#include <math.h>
#include <stdarg.h>
#include <stddef.h>
#include <string.h>

Functions
void	printk (const char *fmt,...)
	Print a formatted message to the device console.

void	vprintk (const char *fmt, va_list ap)
	`printk` taking an already-started variadic argument list.

Detailed Description

Declarations for the system functions an extension is allowed to call — include this instead of hand-writing a prototype.

The rgbx ABI itself (rgbx_api.h) has zero function imports: the host writes rgbx_inputs before each tick and reads rgbx_framebuffer after it. But an extension still runs on top of a small sanctioned set of C library and Zephyr functions, listed in fw/sdk/arm/allowed-symbols.txt — the subset of the firmware's llext export table the device will resolve at load time. This header is where those get their declarations.

Why this file exists (issue #351)

The SDK used to ship the allow-list without prototypes, so every author who wanted printk wrote their own. A wrong one is not caught anywhere obvious: extern "C" mangles on the name alone, so a mismatched signature still links. On ARM a wrong return type is usually survivable (AAPCS leaves the value in r0 and an ignoring caller is fine); in the WebAssembly simulator calls are typed by full signature, so the same source traps on first call with RuntimeError: unreachable. A wrong parameter type (float where the callee takes double, or vice versa) is a genuine ABI bug on both.

Including this header turns all of that into a compile error at the point of the mistake. Do not declare these functions yourself.

What is declared, and from where

The standard functions come from the real <string.h> and <math.h>, pulled in below, rather than being re-declared here. Both the ARM (arm-none-eabi / picolibc) and wasm (wasi-libc) toolchains ship them with the standard prototypes, and a hand-written copy would risk conflicting with the toolchain's own — restrict qualifiers and the noexcept that C++ headers attach to libc declarations make an exact re-declaration fragile for no benefit. Only printk/vprintk, which have no standard header, are declared here.

Note that <string.h> and <math.h> declare far more than the allow-list (strtol, snprintf, double-precision sin/pow, …). Calling those still compiles and still fails — at llext load on the device, and at build time in the SDK's check-llext.sh undefined-symbol gate, which is the check that governs. This header fixes wrong prototypes for sanctioned symbols; it is not, and cannot be, the gate on which symbols exist.

The __aeabi_* entries on the allow-list (64-bit division, 64-bit-int <-> float conversion, variable-count 64-bit shifts) are deliberately not declared. They are ARM run-time ABI helpers the compiler emits on your behalf — their real signatures return multi-register pairs that C cannot express, and no extension calls them by name. They are exported so that ordinary uint64_t arithmetic links; nothing to include for them.

See also: rgbx_api.h for the ABI itself.

Function Documentation

◆ printk()

void printk	(	const char *	fmt,
			... )

Print a formatted message to the device console.

On the glasses this goes to the UART shell console; in the simulator it goes to the printk pane. Available from inside the sandbox and the usual way to see what an extension is doing when there is no debugger.

Returns void — this is Zephyr's printk (zephyr/lib/os/printk.c), not printf. Nothing reports how many bytes were written.

Float conversions are not formatted. The device builds Zephyr with CONFIG_CBPRINTF_FP_SUPPORT=n, so %f, %g and %e emit the literal specifier rather than a number, and the simulator reproduces that deliberately. Scale to an integer instead:

printk("v=%d\n", (int)(v * 1000.0f));

printk

void printk(const char *fmt,...)

Print a formatted message to the device console.

Parameters

fmt	printf-style format string. Supported conversions are %c %s %p %d %i %u %x %X and %%, with `l`/`ll`/`z`/`h` length modifiers and numeric width / zero padding.
...	values matching the conversions in `fmt`.

◆ vprintk()

void vprintk	(	const char *	fmt,
		va_list	ap )

printk taking an already-started variadic argument list.

The same formatting rules and the same void return as printk(); see there. Useful for wrapping printk in your own diagnostic helper.