Clang compiler couldn't find C/C++ standard libraries when I gave a specific target although it works without giving target

Question

I am currently using clang11 on ubuntu to compile any c/c++ code and it works fine but when I tried to compile any code (including any standard library) to assembly code for any specific target like x86_64 (even I have x86_64) riscv with giving a flag that --target=x86_64 or --target=riscv32 I got errors for any standard library that I included in my code. A simple example:

// ex.cpp
#include<iostream>
int main(){
    int a = 5;
    int b = 3;
    std::cout << a - b;
}

Without giving flag for a spesific target works fine:

clang++-11 -S ex.cpp -o ex.s

With --target=riscv32 flag:

clang++-11 --target=riscv32 -S ex.cpp -o ex.s

gives this error: ex.cpp:1:9: fatal error: 'iostream' file not found

also without standard libraries gives no error even I give a spesific target.

I am searching for a solution for days but I couldn't find any proper solution for this problem, most of them says try to include gnu libraries and subfolders like -I/usr/include/x86_64-linux-gnu/c++/ but it doesn't work for me.

Please don't say use g++ compiler, for adding an optimization I need clang.

Actually I am trying to compile my codes for riscv target, linking with g++ and running with spike (doesn't differ --target=... or -target ...):

clang++-11 -target riscv32-unknown-elf -march=rv32gc -fno-addrsig -S ex.cpp -o ex.s
~/riscv/bin/riscv32-unknown-elf-g++ ex.s -o ex
~/riscv/riscv-isa-sim/build/spike --isa=RV32GC ~/riscv/riscv-pk/build/pk ex

And it works fine without include a standard library. Now, I want to ask that

Can I solve this problem simply?

or

Can I use clang directly from riscv bin utils like ~/riscv/bin/riscv32-unknown-elf-clang++ (I saw something like this on the net but couldn't find) adding and building a submodule to my riscv directory?

Edit: As @NateEldredge said, for x86_64 target triple should --target=x86_64-linux-gnu but for riscv as a target triple riscv32-unknown-elf I still have the same errors. Is there a proper target flag for riscv any other than --target=riscv32-unknown-elf? Maybe I am missing that point.

Testa

Are you compiling as C or C++? Is your code C or C++? They are distinct languages. For example, C++ has std::string and std::vector, which would be different libraries than C. Please update your tags and question about which language you are using. – Florineflorio

You probably need to install heaers and libraries for the other targets – Leadbelly

@Leadbelly How can I install, especially for riscv? Also, as I said even I give x86_64 for target I get same error although I am working on x86_64 and also I can compile without no error without no target flag. – Testa

@ThomasMatthews Sorry but as you can see, I just gave an example for C++, this is a general problem for me even if compiler is clang (for C) or clang++ (for C++) because of that I am asking general. For example, if I try to compile a C code with stdio.h library I get same error like fatal error: 'stdio.h' file not found and so forth for any other library. – Testa

Shouldn't the argument to --target be a target triplet, not just an architecture? So --target=x86_64-linux-gnu instead of just x86_64. Running clang -v should show you what the default target triplet is. – Hoeve

More generally, you can use the verbosity flag (-v), to see what libraries and include paths are implicitly used. – Turmoil

@NateEldredge Right, I will edit my post, missed that thing target triple should --target=x86_64-linux-gnu and it works but it is weird that without including a library, codes are compiling with just like --target=x86_64 but this is not the case :) Actually I need specify a target because I don't use on my x86, I need for riscv and I tried for riscv as a target triple riscv32-unknown-elf or riscv32-none-elf and also tried for arm like arm-none-eabi but noone works. Probably a proper target triple will work but couldn't find for riscv just find riscv32-unknown-elf that doesnt work. – Testa

How did you build and install your riscv32 cross compiler? Using --target with the installed clang binary will search for its files in a specific place, which maybe is not where you have them installed. – Hoeve

OT but quite irritating: a header file is not a library. It may define parts or all of the public API of one or more libraries, but not necessarily. It can even be empty. -- A library might be needed later when linking, but you are just compiling. Please get used to the proper terms. – Bulletproof

@NateEldredge Actually I didn't build a cross compiler firstly, just built llvm-project with -DLLVM_TARGETS_TO_BUILD=all option and used clang compiler from this built, because after llvm 9 i know llvm support came for riscv target. (All supported targets can be seen here.) I mean as llvm promised I should have been able to used clang for all targets with llvm in built but didn't work for me. – Testa

I just solved my problem cross compilation with riscv-llvm repo which has not updated for a long time and is not official for now. I didn't understand that why llvm has a support for riscv but I couldn't use its clang compiler for this spesific target, why I need a cross compilation or if I need that how I can cross compile riscv gnu toolchain with official llvm. – Testa

Answer 1

I solved my problem by linking compilations with riscv-gnu-toolchain built and also answered a similar question here in detailed: Using Clang to compile for RISC-V

Simply we need cross-compilation. Further information you can also look here: https://github.com/lowRISC/riscv-llvm#how-can-i-build-upstream-llvmclang-and-use-it-to-cross-compile-for-a-riscv32-target

Testa