From: Arnd Bergmann

Sent: 03 May 2021 10:25

...

One scenario that I've seen previously is where user space and
kernel are built together as a source based distribution (OE, buildroot,
openwrt, ...), and the compiler is picked to match the original sources
of the user space because that is best tested, but the same compiler
then gets used to build the kernel as well because that is the default
in the build environment.

If you are building programs for release to customers who might
be running then on old distributions then you need a system with
the original userspace headers and almost certainly a similar
vintage compiler.
Never mind RHEL7 we have customers running RHEL6.
(We've managed to get everyone off RHEL5.)
So the build machine is running a 10+ year old distro.

I did try to build on a newer system (only 5 years old)
but the complete fubar of memcpy() makes it impossible
to compile C programs that will run on an older libc.
And don't even mention C++, the 'character traits' is just
plain horrid - enough to make me want to remove every
reference to CString from the small amount of C++ we have.

To quote our makefile:
# C++ is fighting back.
# I'd like to be able to compile on a 'new' system and still be able to run
# the binaries on RHEL 6 (2.6.32 kernel 2011 era libraries).
# But even linking libstdc++ static still leaves
# an undefined C++ symbol that the dynamic loader barfs on.
# The static libstdc++ also references memcpy@GLIBC_2.14 - but that can be
# 'solved' by adding an extra .so that defines the symbol (and calls memmove()).
# I've also tried pulling a single .o out of libstc++.a. This might work if
# the .o is small and self contained.
#
# For now we statically link libstc++ and continue to build on an old system.
C++LDLIBS := -Wl,-Bstatic -lstdc++ -Wl,-Bdynamic

It would be nice to be able to build current kernels (for local
use) on the 'new' system - but gcc is already too old.

	David

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