Re: Cross Compile Process

["Yann E. MORIN" <yann dot morin dot 1998 at anciens dot enib dot fr>]

Xun, All,

On Friday 01 April 2011 035611 Xun Li wrote:
> I have been using crosstoll-NG for a while and it is a great tool!
> Now I am trying to understand the internals of crosstool about how it
> actually works so that I will be able to contribute and help others in
> the future.
> I looked into the script and the steps but I was getting confused
> about the complicated process of building a cross compiler.
> Are there any good documents explaining this well or would enable me
> better understand how gcc actually get built?

It's not yet in the released versions, but there is a tentative tutorial
in the docs/ directory. You can look at it on-line at:
  http://ymorin.is-a-geek.org/hg/crosstool-ng/file/tip/docs/9%20-%20How%20is%20a%20toolchain%20constructed.txt

Or you can clone the repository and look locally:
  hg clone http://ymorin.is-a-geek.org/hg/crosstool-ng

> Specifically, I am confused at the following questions:
> 1. Why does it need to fetch the linux kernel? I searched around about
> how to build a cross compiler, the methodologies are all different.

It's explained in the above doc.

> Some need to get the linux-libc headers, some don't need to get
> anything.

The doc is still not complete. You need the kernel headers iff you are
targetting a Linux kernel based system. For bare-metal (with or without
newlib), you do not need the Linux kernel headers, as you are not using
Linux on the target.

For Mingw targets, you do not need the Mingw 'kernel' headers, as those
are not a moving target.

> 2. What is the major difference of using newlib instead of glibc?

The glibc, eglibc and uClibc C libraries all target Linux-based systems.
glibc and its eglibc fork can also target Hurd, Solaris etc...

Newlib is meant to be run on bare-metal systems, where there is no
kernel at all (eg. a bootloader).

So the choice is not between glibc/eglibc/uClibc on one side, and newlib
on the other side. It rather depends on the target system: Linux kernel,
or bare-metal.

Note: you can also build a bare-metal toolchain that does not have newlib
at all. Then, your programs will have to provide everything.

> 3. Why building gcc needs to be split into different stages?

It's in the above doc.

> 3. If I am to configure the cross compiler to enable soft-float, where
> exactly do crosstool set the flags?

Not really documented. You can search for 'CT_DoBuildTargetTuple' in
scripts/functions

Regards,
Yann E. MORIN.

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