Raspberry Pi 3 Model B: How to change the Byte Order to Big Endian?

Question

I have a Raspberry Pi 3 Model B and I switched from the 32-bit Raspbian to the 64-bit Gentoo Linux as I wanted to test some 64-bit applications. Now, I want to test an application which would access data bytes in big-endian order. I know I can do this manually by swapping the bytes in the word. But my main question is how to change the byte order to big-endian permanently on Raspberry Pi and may be later again switch back to little-endian after all the tests are done. Any leads would be greatly appreciated.

Answer 1

I have been reading quite a lot since seeing your question for the first time, and my answer is that you cannot execute a big-endian Linux application on a little-endian Linux kernel/distribution.
I even tried on my Armbian-based Orangepi-PC2 aarch64 system, which simply refused loading a sample program cross-compiled using ARM'latest big-endian toolchain for aarch64.

Now, if you really don't want to convert your data, and if performance is not an issue - it probably is not because you want to make your processing on a Raspberry-pi instead of on a PC - I would suggest you to:

• install qemu (on a Linux PC or a virtual machine) for a big-endian 64 bits platform, say ppc64,
• install a cross-compiler for ppc64,
• compile your program and process your data.

On an Ubuntu 18.04 system, you would execute the following command:

sudo apt-get install gcc-8-powerpc64-linux-gnu qemu

Here is small example:

Create a binary file containing the 64 bits number 0x1122334455667788:

printf "\x11\x22\x33\x44\x55\x66\x77\x88">  example.dat
hexdump -C example.dat 
00000000  11 22 33 44 55 66 77 88                           |."3DUfw.|
00000008

Create this minimal, demo-only program:

// example.c

#include <stdio.h>
#include <stdlib.h>

int main()
{
    // demo code only - don't do this in real life...
    unsigned int value = 0;
    FILE* fp = fopen("example.dat", "rb");
    fread(&value, sizeof(value), 1, fp);
    printf("%d\n", value);
}

Compile it:

powerpc64-linux-gnu-gcc-8 -static -o example example.c

Execute it:

qemu-ppc64 ./example
0x1122334455667788

The same program compiled for x86_64 would display 0x8877665544332211, which is fine:

gcc -o example example.c
./example 
0x8877665544332211

qemu-ppc64 is emulating the ppc64, but system calls are being performed by your x86_64 linux kernel. I hope this help solving your original problem.