On arm linux sys/ucontext.h heavilly polloutes the global namespace firstly by including sys/user.h (which defines among other things a type called "struct user" and secondly by defining symbols and #defines named R? to represent the processor registers.
That issue in itself is nothing new but fairly recently* signal.h started including sys/ucontext.h by default causing programs that (quite reasonable IMO) used those names for their own symbols to fail to build on arm. This has been especially noticable recently because debian is trying to build the new "armhf" port.
After discussions on debian-arm/debian-glibc/linaro-dev (I do not know which responders came from which list) I was given the following advice on the "struct user" issue by Ulrich Weigand.
Now, glibc also provides a file <sys/ucontext.h> that defines layouts of register sets for use with signal handlers as well as the makecontext/setcontext/getcontext family of routines.
Usually, those layouts tend to be in fact identical to the layouts described in <sys/user.h> / <sys/procfs.h>. Apparently, whoever implemented the ARM version of <sys/ucontext.h> was trying to avoid some seemingly unnecessary code duplication by making <sys/ucontext.h> *include* <sys/procfs.h> and using the information from there directly. This is not done on other platforms, for precisely the reason that the <sys/procfs.h> and <sys/user.h> headers do pollute the name space ...
So I think the right thing to do in the short term would be to stop <sys/ucontext.h> including <sys/procfs.h>, and instead add the register set information there directly, even if that means some duplication of code. (Again, since this is never-changing ABI, duplication isn't actually all that bad. Also, all the other platforms do it that way too, so why should ARM be different ...)
On the issue of the R? definitions I proposed renaming them to REG_R?. The use of a REG_ prefix is consistent with x86, x64 and sparc (I couldn't find any comparable definitions at all on other architectures I looked at) I asked what the impact of this change would be on the aforementioned mailing lists and got the following reply from Konstantinos Margaritis
at worst the packages that had to be workaround on arm* for this, can have the workaround removed.
The attached patch implements these changes.
My tests do not show any new failures in the libc testsuite (though I do get failures that debian considers "unexpected" regardless of whether this patch is applied or not)
* I have not investigated exactly when this change occoured but it was somewhere between the version in debian squeeze and the version in debian wheezy.