Compiling as 32bit on MacOSX

[Ned Deily]

In article <8hpgn7FhomU1 at mid.individual.net>,
 Gregory Ewing <greg.ewing at canterbury.ac.nz> wrote:

> Ned Deily wrote:
> > Perhaps you're 
> > calling ld(1) directly?  To link multiple-arch executables (etc), the 
> > Apple gcc driver does the dirty work of calling ld multiple times and 
> > lipo-ing the results.
> 
> Is this something that only works at link time, then? The
> gcc man page says:
> 
>    "Multiple options work, and
>    direct the compiler to produce "universal" binaries including
>    object code for each architecture specified with -arch."
> 
>  From this I was hoping to be able to do
> 
>     gcc -arch i386 -arch x86_64 -c foo.c
> 
> and get dual-architecture .o files that could then be linked
> into dual-architecture libraries. But if I do the above, I
> just get an x86_64 .o file.
> 
> Are you saying that I need to compile separate sets of .o
> files and then combine them at link time? That sounds like
> an awkward thing to retrofit onto a library's existing
> build system.

No, it's supported both at compile and link time.  Here's a compile on 
10.6:

$ /usr/bin/gcc --version
i686-apple-darwin10-gcc-4.2.1 (GCC) 4.2.1 (Apple Inc. build 5664)
Copyright (C) 2007 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is 
NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR 
PURPOSE.

$ /usr/bin/gcc -arch i386 -arch x86_64 -c foo.c
$ file foo.o
foo.o: Mach-O universal binary with 2 architectures
foo.o (for architecture i386):   Mach-O object i386
foo.o (for architecture x86_64): Mach-O 64-bit object x86_64

BTW, if, for some reason, you *do* need to extract a particular 
architecture from a multi-arch file, you can use lipo to do it:

$ lipo foo.o -output foo_32.o -extract i386
$ lipo foo.o -output foo_64.o -extract x86_64
$ file foo_32.o
foo_32.o: Mach-O universal binary with 1 architecture
foo_32.o (for architecture i386):   Mach-O object i386
$ file foo_64.o
foo_64.o: Mach-O universal binary with 1 architecture
foo_64.o (for architecture x86_64): Mach-O 64-bit object x86_64

-- 
 Ned Deily,
 nad at acm.org