Question about std::string

[PhilDay-CT]

This probably shows the depth of my ignorance about the compiler system, and happy to move it to a discussion group instead if that's more appropriate.

I was starting work on an example of how we might do dynamic configuration of compartments, and hit what I'm sure is noob issue with std:;string

If I change the examples/01.hello_world/hello.cc to try an use std::string

// Copyright Microsoft and CHERIoT Contributors.
// SPDX-License-Identifier: MIT

#include <compartment.h>
#include <debug.hh>
#include <fail-simulator-on-error.h>
#include <string>

/// Expose debugging features unconditionally for this compartment.
using Debug = ConditionalDebug<true, "Hello world compartment">;

/// Thread entry point.
void __cheri_compartment("hello") say_hello()
{
	// Print hello world, along with the compartment's name to the default UART.
	std::string msg = "Hello world";
	Debug::log("{}", msg);
}

xmake gives:

error: /workspaces/cheriot-rtos/sdk/include/debug.hh:388:4: error: implicit instantiation of undefined template 'DebugFormatArgumentAdaptor<std::string>'
          DebugFormatArgumentAdaptor<
...

If I remove the call to Debug so I just have the definition

    std::string msg = "Hello world";

xmake fails with:

error: ld.lld: error: undefined symbol: strlen
>>> referenced by hello.cc:14
>>>               build/cheriot/cheriot/release/hello.compartment:()

compile_commands.json includes the following - that I assume should be picking up strlen ?

{
  "directory": "/workspaces/cheriot-rtos/examples/01.hello_world",
  "arguments": ["/cheriot-tools/bin/clang", "-c", "-std=c2x", "-Qunused-arguments", "-target", "riscv32-unknown-unknown", "-mcpu=cheriot", "-mabi=cheriot", "-mxcheri-rvc", "-mrelax", "-fshort-wchar", "-nostdinc", "-Oz", "-g", "-ffunction-sections", "-fdata-sections", "-fomit-frame-pointer", "-fno-builtin", "-fno-exceptions", "-fno-asynchronous-unwind-tables", "-fno-c++-static-destructors", "-fno-rtti", "-Werror", "-I/workspaces/cheriot-rtos/sdk/include/c++-config", "-I/workspaces/cheriot-rtos/sdk/include/libc++", "-I/workspaces/cheriot-rtos/sdk/include", "-fvisibility=hidden", "-DNDEBUG", "-o", "build/.objs/string/cheriot/cheriot/release/__/__/sdk/lib/string/strlen.c.o", "../../sdk/lib/string/strlen.c"],
  "file": "../../sdk/lib/string/strlen.c"
},

Feels like I'm missing something obvious about using std::string ?

[davidchisnall]

Hi, the strlen function is provided by the string library.

We pay almost no penalty from splitting things into different libraries and so we’ve split the traditional libc up quite a lot, so you can build small, firmware images with only the subset that you need.

We should add an overload for the debug adaptor (patches welcome!) that forwards to the std::string_view specialisation.

Putting add_deps(“string”) in the firmware bit of the xmake.lua file should make the string bit work.

[davidchisnall]

It looks as if there’s also a compiler bug. The strlen is being lowered as a compiler intrinsic but is then not having the correct calling convention applied when it’s expanded.

[davidchisnall]

Tracked here:

CHERIoT-Platform/llvm-project#29

I am out this week, but will try to fix next week if no one gets there first.

[PhilDay-CT]

It's not urgent - don't let it get in the way of your holiday

[davidchisnall]

As of #260, std::string is expected to work.