linux-kernel - Re: [PATCH v2 next 03/11] tools/nolibc/printf: Add buffering to vfprintf() callback.

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Message-ID: <aYeSnohSJ78ADFUZ@1wt.eu>
Date: Sat, 7 Feb 2026 20:29:34 +0100
From: Willy Tarreau <w@....eu>
To: david.laight.linux@...il.com
Cc: Thomas Weißschuh <linux@...ssschuh.net>,
        linux-kernel@...r.kernel.org, Cheng Li <lechain@...il.com>
Subject: Re: [PATCH v2 next 03/11] tools/nolibc/printf: Add buffering to
 vfprintf() callback.

On Fri, Feb 06, 2026 at 07:11:13PM +0000, david.laight.linux@...il.com wrote:
> From: David Laight <david.laight.linux@...il.com>
> 
> Add per-call buffering to the vprintf() callback.
> While this adds some extra code it will speed things up and
> makes a massive difference to anyone looking at strace output.

This patch alone adds more than 200 extra bytes to the smallest binary
for something that was never expressed as a need by users:

 $ size hello-patch*
   text    data     bss     dec     hex filename
   1859      48      24    1931     78b hello-patch1
   2071      48      24    2143     85f hello-patch2

I doubt it would make sense to have a build option to choose this.
Or alternately one could decide do disable it when __OPTIMIZE_SIZE__
is defined. I just tried quickly and it does the job:

  @@ -390,6 +390,7 @@ struct __nolibc_fprintf_cb_state {
   
   static int __nolibc_fprintf_cb(void *v_state, const char *buf, size_t size)
   {
  +#if !defined(__OPTIMIZE_SIZE__)
          struct __nolibc_fprintf_cb_state *state = v_state;
          unsigned int off = state->buf_offset;
   
  @@ -407,16 +408,24 @@ static int __nolibc_fprintf_cb(void *v_state, const char *buf, size_t size)
                  memcpy(state->buf + off, buf, size);
          }
          return 0;
  +#else
  +       /* v_state is the stream */
  +       return size ? _fwrite(buf, size, v_state) : 0;
  +#endif
   }
   
   static __attribute__((unused, format(printf, 2, 0)))
   int vfprintf(FILE *stream, const char *fmt, va_list args)
   {
  +#if !defined(__OPTIMIZE_SIZE__)
          struct __nolibc_fprintf_cb_state state;
   
          state.stream = stream;
          state.buf_offset = 0;
          return __nolibc_printf(__nolibc_fprintf_cb, &state, fmt, args);
  +#else
  +       return __nolibc_printf(__nolibc_fprintf_cb, stream, fmt, args);
  +#endif
   }
   
   static __attribute__((unused, format(printf, 1, 0)))

> Signed-off-by: David Laight <david.laight.linux@...il.com>
> ---
> 
> Changes for v2:
> Formally patch 2, unchanged.
> 
>  tools/include/nolibc/stdio.h | 32 +++++++++++++++++++++++++++++---
>  1 file changed, 29 insertions(+), 3 deletions(-)
> 
> diff --git a/tools/include/nolibc/stdio.h b/tools/include/nolibc/stdio.h
> index 36733ecd4261..552f09d51d82 100644
> --- a/tools/include/nolibc/stdio.h
> +++ b/tools/include/nolibc/stdio.h
> @@ -382,15 +382,41 @@ int __nolibc_printf(__nolibc_printf_cb cb, void *state, const char *fmt, va_list
>  	return written;
>  }
>  
> -static int __nolibc_fprintf_cb(void *stream, const char *buf, size_t size)
> +struct __nolibc_fprintf_cb_state {
> +	FILE *stream;
> +	unsigned int buf_offset;
> +	char buf[128];
> +};

So that's the other state I was wondering if we could merge with the first
one.

> +static int __nolibc_fprintf_cb(void *v_state, const char *buf, size_t size)
>  {
> -	return size ? _fwrite(buf, size, stream) : 0;
> +	struct __nolibc_fprintf_cb_state *state = v_state;
> +	unsigned int off = state->buf_offset;
> +
> +	if (off + size > sizeof(state->buf) || buf == NULL) {

Please mention that special case of buf==NULL in a comment above the
function. That's an internal API choice.

> +		state->buf_offset = 0;
> +		if (off && _fwrite(state->buf, off, state->stream))
> +			return -1;
> +		if (size > sizeof(state->buf))
> +			return _fwrite(buf, size, state->stream);
> +		off = 0;
> +	}
> +
> +	if (size) {
> +		state->buf_offset = off + size;
> +		memcpy(state->buf + off, buf, size);
> +	}
> +	return 0;
>  }
>  
>  static __attribute__((unused, format(printf, 2, 0)))
>  int vfprintf(FILE *stream, const char *fmt, va_list args)
>  {
> -	return __nolibc_printf(__nolibc_fprintf_cb, stream, fmt, args);
> +	struct __nolibc_fprintf_cb_state state;
> +
> +	state.stream = stream;
> +	state.buf_offset = 0;
> +	return __nolibc_printf(__nolibc_fprintf_cb, &state, fmt, args);
>  }
>  
>  static __attribute__((unused, format(printf, 1, 0)))

Thanks,
Willy