binary files.

By reading this number and comparing it against the expected value, you can determine the endianness.  Here's a practical example:

```c++
#include <iostream>
#include "SDL.h"

int main() {
  // When writing the file:
  SDL_RWops* WriteHandle{
    SDL_RWFromFile("data.bin", "wb")};
  if (!WriteHandle) {
    std::cout <<
      "Error opening file for writing: "
      << SDL_GetError() << '
';
    return 1;
  }

// Write a known magic number (0x12345678)
  Uint32 MagicNumber{0x12345678};
  SDL_WriteLE32(WriteHandle, MagicNumber);

// Write your actual data...

SDL_RWclose(WriteHandle);

// When reading the file:
  SDL_RWops* ReadHandle{
    SDL_RWFromFile("data.bin", "rb")};
  if (!ReadHandle) {
    std::cout <<
      "Error opening file for reading: "
      << SDL_GetError() << '
';
    return 1;
  }

// Try reading as little-endian first
  Uint32 ReadMagic{SDL_ReadLE32(ReadHandle)};

if (ReadMagic == 0x12345678) {
    std::cout <<
      "File is in little-endian format
";
  } else {
    // Rewind and try big-endian
    SDL_RWseek(ReadHandle, 0, RW_SEEK_SET);
    ReadMagic = SDL_ReadBE32(ReadHandle);

if (ReadMagic == 0x12345678) {
      std::cout <<
        "File is in big-endian format
";
    } else {
      std::cout << "Unknown file format
";
    }
  }

SDL_RWclose(ReadHandle);
  return 0;
}
```

This approach is commonly used in file formats like PNG, which starts with a specific sequence of bytes (89 50 4E 47). When you read these bytes, their order tells you about the endianness.

Another common technique is to include a version number or format identifier at the start of your file. You can then maintain a record of which versions used which endianness. This is particularly useful when your file format evolves over time.

Remember that if you're working with an established file format (like PNG, WAV, etc.), you should consult its specification - most formats have predetermined endianness requirements that you need to follow.

Byte Order and Endianness

Detecting File Endianness

Byte Order and Endianness

Game Development with SDL2

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