Function apr_vsnprintf 

pub unsafe extern "C" fn apr_vsnprintf(
    buf: *mut c_char,
    len: apr_size_t,
    format: *const c_char,
    ap: *mut __va_list_tag,
) -> c_int

vsnprintf routine based on apr_vformatter. This means it understands the same extensions. @param buf The buffer to write to @param len The size of the buffer @param format The format string @param ap The arguments to use to fill out the format string.