Variadic arguments bug Macbook Air M1

Question

I'm currently creating a printf function from scratch which is working on a iMac 2020 and not on Air M1

If I try to test with the same % and different results, it will always show the first one.

For example: ft_printf(“test %s %s”, “test2", “test3”) Output: test test2 test2

But the output on the iMac is the correct one: test test2 test3

this is the main function:

int main()
{
    ft_printf("test %s %s\n", "test2", "test3");
    printf("test %s %s\n", "test2", "test3");
    return (0);
}

int ft_printf(const char *str, ...)
{
    va_list ap;
    int     cont;
    int     ret;

    cont = 0;
    va_start(ap, str);
    while (str[cont] != '\0')
    {
        if (str[cont] == '%')
        {
            ft_after_percentage(ap, str[cont + 1]);
            cont += 2;
            continue ;
        }
        write(1, &str[cont], 1);
        ft_count_return(0);
        cont++;
    }
    va_end(ap);
    ret = ft_count_return(0) - 1;
    ft_count_return(1);
    return (ret);
}

void    ft_after_percentage(va_list ap, char c)
{
    if (c == 'c')
        ft_putchar(va_arg(ap, int));
    else if (c == 's')
        ft_putstr(va_arg(ap, char *));
    else if (c == 'p')
        ft_percentage_p(va_arg(ap, long unsigned));
    else if (c == 'd')
        ft_putnbr(va_arg(ap, int));
    else if (c == 'i')
        ft_putnbr(va_arg(ap, int));
    else if (c == 'u')
        ft_uns_putnbr(va_arg(ap, unsigned int));
    else if (c == 'x')
        ft_puthex(va_arg(ap, int), "0123456789abcdef");
    else if (c == 'X')
        ft_puthex(va_arg(ap, int), "0123456789ABCDEF");
    else if (c == '%')
        ft_putchar('%');
}

Answer 1

Once you have passed the va_list variable by value to another function which uses va_arg() on it, you are not supposed to use that va_list variable in the calling function again.

The C99 standard, in the section for the stdarg.h header, says:

The type declared is va_list which is an object type suitable for holding information needed by the macros va_start, va_arg, va_end, and va_copy. If access to the varying arguments is desired, the called function shall declare an object (generally referred to as ap in this subclause) having type va_list. The object ap may be passed as an argument to another function; if that function invokes the va_arg macro with parameter ap, the value of ap in the calling function is indeterminate and shall be passed to the va_end macro prior to any further reference to ap.²¹²⁾

So here, you pass ap by value to ft_after_percentage(), which calls va_arg() on it, and then you continue to use ap in the calling function (by passing it to ft_after_percentage() for the second variable argument). This causes undefined behavior. It might happen to work on one architecture, but not another, due to differences in how va_list is implemented between architectures.

The footnote from above suggests for your ft_after_percentage() to instead take a va_list *, and pass &ap to it:

: 212) It is permitted to create a pointer to a va_list and pass that pointer to another function, in which case the original function may make further use of the original list after the other function returns.