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

i've noticed both -std , --std works setting standard compiling. difference between using - , -- before std?

i've googled , found this, doesn't seem mention single hyphen vs double hyphen before std.

-std=c99 ok -std c99 error. --std c99 valid --std=c99. that's difference.

you can see map same action in opts-common.c:

struct option_map {   /* prefix of option on command line.  */   const char *opt0;   /* if 2 argv elements considered merged 1 option,      prefix second element, otherwise null.  */   const char *opt1;   /* new prefix map to.  */   const char *new_prefix;   /* whether @ least 1 character needed following opt1 or opt0      mapping used.  (--optimize= valid -o,      --warn- not valid -w.)  */   bool another_char_needed;   /* whether original option negated form of option      resulting map.  */   bool negated; };  static const struct option_map option_map[] =   {    ...     { "--std=", null, "-std=", false, false },     { "--std", "", "-std=", false, false },    ...   };