Subject: Re: [PATCH v2 2/7] test-tool genzeros: generate large amounts of data more efficiently

From: Junio C Hamano <[email-masked]>
Date: Thu, 28 Oct 2021 15:55:31 -0700

"Johannes Schindelin via GitGitGadget" <[email-masked]>
writes:

>  int cmd__genzeros(int argc, const char **argv)
>  {
> +	/* static, so that it is NUL-initialized */
> +	static char zeros[256 * 1024];

Of course, another benefit is that you do not waste 256kB of
stackspace for this array.  We could probably even mark it as
"const" to emphasize that it is initialized with and will stay
to be bunch of NULs.

>  	intmax_t count;
> +	ssize_t n;
>  
>  	if (argc > 2) {
>  		fprintf(stderr, "usage: %s [<count>]\n", argv[0]);
> @@ -12,9 +15,19 @@ int cmd__genzeros(int argc, const char **argv)
>  
>  	count = argc > 1 ? strtoimax(argv[1], NULL, 0) : -1;
>  
> -	while (count < 0 || count--) {
> -		if (putchar(0) == EOF)

> +	/* Writing out individual NUL bytes is slow... */
> +	while (count < 0)
> +		if (write(1, zeros, ARRAY_SIZE(zeros) < 0))
>  			return -1;

Be careful where you close your parentheses.  

I wonder if your compiler didn't warn that ARRAY_SIZE(x) will never
be negative?

> +	while (count > 0) {
> +		n = write(1, zeros, count < ARRAY_SIZE(zeros) ?
> +			  count : ARRAY_SIZE(zeros));

This side looks OK.

> +		if (n < 0)
> +			return -1;
> +
> +		count -= n;
>  	}
>  	return 0;