getopt_long_only() is like getopt_long(), but "-" as well as "--" can indicate a long option. If an option that starts with "-" (not "--") doesn't match a long option, but does match a short option, it is parsed as a short option instead.
Note: The getopt_long_only() function is intended only for supporting certain programs whose command line syntax was designed before the Utility Syntax Guidelines of ISO POSIX (2003) were developed. New programs should generally call getopt_long() instead, which provides the --option syntax for long options, which is preferred by GNU and consistent with ISO POSIX (2003).
getopt_long_only() returns the option character if the option was found successfully, or ":" if there was a missing parameter for one of the options, or "?" for an unknown option character, or -1 for the end of the option list.
getopt_long_only() also returns the option character when a short option is recognized. For a long option, they return val if flag is NULL, and 0 otherwise. Error and -1 returns are the same as for getopt(), plus "?" for an ambiguous match or an extraneous parameter.