linux-kernel - Re: [PATCH 1/4] lib/sprintf: add scnprintf

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Message-ID: <aQ29CSOtOG_tmeIp@smile.fi.intel.com>
Date: Fri, 7 Nov 2025 11:34:01 +0200
From: Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
To: Junrui Luo <moonafterrain@...look.com>
Cc: linux-kernel@...r.kernel.org, pmladek@...e.com, rostedt@...dmis.org,
	akpm@...ux-foundation.org, tiwai@...e.com, perex@...ex.cz,
	linux-sound@...r.kernel.org, mchehab@...nel.org,
	awalls@...metrocast.net, linux-media@...r.kernel.org,
	davem@...emloft.net, edumazet@...gle.com, kuba@...nel.org,
	pabeni@...hat.com, netdev@...r.kernel.org
Subject: Re: [PATCH 1/4] lib/sprintf: add scnprintf_append() helper function

On Fri, Nov 07, 2025 at 01:16:13PM +0800, Junrui Luo wrote:
> Add a new scnprintf_append() helper function that appends formatted
> strings to an existing buffer.
> 
> The function safely handles buffer bounds and returns the total length
> of the string, making it suitable for chaining multiple append operations.

This won't work as expected in the *printf() functions.

...

> +/**
> + * scnprintf_append - Append a formatted string to a buffer

The name with _cat would be probably closer to the strcpy()/strcat().
OTOH we have pr_cont(), so perhaps I would name this scnprintf_cont().

> + * @buf: The buffer to append to (must be null-terminated)

Yeah, and must be not NULL which is no go. The *printf() should tolerate
the (buf = NULL, size = 0) input.

> + * @size: The size of the buffer
> + * @fmt: Format string
> + * @...: Arguments for the format string
> + *
> + * This function appends a formatted string to an existing null-terminated

Use the form of "null-terminated" used in the file. Perhaps you want to update
the existing one(s), just make sure there is some consistency.

> + * buffer. It is safe to use in a chain of calls, as it returns the total
> + * length of the string.
> + *
> + * Returns: The total length of the string in @buf
> + */
> +int scnprintf_append(char *buf, size_t size, const char *fmt, ...)
> +{
> +	va_list args;
> +	size_t len;

> +	len = strnlen(buf, size);
> +	if (len >= size)

How can it be '>' ?

> +		return len;

> +	va_start(args, fmt);
> +	len += vscnprintf(buf + len, size - len, fmt, args);
> +	va_end(args);
> +	return len;
> +}

-- 
With Best Regards,
Andy Shevchenko