A basic guide on cross compiling on an Ubuntu PC for an ARM linux environment
Mainly to document what I have learnt from the web. I do not claim to be an expert.

Reference : http://embedonix.com/articles/linux/emulating-raspberry-pi-on-linux/

QEMU is used to emulate the raspberry pi hardware. To install, use sudo apt-get install qemu

Download the kernel here: https://github.com/dhruvvyas90/qemu-rpi-kernel/blob/master/kernel-qemu-4.4.34-jessie?raw=true
Download the image (raspbian jessie lite) here: https://www.raspberrypi.org/downloads/raspbian/

Copy the command below into a text editor and name the file config

#!/bin/bash
# Starts raspberry pi image in configuration mode

qemu-system-arm -kernel ./kernel-qemu-4.4.34-jessie -cpu arm1176 -m 256 -M versatilepb -no-reboot -serial stdio -append "root=/dev/sda2 panic=1 rootfstype=ext4 rw init=/bin/bash" -hda rpi.img

Make the script executable using chmod +x config and run it using ./config. This launches QEMU raspberry pi in bash terminal mode.

Now in QEMU, use nano /etc/ld.so.preload to edit the file and comment out the only line using #. (i.e #/usr/lib/arm-linux-gnueabihf/libarmmem.so) Use CTRL+O and Enter to save the changes to the file and CTRL+X to exit.

Since the actual Pi system uses SD Card but QEMU does not have that (we defined the root drive to be sda2 as seen in the config script), we have to redirect calls to the SD Card to sda2. To do that we need to define rules for the symbolic link.

In QEMU, use nano /etc/udev/rules.d/90-qemu.rules to create a new rule file. In the nano editor, type in the following:

KERNEL=="sda", SYMLINK+="mmcblk0"
KERNEL=="sda?", SYMLINK+="mmcblk0p%n"
KERNEL=="sda2", SYMLINK+="root"

Use the following script to launch QEMU pi with raspbian OS

#!/bin/bash
# Start the Raspberry Pi in fully functional mode!
 
qemu-system-arm -kernel ./qemu-rpi-kernel/kernel-qemu -cpu arm1176 -m 256 -M versatilepb -no-reboot -serial stdio -append "root=/dev/sda2 panic=1 rootfstype=ext4 rw" -hda rpi.img

Username: pi
Password: raspberry

Reference: http://gnutoolchains.com/raspberry/tutorial/

Install the GCC compiler toolchain for ARM linux using sudo apt-get install gcc-6-arm-linux-gnueabi

Save the following code as helloworld.c

#include <stdio.h>

int main()
{
    printf("Hello world!\n");
    return 0;
}

Compile the code using arm-linux-gnueabi-gcc-6 helloworld.c -o test

Reference: https://www.youtube.com/watch?v=Hj7UWfhv7Gc

Now we need to transfer this test file to the QEMU pi. But first we need to make changes to the run script

#!/bin/bash
# Start the Raspberry Pi in fully functional mode!

qemu-system-arm -kernel ./kernel-qemu-4.4.34-jessie -cpu arm1176 -m 256 -M versatilepb -no-reboot -serial stdio -append "root=/dev/sda2 panic=1 rootfstype=ext4 rw" -redir tcp:2222::22 -hda rpi.img

The difference between this and the old script is additional the -redir tcp:2222::22, which fowards port 2222 of the host machine to port 22 of the virtual machine (QEMU pi). This allows us to call port 2222 (using SCP) on the local machine to make transfers to the QEMU pi.

Use sudo raspi-config to bring up the raspberry configuration menu

Follow the screenshots to enable SSH:

Press ESC to exit the config menu

The following command transfers the test file to the /home/test directory in pi

scp -P 2222 test pi@localhost:/home/pi/test

Inside QEMU pi test directory, use chmod +x test to make the test file executable. Run it using ./test to see Hello world! printed