Cross compiling and sysroot

Question

I've been following instructions to cross compile code for the Raspberry Pi but I need some clarification regarding the tool chain and sysroot.

My setup is as follows:
Host: 4.15.0-76-generic x86_64
Target Pi: 4.19.93+ armv6l

I created a directory and downloaded the toolchain as follows:

$ mkdir rpi-cross
$ cd rpi-cross
$ git clone https://github.com/raspberrypi/tools

Within rpi-cross I created a sysroot directory and then used rsync to copy files from the target.

$ mkdir sysroot
$ rsync -avz pi@raspberrypi_ip:/lib sysroot
$ rsync -avz pi@raspberrypi_ip:/usr/include sysroot/usr
$ rsync -avz pi@raspberrypi_ip:/usr/lib sysroot/usr

I also repaired any absolute symlinks with relative ones as follows

$ wget https://raw.githubusercontent.com/riscv/riscv-poky/master/scripts/sysroot-relativelinks.py
$ chmod +x sysroot-relativelinks.py
$ ./sysroot-relativelinks.py sysroot

I then exported the path to the toolchain and sysroot

export CROSS_COMPILE=/home/j/rpi-cross/tools/arm-bcm2708/arm-linux-gnueabihf/bin/arm-linux-gnueabihf-
export SYSROOT=/home/j/rpi-cross/sysroot

To test this I then created a simple hello-world application with the following make file

CC=$(CROSS_COMPILE)gcc

hello-world : main.c
    $(CC) --sysroot=$(SYSROOT) -march=armv6 -mfpu=vfp -mfloat-abi=hard -o hello-world main.c

But I got the following output:

In file included from main.c:1:0:
/home/j/rpi-cross/sysroot/usr/include/stdio.h:27:36: fatal error: bits/libc-header-start.h: No such file or directory
 #include <bits/libc-header-start.h>

As an additional test I made an even simpler main.c

//#include <stdio.h>
int main (int argc, char ** argv)
{
//    printf("Hello, world \r\n");
    return 0;
}

Which gave a different set of errors

/home/j/rpi-cross/tools/arm-bcm2708/arm-rpi-4.9.3-linux-gnueabihf/bin/../lib/gcc/arm-linux-gnueabihf/4.9.3/../../../../arm-linux-gnueabihf/bin/ld: cannot find crt1.o: No such file or directory
/home/j/rpi-cross/tools/arm-bcm2708/arm-rpi-4.9.3-linux-gnueabihf/bin/../lib/gcc/arm-linux-gnueabihf/4.9.3/../../../../arm-linux-gnueabihf/bin/ld: cannot find crti.o: No such file or directory

I then edited the makefile as follows, removing the sysroot option, and put the call to printf back into main.c.

CC=$(CROSS_COMPILE)gcc

hello-world : main.c
    $(CC) -march=armv6 -mfpu=vfp -mfloat-abi=hard -o hello-world main.c 

At which point the build was succesful and after copying the application onto the target I was able to run it.

$ file hello-world 
hello-world: ELF 32-bit LSB executable, ARM, EABI5 version 1 (SYSV), dynamically linked, interpreter /lib/ld-, for GNU/Linux 2.6.32, not stripped

Further investigation reveals that there is a sysroot folder under /home/james/rpi-cross/tools/arm-bcm2708/arm-rpi-4.9.3-linux-gnueabihf/arm-linux-gnueabihf/ which I assume the toolchain uses by default. This brings me to the crux of the question; what is wrong with the sysroot that I created by copying files from my target device? Is the best approch to use the sysroot that comes as part of the toolchain?

Answer 1

I can't answer your question, but I've been trying to do the same thing, and removing the SYSROOT setting (despite everything else I've read saying it was needed) was the key I needed. I can now compile for pi using just the pi tools from github, using WSL from Visual Studio. This is my toolchain file:

# Define our host system
SET(CMAKE_SYSTEM_NAME Linux)
set(CMAKE_SYSTEM_PROCESSOR arm)
# Define the cross compiler locations
set(tools /home/david/raspi/tools/arm-bcm2708/gcc-linaro-arm-linux-gnueabihf-raspbian-x64)
set(CMAKE_C_COMPILER ${tools}/bin/arm-linux-gnueabihf-gcc)
set(CMAKE_CXX_COMPILER ${tools}/bin/arm-linux-gnueabihf-g++)
set(CMAKE_STAGING_PREFIX /home/david/stage)

# Don't mess with the sysroot, despite every post saying you should
#set(CMAKE_SYSROOT ${tools}/arm-linux-gnueabihf)

# Use our definitions for compiler tools
SET(CMAKE_FIND_ROOT_PATH_MODE_PROGRAM NEVER)
# Search for libraries and headers in the target directories only
SET(CMAKE_FIND_ROOT_PATH_MODE_LIBRARY ONLY)
SET(CMAKE_FIND_ROOT_PATH_MODE_INCLUDE ONLY)
set(CMAKE_FIND_ROOT_PATH_MODE_PACKAGE ONLY)