Enabling LVGL breaks `snprintf`

[Finomnis]

Describe the bug

It seems LVGL by default ships with its own version of sprintf, overwriting the picolibc one via linker order priority.

This is a problem because the one provided by LVGL does not support floating point formatting.

Reproduce

Replace the code in samples/hello_world/src/main.c with:

#include <stdio.h>

int main(void)
{
	printf("Hello World! %s\n", CONFIG_BOARD_TARGET);

	double pi = 355.0 / 113.0;

	char data[20];
	snprintf(data, sizeof(data), "snprintf: %f", pi);

	printf("%s\n", data);
	printf("printf: %f\n", pi);

	return 0;
}

west build -b qemu_cortex_a53 samples/hello_world -t run -- \
    -DCONFIG_LVGL=y \
    -DCONFIG_DISPLAY=y \
    -DCONFIG_DUMMY_DISPLAY=y \
    -DDTC_OVERLAY_FILE=../drivers/display/dummy_dc.overlay \
    -DCONFIG_MAIN_STACK_SIZE=8192 \
    -DCONFIG_HEAP_MEM_POOL_SIZE=8192 \
    -DCONFIG_LV_Z_MEM_POOL_SIZE=65536

Result:

*** Booting Zephyr OS build v4.2.1 ***
Hello World! qemu_cortex_a53/qemu_cortex_a53
snprintf: f
printf: 3.141593

When I add -DCONFIG_LV_USE_CUSTOM_SPRINTF=y, then I get:

*** Booting Zephyr OS build v4.2.1 ***
Hello World! qemu_cortex_a53/qemu_cortex_a53
snprintf: 3.141593
printf: 3.141593

Fixing the problem.

Regression

• This is a regression.

Impact

Functional Limitation – Some features not working as expected, but system usable.

Environment

• Windows
• Zephyr 4.2.1
• Zephyr SDK 0.17.4

Additional Context

No response

[Finomnis]

Suggestion: Make CONFIG_LV_USE_CUSTOM_SPRINTF the default.

[faxe1008]

Not sure, when I set CONFIG_LV_USE_CUSTOM_SPRINTF I get a ton of undefined reference errors for the symbols.

west build -b native_sim/native/64 samples/subsys/display/lvgl -- -DCONFIG_LV_USE_CUSTOM_STRING=y

Deleting the setting in lv_conf.h first of course.

[Finomnis]

@faxe1008 LV_USE_CUSTOM_SPRINTF, not STRING.

[Finomnis]

@faxe1008 Added reproducible example for qemu in description.

[keith-packard]

I'd suggest setting all of these C function options to their LV_USE_CLIB_ variant -- let Zephyr choose the C API, not some external module. I'm not sure they should even be configurable...

[Finomnis]

It's because lvgl strives to be self sufficient. Setting CONFIG_LV_USE_CLIB_SPRINTF crashes for me though. Zephyr provides the necessary custom glue code to use LV_USE_CUSTOM_SPRINTF, and that works for me and seems lightweight. The glue code simple forwards it to the system sprintf.

[keith-packard]

Setting CONFIG_LV_USE_CLIB_SPRINTF crashes for me though.

With zephyrproject-rtos/lvgl#69, I was able to build the hello-world sample (modified as above) and it worked for me?

[Finomnis]

Let me try on real hardware :)

[Finomnis]

Sadly I won't get to try until Monday :/

[keith-packard]

It's because lvgl strives to be self sufficient.

Perhaps that's useful in other environments, but when used with Zephyr, we shouldn't use the lvgl versions as Zephyr expects to provide those functions and requires that they conform to application needs, including support for floating point and the like.