Hi наб,

On 2026-02-14T21:48:01+0100, наб wrote:
[...]
> +static inline long
> +my_futex_wait_private(_Atomic uint32_t *uaddr, uint32_t val)

I just noticed this should be my_futex_wa{it => ke}_private().
Probably caused by my earlier confusion while reading these; sorry!

> +{
> +	return syscall(SYS_futex, uaddr, FUTEX_WAKE_PRIVATE, val);
> +}
> +\&
> +static inline long
> +my_futex_waitv(unsigned int n;
> +               struct futex_waitv waiters[n], unsigned int n,
> +               unsigned int flags, const struct timespec *timeout,
> +               clockid_t clockid)
> +{
> +	return syscall(SYS_futex_waitv, waiters, n, flags, timeout, clockid);
> +}
> +\&
> +void *
> +worker(void *arg)
> +{
> +	_Atomic uint32_t  *futex = arg;
> +\&
> +	usleep(*futex * 10000);
> +	*futex *= 2;
> +	my_futex_wait_private(futex, 1);
> +	return NULL;
> +}
> +\&
> +int
> +main(void)
> +{
> +	_Atomic uint32_t  futexes[10];
> +	uint8_t  init[countof(futexes)];
> +	struct futex_waitv waiters[countof(futexes)] = {};
> +	int  i;
> +\&
> +	if (getentropy(init, sizeof(init)))
> +		err(EXIT_FAILURE, "getentropy");
> +	init[0] = init[1] = init[2];
> +	for (i = 0; i < countof(futexes); ++i) {
> +		printf("%" PRIu8 "\[rs]t", init[i]);
> +		atomic_init(&futexes[i], init[i]);
> +		pthread_create(&(pthread_t){}, NULL, worker, &futexes[i]);
> +	}
> +	putchar(\[aq]\[rs]n\[aq]);
> +\&
> +	for (i = 0; i < countof(futexes); ++i) {
> +		waiters[i].val   = futexes[i];
> +		waiters[i].uaddr = (uintptr_t) &futexes[i];
> +		waiters[i].flags = FUTEX2_SIZE_U32 | FUTEX2_PRIVATE;
> +	}
> +	for (;;) {
> +		struct timespec  timeout;
> +		int  woke;
> +\&
> +		clock_gettime(CLOCK_MONOTONIC, &timeout);
> +		timeout.tv_sec += 1;
> +\&
> +		woke = my_futex_waitv(waiters, countof(futexes), 0, &timeout, CLOCK_MONOTONIC);
> +		if (woke == \-1 && (errno != EAGAIN && errno != EWOULDBLOCK))
> +			err(EXIT_FAILURE, "my_futex_waitv");
> +\&
> +		for (i = 0; i < countof(futexes); ++i) {
> +			if (futexes[i] != waiters[i].val)
> +				printf("%" PRIu32 "%s", futexes[i], i == woke ? "!" : "");

This line goes past the right margin, and could be trivially narrowed
by using the new (C23) fixed-width length modifiers:

	alx@devuan:~/tmp$ cat pf.c 
	#include <stdint.h>
	#include <stdio.h>
	int
	main(void)
	{
		int32_t  i = 7;

		printf("%w32d\n", i);
	}
	alx@devuan:~/tmp$ gcc -Wall -Wextra pf.c 
	alx@devuan:~/tmp$ ./a.out 
	7

So, the line would be:

				printf("%w32d%s", futexes[i], i == woke ? "!" : "");

> +			putchar(\[aq]\[rs]t\[aq]);
> +		}
> +		putchar(\[aq]\[rs]n\[aq]);
> +\&
> +		for (i = 0; i < countof(futexes); ++i)
> +			waiters[i].val = futexes[i];
> +	}
> +}


Have a lovely night!
Alex

-- 
<https://www.alejandro-colomar.es>