Re: x86 crosscompile

[Ralf Corsepius]

On Wed, 2004-05-19 at 23:02, Marcus Furlong wrote:
> Hi,
> 
> I've seen conflicting information about what configure flags to use when 
> cross-compiling for a lower x86 architecture on an x86.

Yes, because there exist different and partially inconsistent
conventions and .. because many packages are not ready for
cross-compilation.

>  For example 
> cross-compiling for a i486 on an i686. Not even sure if this should be 
> called crosscompiling?
It's a grey zone :)

> For most packages you just need to set CFLAGS or similar to include -march 
> for the lower platform, but for some packages setting --host= and --target= 
> is needed, especially gcc, glibc and binutils.
> 
> see 
> http://www.linuxfromscratch.org/hints/downloads/files/crosscompiling-x86.txt
> for examples.
Well, this document is questionable in details. In particular, it
generalizes too much.

In most cases you will have to inspect a package's configure script
thoroughly, before trying to cross-build it.

> This gives a warning message at the start of each compilation because a 
> crosscompiler is not detected, and I was wondering if --build= is the 
> appropriate arg to use instead (seeing as we're already probably already 
> lying about the build architecture via uname).
> 
> e.g. building for an i486 on a i686
> is the following correct?
This question can't be answered in general. 
It depends on the package:

* build .. the system you are building on.
* host .. the system the binaries your are building are supposed to run
on
* target .. the system the binaries running on "host" are targeting.

> ./configure --build=i486-pc-linux-gnu

Normally you don't want to set --build.

With autoconf-2.5x generated configure scripts, to trigger
cross-compilation (Which in autoconf terms means to apply a specialized
cross-toolchain), you almost always will want to use
configure --host=i486-pc-linux-gnu

However, many packages, esp. those using autoconf-2.13 use all kinds of
"weird" custom conventions on processing "host/build/target", some even
expect you to provide special settings on environment variables etc.

> instead of
> 
> ./configure --host=i486-pc-linux-gnu --target=i486-pc-linux-gnu
Cf. above. 

For complex packages which are prepared for Canadian Cross Compilation
(gcc/binutils/gdb/glibc etc), this can be right.

In most cases,  
configure --target=i486-pc-linux-gnu
will be what you probably want (This is what gcc, binutils, gdb, newlib
expect. I am not familiar enough with glibc to comment on it).

[You are building packages on an i686 (build), which shall contain
binaries to run on an i686 (host), which shall generate code for i486s
(target).]

Ralf