Embedded C++ Cookbook

WIP

Cheatsheet for working with C++ in an embedded bare-metal environment.

• Intro
• CPUs
  • ARM
    • ARM Architecture Flags
  • AVR
    • AVR Architecture Flags
  • RISCV
• Hosted vs. Freestanding
• Compilers
  • GCC
  • Clang
  • Language Flags
    • fno-exceptions
    • fno-rtti
  • Optimization Flags
  • Debug Flags
  • Warning Flags
• Assembler
  • Assembler Flags
• Linker
  • Linker Flags
• Tools
  • ar
  • nm
  • objcopy
  • objdump
  • ranlib
  • readelf
  • size
  • strings
  • strip
• Build System
  • Makefile vs. CMake
  • Makefile
  • CMake
• Libraries
• IDE
• Debugging
  • JTAG / SWD
  • GDB
  • QEMU
• Examples

Intro

CPUs

• No MMU (most of the time)

ARM

• 32 Bit
• Cortex-M Series

Numbers for Frequency, Flash & RAM where taken from the STM product lineup. Others manufacturers may vary, but will probably have a similar offering.

Feature	M0/M0+	M3	M4	M7
Microarchitecture	ARMv6-M	ARMv7-M	ARMv7E-M	ARMv7E-M
Interrupts	1-32	1-240	1-240	1-240
Frequency	32-64 Mhz	24-120 Mhz	48-180 Mhz	200-600 Mhz
Flash	8-256 kBytes	16-1024 kBytes	16-2048 kBytes	64-2048 kBytes
RAM	2-36 kBytes	4-128 kBytes	16-640 kBytes	128-1024 kBytes
Floating Point (single)	❌	❌	✔️ (optional)	✔️ (optional)
Floating Point (double)	❌	❌	❌	✔️ (optional)
STM Series	L0, G0	F1, F2, L1	F3, F4, L4	F7, H7

ARM Architecture Flags

For the complete list see: GCC manual

Flag	Description	gcc	clang
-mcpu=	This specifies the name of the target ARM processor.	✔️	✔️
-mthumb	Use thumb instructions.	✔️	✔️
-mfloat-abi	Specifies which floating-point ABI to use.	✔️	✔️

AVR

• 8 Bit
• Arduino

AVR Architecture Flags

Flag	Description	gcc	clang
-mmcu=	Select the MCU chip. See GCC manual	✔️	✔️
-mint8	Assume int to be 8-bit integer. Not standard! See GCC manual	✔️	❌

RISCV

• 32 Bit
• Open Source ISA

Hosted vs. Freestanding

• Headers
• Exceptions
• RTTI

Compilers

GCC

Clang

Language Flags

Flag	Description	gcc	clang
-std=c++17	Set the language standard for C++. Possible values: 98, 11, 14, 17, 20	✔️	✔️
-std=gnu++17	Same as above, but with GNU extensions enabled.	✔️	✔️
-ffreestanding	Enables freestanding C/C++. Default is hosted.	✔️	✔️
-fno-exceptions	See fno-exceptions.	✔️	✔️
-fno-rtti	See fno-rtti.	✔️	✔️
-nostdinc++	Do not search for header files in the standard directories specific to C++.	✔️	✔️

fno-exceptions

Disable exception handling. See GCC manual for more details.

fno-rtti

Disable generation of information about every class with virtual functions for use by the C++ run-time type identification features dynamic_cast and typeid. Can save space if not needed. Mixing code compiled with -frtti with that compiled with -fno-rtti may not work.

Optimization Flags

Flag	Description	gcc	clang
-O	Sets optimization level. Possible values: 0,1,2,3,s,g & z on clang.	✔️	✔️
-flto	Enables link time optimization.	✔️	✔️
-fno-unroll-loops	Disable loop unrolling. Can save space.	✔️	✔️

Debug Flags

Flag	Description	gcc	clang
-Og	Enable optimization level which is tuned for best debuggability while still performing OK.	✔️	✔️
-g	Include debug symbols.	✔️	✔️
-ggdb		✔️	✔️

Warning Flags

Flag	Description	gcc	clang
-Wall	Basic warnings. Should always be used.	✔️	✔️
-Wextra	Everything from -Wall plus more.	✔️	✔️
-Wpedantic	Issue all the warnings demanded by strict ISO C and ISO C++.	✔️	✔️
-Werror	Make all warnings into errors.	✔️	✔️

Assembler

Assembler Flags

Flag	Description	gcc	clang
-x assembler-with-cpp	Enables the C preprocessor for assembly files.	✔️	✔️

Linker

Linker Flags

When using g++ or clang++ as the linker front end prefix all flags with -Wl. Example: -Wl,--no-undefined

Flag	Description	gcc	clang
-Tpath/to/linkerscript	Path to the location of the linker script	✔️	✔️
-fstack-usage		✔️	❌
--print-memory-usage		✔️	❌
-static		✔️	✔️

Tools

ar

nm

objcopy

objdump

ranlib

readelf

size

strings

strip

Build System

In this section will take a detailed look at a couple of build system, that can be used for embedded bare-metal development. The two most common right now are Makefile or CMake projects, so let's start with those.

Makefile vs. CMake

First a quick comparison:

Makefile	CMake
Easy initial setup	The first CMake toolchain file can be a little tricky.
Portable across Unix-like development hosts	Portable across all development hosts (Linux,macOS,Unix,Windows)
Clear output showing the set flags & options	Default output does not show direct compiler invocation
	Easy management of multiple executables & libraries (shared code)
	Combine host tests with device code in same project
Some IDE support	Supported by many IDEs and editors
Generate compile-commands.json via the bear cli utility	Generate compile-commands.json directly with CMake

Makefile

CMake

Libraries

• STL
• STL-like
• Boost

IDE

• Terminal + Editor
• VS Code
• CLion

Debugging

JTAG / SWD

GDB

QEMU

# Launches QEMU as an Arduino Uno and printing USART to stdio.
qemu-system-avr -machine uno -bios firmware.elf -serial mon:stdio -nographic

Examples

The examples folder contains some basic Makefile & CMake based projects.