Unresolved external symbol using GNU library (getopt) on Microsoft Visual Studio 2013 C++

Question

I don't have that much experience programming. What I'm trying to do is implement getopt in my program so that I can run it using arguments.

I have included getopt.h, getopt_long.c and all necessary libraries.

I get the errors LNK2001: unresolved external symbol opterr, optarg, optind, optopt.

I know I haven't given much information about the problem, but I don't really know what more to add. The files are working for other people, so assume there is nothing wrong with them.

What is likely the problem here?

Edit: Here is the code, although this is probably not where the problem is

/* Flag set by ‘--verbose’. */
static int verbose_flag;

int
main(int argc, char **argv)
{
    int c;

    while (1)
    {
            static struct option long_options[] =
            {
                    /* These options set a flag. */
                    { "verbose", no_argument, &verbose_flag, 1 },
                    { "brief", no_argument, &verbose_flag, 0 },
                    /* These options don't set a flag.
                    We distinguish them by their indices. */
                    { "add", no_argument, 0, 'a' },
                    { "append", no_argument, 0, 'b' },
                    { "delete", required_argument, 0, 'd' },
                    { "create", required_argument, 0, 'c' },
                    { "file", required_argument, 0, 'f' },
                    { 0, 0, 0, 0 }
            };
            /* getopt_long stores the option index here. */
            int option_index = 0;

            c = getopt_long(argc, argv, "abc:d:f:",
                    long_options, &option_index);

            /* Detect the end of the options. */
            if (c == -1)
                    break;

            switch (c)
            {
            case 0:
                    /* If this option set a flag, do nothing else now. */
                    if (long_options[option_index].flag != 0)
                            break;
                    printf("option %s", long_options[option_index].name);
                    if (optarg)
                            printf(" with arg %s", optarg);
                    printf("\n");
                    break;

            case 'a':
                    puts("option -a\n");
                    break;

            case 'b':
                    puts("option -b\n");
                    break;

            case 'c':
                    printf("option -c with value `%s'\n", optarg);
                    break;

            case 'd':
                    printf("option -d with value `%s'\n", optarg);
                    break;

            case 'f':
                    printf("option -f with value `%s'\n", optarg);
                    break;

            case '?':
                    /* getopt_long already printed an error message. */
                    break;

            default:
                    abort();
            }
    }

    /* Instead of reporting ‘--verbose’
    and ‘--brief’ as they are encountered,
    we report the final status resulting from them. */
    if (verbose_flag)
            puts("verbose flag is set");

    /* Print any remaining command line arguments (not options). */
    if (optind < argc)
    {
            printf("non-option ARGV-elements: ");
            while (optind < argc)
                    printf("%s ", argv[optind++]);
            putchar('\n');
    }

    exit(0);
}

Answer 1

Since they're is a linker error, either the source files are missing, or the library isn't being linked.

opterr, optarg, optind, and optopt are externals defined in the getopt header.

In getopt_long.c they are also marked as external. They're initialized (created? actually exist?) in getopt.c, which isn't a file you listed.