下面看一下
getopt_long和
getopt_long_only（参考
getopt(3) – Linux man page）：
#include <getopt.h>
int getopt_long(int argc, char * const argv[],
const char *optstring,
const struct option *longopts, int *longindex);
int getopt_long_only(int argc, char * const argv[],
const char *optstring,
const struct option *longopts, int *longindex);
getopt_long除了可以处理short option外，还可以处理long option（以--开头）。关于struct option定义如下：
struct option {
const char *name;
int has_arg;
int *flag;
int val;
};
The meanings of the different fields are:
name
is the name of the long option.
has_arg
is: no_argument (or 0) if the option does not take an argument; required_argument (or 1) if the option requires an argument; or optional_argument (or 2) if the option takes an optional argument.
flag
specifies how results are returned for a long option. If flag is NULL, then getopt_long() returns val. (For example, the calling program may set val to the equivalent short option character.) Otherwise, getopt_long() returns 0, and flag points to a variable which is set to val if the option is found, but left unchanged if the option is not found.
val
is the value to return, or to load into the variable pointed to by flag.
如果flag是NULL，getopt_long会返回val的值，因此通常会把flag置成NULL，把val置成与long option对应的short option。否则getopt_long会返回0，并把val的值赋给flag。
参考下列代码（选自GNU binutils中的size命令）可以更好地了解getopt_long：
#define OPTION_FORMAT (200)
#define OPTION_RADIX (OPTION_FORMAT + 1)
#define OPTION_TARGET (OPTION_RADIX + 1)
static struct option long_options[] =
{
{"common", no_argument, &show_common, 1},
{"format", required_argument, 0, OPTION_FORMAT},
{"radix", required_argument, 0, OPTION_RADIX},
{"target", required_argument, 0, OPTION_TARGET},
{"totals", no_argument, &show_totals, 1},
{"version", no_argument, &show_version, 1},
{"help", no_argument, &show_help, 1},
{0, no_argument, 0, 0}
};
while ((c = getopt_long (argc, argv, "ABHhVvdfotx", long_options,
(int *) 0)) != EOF)
switch (c)
{
case OPTION_FORMAT:
switch (*optarg)
{
case 'B':
case 'b':
berkeley_format = 1;
break;
case 'S':
case 's':
berkeley_format = 0;
break;
default:
non_fatal (_("invalid argument to --format: %s"), optarg);
usage (stderr, 1);
}
break;
......
case 0:
break;
......
}
{"format", required_argument, 0, OPTION_FORMAT}的flag是NULL，所以getopt_long返回值是OPTION_FORMAT；根据optarg确定应该使用哪种format。而{"totals", no_argument, &show_totals, 1}的flag非NULL，getopt_long返回值是0，show_totals的值为1。
getopt_long和getopt_long_only的区别：
getoptlongonly() 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.