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These module docs are in beta and may be incomplete.

modm:build: Build System Generators

This parent module defines a common set of functionality that is independent of the specific build system generator implementation. This includes straight-forward options like project name and build path but also more complicated configuration for programming your target via AvrDude/OpenOCD and debugging via GDB.

Note that this module does not compile your project, you will need to choose the modm:build:scons or modm:build:cmake submodule for that, or provide your own build system.

Using OpenOCD

For accessing your ARM Cortex-M based device, we use OpenOCD and generate a modm/openocd.cfg file with the target specific configuration:

  • Search directories passed via the metadata key.
  • User configuration files passed via the openocd.configfile metadata key. Your custom modm:build:openocd.cfg is added here too.
  • Two programming commands:
    • program_release for uploading the release profile.
    • program_debug for uploading your debug profile.

You can now upload your program in a one-line command:

openocd -f modm/openocd.cfg -c "program_release"
openocd -f modm/openocd.cfg -c "program_debug"
Open On-Chip Debugger 0.10.0
adapter speed: 2000 kHz
adapter_nsrst_delay: 100
none separate
srst_only separate srst_nogate srst_open_drain connect_deassert_srst
Info : clock speed 1800 kHz
Info : STLINK v2 JTAG v28 API v2 SWIM v18 VID 0x0483 PID 0x374B
Info : using stlink api v2
Info : Target voltage: 3.260972
Info : stm32f4x.cpu: hardware has 6 breakpoints, 4 watchpoints
adapter speed: 1800 kHz
** Programming Started **
auto erase enabled
Info : device id = 0x10006434
Info : flash size = 2048kbytes
wrote 16384 bytes from file path/to/project.elf in 0.589551s (27.139 KiB/s)
** Programming Finished **
** Verify Started **
verified 13064 bytes in 0.328969s (38.781 KiB/s)
** Verified OK **
adapter speed: 1800 kHz
target halted due to debug-request, current mode: Thread
xPSR: 0x01000000 pc: 0x0800263c msp: 0x10000be0
shutdown command invoked

Using GDB

For debugging your program on ARM Cortex-M device, we provide the modm/gdbinit file for the arm-none-eabi-gdb debugger connected to your target via OpenOCD.

Four commands are provided for convenience:

  • file_debug loads the debug profile ELF file.
  • file_release loads the release profile ELF file.
  • restart resets the device and halts.
  • rerun resets the device and continues execution.

GDB is configured in TUI mode and continues running the target after attaching, but does not load an ELF file! Please call file_{debug, release} first or pass the ELF file as a command line argument.

You can start your debug session by launching OpenOCD and then GDB:

# Run OpenOCD without any commands in the background
openocd -f modm/openocd.cfg
# Open another terminal
arm-none-eabi-gdb -x modm/gdbinit
# OR optionally with your custom ELF file
arm-none-eabi-gdb -x modm/gdbinit path/to/project.elf
  >│194             DRAW(x+1, y+3);                                  │
   │195             DRAW(x+2, y+3);                                  │
   │196     #else                                                    │
   │197             DRAW(x  , y  );                                  │
   │198     #endif                                                   │
   │199     #undef DRAW                                              │
   │200     }                                                        │
   │201                                                              │
   │202     static inline void drawScreen(framebuffer_t before, frame│
  >│0x80017a0 <game_of_life()+1692> strh.w r3, [r4, r12, lsl #1]     │
   │0x80017a4 <game_of_life()+1696> add    r0, lr                    │
   │0x80017a6 <game_of_life()+1698> ldr    r2, [r2, #0]              │
   │0x80017a8 <game_of_life()+1700> strh.w r3, [r2, r0, lsl #1]      │
   │0x80017ac <game_of_life()+1704> ldr    r3, [sp, #12]             │
   │0x80017ae <game_of_life()+1706> ldr    r2, [sp, #0]              │
   │0x80017b0 <game_of_life()+1708> add    r2, r3                    │
   │0x80017b2 <game_of_life()+1710> ldrb   r3, [r7, r1]              │
   │0x80017b4 <game_of_life()+1712> strb   r3, [r2, r1]              │
extended-r Remote target In: game_of_life         L194  PC: 0x80017a0

Program received signal SIGINT, Interrupt.
0x080017a0 in drawPixel (color=<optimized out>, y=42, x=578) at main.c

Be careful attaching to a running target

Due to the OpenOCD implementation, the target is halted for a very short period of time, while the device's debug peripheral is initialized. This time is dependent on the debug adapter and may range from just a few milliseconds to hundreds. Make sure your hardware can handle that!

Using AvrDude

Unfortunately AvrDude does not support a user configuration file like OpenOCD does, so there is no convenient one-line command to issue. You have to use the support of the specific build system.

Compiler Options

We maintain a common set of compiler options for all build system generator, so that they all have feature parity. We currently only support compiling modm with GCC for AVR, ARM Cortex-M and x86/posix with the options mentioned in the offical GCC options documentation.

You can find all compiler options inside the generated build script for your project, the options presented here are only the most important ones.

Shared for C and C++

  • -W{all, extra}: a basic set of warnings.
  • -Werror={format, maybe-uninitialized, overflow, sign-compare}: these warnings are treated as errors.
  • -f{data, function}-sections: puts data and functions into their own linker section.
  • -funsigned-{char, bitfields}: modm tries to use stdint.h types everywhere, but just in case.

For release builds:

  • -Os: optimize for smaller size.

For debug builds:

  • -Og: optimize for debugging experience.

Only C

  • -std=gnu11: use C11 with GNU extensions (for asm volatile).
  • -Wnested-externs: shouldn't use extern inside a function body.

Only C++

  • -std=c++17: use C++17
  • -fno-exceptions: no exceptions.
  • -fno-rtti: no run-time type information.


  • --gc-section: garbage collecting sections throws out a lot of unused data/code.
  • -L{linkdir} -Tlinkerscript.ld: modm uses a custom linkerscript.

Only AVR

  • -mmcu={target}: the target to compile for.

Only ARM Cortex-M

  • -mcpu=cortex-m{type}: the target to compile for.
  • -mthumb: only Thumb2 instruction set is supported.
  • -mfloat-abi=hard: if FPU available use the fastest ABI available.
  • -mfpu=fpv{4, 5}-{sp}-d16: single or double precision FPU.
  • -fsingle-precision-constant: if SP-FPU, treat all FP constants as SP.
  • -Wdouble-promotion: if SP-FPU, warn if FPs are promoted to doubles.

In addition, these linker options are added:

  • -nostartfiles: modm implements its own startup script.
  • --specs=nano.specs: use Newlib Nano.
  • --specs=nosys.specs: modm does not use C stdlib features.
  • -wrap,_{calloc, malloc, realloc, free}_r: reimplemented Newlib with our own allocator.


Project Name: blink{ String }

The project name defaults to the folder name you're calling lbuild from.

It's used by your build system to name the executable and it may also be passed to your application via a string constant or CPP define.


Build Path: ../../../build/stm32f469_discovery/blink{ String }

The build path is defaulted to build/{}.

If you have a lot of embedded projects, you may want to change the build path to a common directory so that you don't have build/ folders everywhere. Remember to add your build path to your .gitignore.

You should use a relative path instead of an absolute one, so that this option still works for other developers.


Path to a custom OpenOCD configuration file: { String }

If you have a custom configuration file for your target, it will get included by the generated modm/openocd.cfg.

This is useful for not having to duplicate your config if you have several projects using the same target (like small bring-up and test projects).

Do not execute commands by default

When providing your own config file, wrap your specific commands into functions and do not execute them by default. A stray init or similar in your script will mess with modm's ability to program and debug a device correctly.


modm:build modm_build modm: build modm_build_cmake modm: build: cmake modm_build_cmake->modm_build modm_build_scons modm: build: scons modm_build_scons->modm_build