avr-gcc

Compile C code for an AVR microcontroller

$ avr-gcc -mmcu=[atmega328p] -o [output.elf] [source.c]

$ avr-gcc -mmcu=[atmega328p] -Os -o [output.elf] [source.c]

$ avr-gcc -mmcu=[atmega328p] -g -O0 -o [output.elf] [source.c]

$ avr-gcc -mmcu=[atmega328p] -Os -o [output.elf] [source.c] && avr-objcopy -O ihex [output.elf] [output.hex]

$ avr-gcc -mmcu=[atmega328p] -Wall -Os -Wl,--relax -o [output.elf] [source.c]

$ avr-gcc -mmcu=[atmega328p] -ffreestanding -Os -o [output.elf] [source.c]

avr-gcc [options] file...

avr-gcc is the GNU Compiler Collection configured for AVR 8-bit microcontrollers. It compiles C and C++ code into machine code for Atmel/Microchip AVR chips used in Arduino and embedded systems.
The -mmcu option is essential as it configures code generation, memory layout, and available instructions for the specific target MCU. Common targets include atmega328p (Arduino Uno), atmega2560 (Arduino Mega), and attiny85.
Output is typically an ELF file, which is converted to Intel HEX format using avr-objcopy for flashing to hardware via tools like avrdude.
The -Os optimization is preferred for embedded development as it minimizes code size while maintaining reasonable performance. The -ffreestanding flag is appropriate since AVR programs run without an operating system.
AVR-GCC is typically installed as part of the avr-libc toolchain, which includes the C library, header files, and additional tools.

-mmcu=mcu

Target MCU (atmega328p, attiny85, atmega2560, etc.); required for correct code generation

Optimize for size (recommended for embedded)

Optimization levels (0=none, 3=maximum)

Include debugging information

Enable all common warnings

Assume freestanding environment (no standard library assumptions)

Disable jump tables (required for bootloaders on >64KB devices)

Enable linker relaxation to use shorter instructions when possible

Pass relaxation option to linker

Remove unused code sections to reduce binary size

Use subroutines for function prologues/epilogues (saves space)

Define CPU frequency in Hz (e.g., -DF_CPU=16000000)

Add include directory

Add library search directory

Link with library

The -mmcu option must match the target hardware exactly. Optimization level -O3 often increases code size significantly on AVR. Programs should avoid heavy stack usage due to limited RAM. Integer math defaults to 16-bit; use explicit types for larger values.

AVR-GCC emerged from the GCC project's support for the AVR architecture, initially developed by Denis Chertykov in the late 1990s. Combined with AVR-LibC, it became the standard open-source toolchain for AVR development. The toolchain gained widespread adoption through the Arduino project starting in 2005, making AVR programming accessible to hobbyists and educators.

gcc(1), avr-objcopy(1), avrdude(1), arduino-cli(1)