clang cross compilation command can not find head files - c++

Question

I'm trying simple cross compilation with clang 10.0 followed the official tutorial, with the option --target=<triple>.
My environment is on 64-bit ubuntu16.04, clang10.0.
My source file is just the a.cpp file:
#include <iostream>
int main() {
std::cout << "hello world" << std::endl;
return 0;
}
If I comile the source for the host machine with clang++ --target=x86_64-unknown-linux-gnu a.cpp, it worked well. But if I use clang++ --target=i386-unknown-linux-gnu a.cpp for 32-bit machine, or clang++ --target=arm-unknown-linux-eabi a.cpp, it got the same errors:
a.cpp:18:10: fatal error: 'iostream' file not found
#include <iostream>
^~~~~~~~~~
1 error generated.
My question is:
1) Whether my compilation command is wrong?
2) What should the host machine prepare for cross compilation to other target machine?
Any answer would be much appreciated!

Answer 1

Apart from the compiler you need the sysroot, where all the header files and runtime libraries are present, to have successful compilation. If the compiler can't find the path to sysroot, it wont know where to find standard library headers etc.
If your system doesn't have a i386 toolchain then you need to download the compiler toolchain that will have the sysroot. e.g. from here: https://github.com/nativeos/i386-elf-toolchain/releases.
clang++ --target=i386-unknown-linux-gnu --sysroot /path/to/sysroot a.cpp
Similarly for arm you can download the toolchain from: https://launchpad.net/gcc-arm-embedded/+download
To learn more about toolchains and sysroot please read: https://elinux.org/Toolchains