Re: [kvm-unit-tests PATCH v2 09/23] lib/printf: Support for precision modifier in printing strings

[Andrew Jones <drjones@redhat.com>]

On Fri, May 06, 2022 at 09:55:51PM +0100, Nikos Nikoleris wrote:
> This follows the typical format of:
> 
> printf("%.Ns", *str);
> 
> Where N might be a decimal digit string or '*'. This feature is used
> by a future change.
> 
> See also: man 3 printf
> 
> Signed-off-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
> ---
>  lib/printf.c | 84 ++++++++++++++++++++++++++++++++++++++++++++--------
>  1 file changed, 71 insertions(+), 13 deletions(-)
> 
> diff --git a/lib/printf.c b/lib/printf.c
> index 1269723..724befa 100644
> --- a/lib/printf.c
> +++ b/lib/printf.c
> @@ -19,6 +19,7 @@ typedef struct strprops {
>      char pad;
>      int npad;
>      bool alternate;
> +    int precision;
>  } strprops_t;
>  
>  static void addchar(pstream_t *p, char c)
> @@ -43,7 +44,7 @@ static void print_str(pstream_t *p, const char *s, strprops_t props)
>  	}
>      }
>  
> -    while (*s)
> +    while (*s && props.precision--)
>  	addchar(p, *s++);
>  
>      if (npad < 0) {
> @@ -147,9 +148,61 @@ static int fmtnum(const char **fmt)
>      return num;
>  }
>  
> +static inline int isdigit(int c)
> +{
> +    return '0' <= c && c <= '9';
> +}

We desperately need to add ctype to our library. We've already got
isblank, isalpha, and isalnum local to argv.c and I see later you
add isspace. I'll post a patch now that introduces ctype.[ch] with
the ones used by argv. Then, when you respin this series you can
add your ctype functions there.

> +
> +/*
> + * Adapted from drivers/firmware/efi/libstub/vsprintf.c
> + */
> +static int skip_atoi(const char **s)
> +{
> +    int i = 0;
> +
> +    do {
> +	i = i*10 + *((*s)++) - '0';
> +    } while (isdigit(**s));
> +
> +    return i;
> +}
> +
> +/*
> + * Adapted from drivers/firmware/efi/libstub/vsprintf.c
> + */
> +static int get_int(const char **fmt, va_list *ap)
> +{
> +    if (isdigit(**fmt)) {
> +	return skip_atoi(fmt);
> +    }
> +    if (**fmt == '*') {
> +	++(*fmt);
> +	/* it's the next argument */
> +	return va_arg(*ap, int);
> +    }
> +    return 0;
> +}
> +
>  int vsnprintf(char *buf, int size, const char *fmt, va_list va)
>  {
>      pstream_t s;
> +    va_list args;
> +
> +    /*
> +     * We want to pass our input va_list to helper functions by reference,
> +     * but there's an annoying edge case. If va_list was originally passed
> +     * to us by value, we could just pass &ap down to the helpers. This is
> +     * the case on, for example, X86_32.
> +     * However, on X86_64 (and possibly others), va_list is actually a
> +     * size-1 array containing a structure. Our function parameter ap has
> +     * decayed from T[1] to T*, and &ap has type T** rather than T(*)[1],
> +     * which is what will be expected by a function taking a va_list *
> +     * parameter.
> +     * One standard way to solve this mess is by creating a copy in a local
> +     * variable of type va_list and then passing a pointer to that local
> +     * copy instead, which is what we do here.
> +     */
> +    va_copy(args, va);
>  
>      s.buffer = buf;
>      s.remain = size - 1;
> @@ -160,6 +213,7 @@ int vsnprintf(char *buf, int size, const char *fmt, va_list va)
>  	strprops_t props;
>  	memset(&props, 0, sizeof(props));
>  	props.pad = ' ';
> +	props.precision = -1;
>  
>  	if (f != '%') {
>  	    addchar(&s, f);
> @@ -172,11 +226,14 @@ int vsnprintf(char *buf, int size, const char *fmt, va_list va)
>  	    addchar(&s, '%');
>  	    break;
>  	case 'c':
> -            addchar(&s, va_arg(va, int));
> +	    addchar(&s, va_arg(args, int));
>  	    break;
>  	case '\0':
>  	    --fmt;
>  	    break;
> +	case '.':
> +	    props.precision = get_int(&fmt, &args);
> +	    goto morefmt;
>  	case '#':
>  	    props.alternate = true;
>  	    goto morefmt;
> @@ -204,54 +261,55 @@ int vsnprintf(char *buf, int size, const char *fmt, va_list va)
>  	case 'd':
>  	    switch (nlong) {
>  	    case 0:
> -		print_int(&s, va_arg(va, int), 10, props);
> +		print_int(&s, va_arg(args, int), 10, props);
>  		break;
>  	    case 1:
> -		print_int(&s, va_arg(va, long), 10, props);
> +		print_int(&s, va_arg(args, long), 10, props);
>  		break;
>  	    default:
> -		print_int(&s, va_arg(va, long long), 10, props);
> +		print_int(&s, va_arg(args, long long), 10, props);
>  		break;
>  	    }
>  	    break;
>  	case 'u':
>  	    switch (nlong) {
>  	    case 0:
> -		print_unsigned(&s, va_arg(va, unsigned), 10, props);
> +		print_unsigned(&s, va_arg(args, unsigned), 10, props);
>  		break;
>  	    case 1:
> -		print_unsigned(&s, va_arg(va, unsigned long), 10, props);
> +		print_unsigned(&s, va_arg(args, unsigned long), 10, props);
>  		break;
>  	    default:
> -		print_unsigned(&s, va_arg(va, unsigned long long), 10, props);
> +		print_unsigned(&s, va_arg(args, unsigned long long), 10, props);
>  		break;
>  	    }
>  	    break;
>  	case 'x':
>  	    switch (nlong) {
>  	    case 0:
> -		print_unsigned(&s, va_arg(va, unsigned), 16, props);
> +		print_unsigned(&s, va_arg(args, unsigned), 16, props);
>  		break;
>  	    case 1:
> -		print_unsigned(&s, va_arg(va, unsigned long), 16, props);
> +		print_unsigned(&s, va_arg(args, unsigned long), 16, props);
>  		break;
>  	    default:
> -		print_unsigned(&s, va_arg(va, unsigned long long), 16, props);
> +		print_unsigned(&s, va_arg(args, unsigned long long), 16, props);
>  		break;
>  	    }
>  	    break;
>  	case 'p':
>  	    props.alternate = true;
> -	    print_unsigned(&s, (unsigned long)va_arg(va, void *), 16, props);
> +	    print_unsigned(&s, (unsigned long)va_arg(args, void *), 16, props);
>  	    break;
>  	case 's':
> -	    print_str(&s, va_arg(va, const char *), props);
> +	    print_str(&s, va_arg(args, const char *), props);
>  	    break;
>  	default:
>  	    addchar(&s, f);
>  	    break;
>  	}
>      }
> +    va_end(args);
>      *s.buffer = 0;
>      return s.added;
>  }
> -- 
> 2.25.1
>

I think I should also post a patches that finally reformat these older
files. The tab+4spaces stuff must go!

When we get ctype we'll want to move out isdigit, but otherwise

Reviewed-by: Andrew Jones <drjones@redhat.com>

Thanks,
drew