clang/llvm compile fatal error: 'cstdarg' file not found

Question

Trying to convert a large gcc/makefile project into clang. Got it roughly working for x86, but now I'm trying to get cross compilation working.

The way it currently works is that we use Linaro's 7.1.1 arm compiler alongside its companion sysroot directory for base libraries/headers. I installed clang-6.0 and then the base clang(not sure if that mattered). I used some commands I found to redirect clang to clang-6.0 and when I execute 'clang -v' and got

clang version 6.0.0-1ubuntu2~16.04.1 (tags/RELEASE_600/final)
Target: x86_64-pc-linux-gnu
Thread model: posix
InstalledDir: /usr/bin
....
Found candidate GCC installation: /usr/bin/../lib/gcc/i686-linux-gnu/9
....
Found candidate GCC installation: /usr/lib/gcc/i686-linux-gnu/6.5.0
....
Selected GCC installation: /usr/bin/../lib/gcc/x86_64-linux-gnu/7.4.0
Candidate multilib: .;@m64
Selected multilib: .;@m64

It does not find the current compiler we use which is at

/usr/local/gcc-linaro-7.1.1-2017.08-i686_arm-linux-gnueabihf/bin/arm-linux-gnueabihf-g++(also a directory for *x86_64*)

I only found references to setting --sysroot, but not to a specific compiler. Definitely still lost about the relationship between clang+llvm+other compilers. I even saw somewhere saying I needed to compile llvm before I could use it?

I very roughly made changes in our make files to get the following output, basically all I had to add was '-target arm-linux-gnueabuhf' and reordered the mcpu/mfloat/marm/march so they came after -target in case it mattered

clang --sysroot=/usr/local/sysroot-glibc-linaro-2.25-2017.08-arm-linux-gnueabihf  -c -std=c++0x 
-g -DDEBUG_ON -target arm-linux-gnueabihf -mcpu=cortex-a7 -mfloat-abi=hard -marm -march=armv7ve
-Wall -fexceptions -fdiagnostics-show-option -Werror .... -I/usr/local/gcc-linaro-7.1.1-2017.08-x86_64_arm-linux-gnueabihf/arm-linux-gnueabihf/include .... and many more

I think the problem probably lies with the change I made which is the actual 'clang' call that replaced

/usr/local/gcc-linaro-7.1.1-2017.08-i686_arm-linux-gnueabihf/bin/arm-linux-gnueabihf-g++ ....

End up with

fatal error: 'cstdarg' file not found
#include <cstdarg>

As said before I can already cross-compile with gcc, so I've already come across issues with std libraries that require 'build-essentials', 'g++-multilibs', etc. So they're already installed.

Looked and really haven't found anything too useful to me, I'm on linux mint 18.3 and the closest things I found were issues people had on mac and windows.

Answer 1

So I came across some posts mentioning setting --gcc-toolchain=/your/choice/of/cross/compiler but they also mention it not working. I discovered that if you combine this with the installation of llvm-6.0-dev(or maybe llvm-6.0-tools, tools installed dev so not 100%) it at least worked for me.

Answer 2

Any compiler clang or gcc needs to know where a header file is defined. The standard headers, standard libraries, c-runtime, and libc are all packaged together for each target e.g., arm64, x86 in a directory called 'sysroot'. When we compile a program we need to pass the path to sysroot for a compiler to know where to look for standard headers during compilation, and where to look for common libraries (libc, libstdc++ etc) during linkage.

Normally when we compile a program for the same machine the compiler uses the standard headers available in '/usr/include' and libraries from '/usr/lib'. When cross-compiling programs we should supply the sysroot as compiler flag. e.g. gcc --sysroot="/path/to/arm64/sysroot/usr" test.cpp. Same for clang. Most often pre-packaged cross compilers come with a script/binary that has 'sysroot' path embedded into it. e.g., aarch64-linux-gnu-gcc (https://packages.ubuntu.com/xenial/devel/gcc-aarch64-linux-gnu).

... the closest things I found were issues people had on mac and windows.

On mac the clang compiler will have the similar configuration as linux. So the details you found there should be totally applicable to yours.

More details on sysroot and cross-compilation:

https://elinux.org/images/1/15/Anatomy_of_Cross-Compilation_Toolchains.pdf