getopt() for optional argument

my_getopt.c

#include <unistd.h>
#include <string.h>
#include <stdio.h>

#define  ALIGN_OPTIONAL_ARGUMENT(_argc, _argv, _optstring, _optarg, _optind) \
do { \
	if (!(_optarg) && (_optind) < (_argc) && \
			(((_argv)[(_optind)][0] != '-') || \
			 !strchr(_optstring, (_argv)[(_optind)][1]))) { \
		(_optarg) = (_argv)[(_optind)++]; \
	} \
} while (0)

int main(int argc, char **argv)
{
	char *optstring = "a:b::c";
	char o;

	while ((o = getopt(argc, argv, optstring)) != -1) {
		switch (o) {
		case 'b':
			ALIGN_OPTIONAL_ARGUMENT(argc, argv, optstring, optarg,
				optind);
		case 'a':
		case 'c':
			printf("-%c got optarg: %s\n", o,
				optarg ? optarg : "N/A");
			break;
		}
	}

	return 0;
}

$ gcc my_getopt.c

$ ./a.out -b    -x
-b got optarg: -x

$ ./a.out -b    -c
-b got optarg: N/A
-c got optarg: N/A

$ ./a.out -b    -a
-b got optarg: N/A
./a.out: option requires an argument -- 'a'

$ ./a.out -b    -a x
-b got optarg: N/A
-a got optarg: x

$ ./a.out -b    -x x
-b got optarg: -x