Cross Development

Native Compilation

  • Compiler runs on a machine, say x86_64

  • Produces code for that machine

  • Uses header files and libraries from that machine

  • Default paths for headers and libraries

    • /usr/include: e.g. #include <stdio.h> is found as /usr/include/stdio.h

    • /usr/lib, /lib64, …: e.g. -lpthread (“linker, please go find me the pthread library”) is resolved to /lib64/libpthread.so

Cross?

  • Compiler runs on host machine (e.g. x86_64)

  • Produces code for the target machine (e.g. armv7l)

Simple? ⟶ No

  • Code must be built against target situation

  • Header files and libraries

  • ⟶ must be available on host

    • ⟶ sysroot

  • Must match the target situation ⟶ binary compatibility

  • Statically linked cross build generally ok

  • But: shared libraries are a continuous source of pain

Challenges

  • Cross compiler: finding or building one that works

    • Trial and error, mostly

    • Building one: crosstool-ng

    • Using a metadistribution like Yocto: give you a cross toolchain as by-product of creating the target rootfilesystem

  • sysroot

    • Target root filesystem, in principle

    • Ideally stripped down to a miniumum

      • Don’t need target executables on the host, for example (they won’t run)

    • Ideally a by-product of root filesystem creation

The sysroot

  • The situation on the target, made available on the host

  • Usually a single tree, containing a subset of the target’s root filesystem

One of a number of sysroots; on the host
$ ls -l ~/cross/sysroots/raspberry/
total 0
drwxrwxr-x. 1 jfasch jfasch 38 Mar 29 19:00 lib
drwxrwxr-x. 1 jfasch jfasch 20 Mar 29 18:59 usr

  • Easy solution: use sshfs to mount the target root directory (that does not scale up to reality though)