It is the behaviour of GNU GetOpt, which is very widely used and understood.
No, it isn't. At least, it's not the default. The GNU libc manual encourages the use of getopt_long(). It also provides an alternative called getopt_long_only() for programs that want to be backward compatible. Here's the relevant bit from the libc manual: Since long option names were used before before the `getopt_long' options was invented there are program interfaces which require programs to recognize options like `-option value' instead of `--option value'. To enable these programs to use the GNU getopt functionality there is one more function available. -- Function: int getopt_long_only (int ARGC, char *const *ARGV, const char *SHORTOPTS, const struct option *LONGOPTS, int *INDEXPTR) The `getopt_long_only' function is equivalent to the `getopt_long' function but it allows to specify the user of the application to pass long options with only `-' instead of `--'. The `--' prefix is still recognized but instead of looking through the short options if a `-' is seen it is first tried whether this parameter names a long option. If not, it is parsed as a short option. Assuming `getopt_long_only' is used starting an application with app -foo the `getopt_long_only' will first look for a long option named `foo'. If this is not found, the short options `f', `o', and again `o' are recognized. -- /jaap