Re: [PATCH 1/2] tools/nolibc: Add fread() to stdio.h

[David Laight <david.laight.linux@gmail.com>]

On Sun,  4 Jan 2026 17:38:36 +0900
Daniel Palmer <daniel@thingy.jp> wrote:

> Add a very basic version of fread() like we already have for fwrite().
> 
> Signed-off-by: Daniel Palmer <daniel@thingy.jp>
> ---
>  tools/include/nolibc/stdio.h | 35 ++++++++++++++++++++++++++++++++++-
>  1 file changed, 34 insertions(+), 1 deletion(-)
> 
> diff --git a/tools/include/nolibc/stdio.h b/tools/include/nolibc/stdio.h
> index 1f16dab2ac88..21569ebae824 100644
> --- a/tools/include/nolibc/stdio.h
> +++ b/tools/include/nolibc/stdio.h
> @@ -170,7 +170,7 @@ int putchar(int c)
>  }
>  
>  
> -/* fwrite(), puts(), fputs(). Note that puts() emits '\n' but not fputs(). */
> +/* fwrite(), fread(), puts(), fputs(). Note that puts() emits '\n' but not fputs(). */
>  
>  /* internal fwrite()-like function which only takes a size and returns 0 on
>   * success or EOF on error. It automatically retries on short writes.
> @@ -204,6 +204,39 @@ size_t fwrite(const void *s, size_t size, size_t nmemb, FILE *stream)
>  	return written;
>  }
>  
> +/* internal fread()-like function which only takes a size and returns 0 on
> + * success or EOF on error. It automatically retries on short reads.
> + */
> +static __attribute__((unused))
> +int _fread(void *buf, size_t size, FILE *stream)
> +{
> +	ssize_t ret;
> +	int fd = fileno(stream);
> +
> +	while (size) {
> +		ret = read(fd, buf, size);
> +		if (ret <= 0)
> +			return EOF;

You need to return a partial length if some data was read before EOF.

> +		size -= ret;
> +		buf += ret;
> +	}
> +	return 0;
> +}
> +
> +
> +static __attribute__((unused))
> +size_t fread(void *s, size_t size, size_t nmemb, FILE *stream)
> +{
> +	size_t readed;

Isn't it enough to just multiply 'size' and 'nmemb' together
and then do a single long read() (maybe with retries for short reads).
The multiply can't overflow (for a valid request) because the buffer
size is also limited to 'size_t'.

OTOH Linux limits the maximum return from read() to INT_MAX - PAGE_SIZE
(even on 64bit).

	David 


> +
> +	for (readed = 0; readed < nmemb; readed++) {
> +		if (_fread(s, size, stream) != 0)
> +			break;
> +		s += size;
> +	}
> +	return readed;
> +}
> +
>  static __attribute__((unused))
>  int fputs(const char *s, FILE *stream)
>  {