Difference between '-std' and '--std' compiler flags

Question

I've noticed that both -std and --std works for setting the standard for compiling. What is the difference between using a - and -- before std?

I've googled and found this, but it doesn't seem to mention anything about a single hyphen vs a double hyphen before a std.

Answer 1

-std=c99 is ok but -std c99 is an error. --std c99 is valid as is --std=c99. That's the only difference.

You can see that they map to the same action in opts-common.c:

struct option_map
{
  /* Prefix of the option on the command line.  */
  const char *opt0;
  /* If two argv elements are considered to be merged into one option,
     prefix for the second element, otherwise NULL.  */
  const char *opt1;
  /* The new prefix to map to.  */
  const char *new_prefix;
  /* Whether at least one character is needed following opt1 or opt0
     for this mapping to be used.  (--optimize= is valid for -O, but
     --warn- is not valid for -W.)  */
  bool another_char_needed;
  /* Whether the original option is a negated form of the option
     resulting from this map.  */
  bool negated;
};

static const struct option_map option_map[] =
  {
   ...
    { "--std=", NULL, "-std=", false, false },
    { "--std", "", "-std=", false, false },
   ...
  };