Question

OK, I have searched and found the following two StackOverflow topics that started me in the right direction:

Argument-parsing helpers for C/UNIX

Pass arguments into C program from command line

NOTE: ALL CODE IS PSEUDO-CODE. WILL POST COMPILABLE CODE WHEN IT WORKS.

However, I’m still completely confused on how to use getopt_long() in C. The program I’m writing is defined as having the following possible tags (but can include as many as you absolutely need, filling the rest in with empty values):

id3tagEd filename -title "title" -artist "artist" -year 1991 -comment "comment" -album "album" -track 1

Now, from what I read, I need to utilize a struct for the long options, correct? If so, I wrote something along the lines of this:

struct fields field =
{
    char *[] title;
    char *[] artist;
    char *[] album;
    int year;
    char *[] comment;
    int track;
}


static struct options long_options[] =
{
    {"title", 0, &field.title, 't'},
    {"artist", 0, &field.artist, 'a'},
    {"album", 0, &field.album, 'b'},
    {"year", 0, &field.year, 'y'},
    {"comment", 0, &field.comment, 'c'},
    {"track", 0, &field.track, 'u'},
    {0, 0, 0, 0}
}

Now, from what I gathered, I would be calling it via this:

int option_index = 0;

int values = getopt_long(argc, argv, "tabycu", long_options, &option_index);

From here, could I strictly use the field struct and do what I need to within my program? However, if this is the case, can someone explain the whole long_options struct? I read the man pages and such, and I’m just utterly confused. By rereading the man pages, I can see I can set variables to null, and should be setting all my option requirements to “required_argument”? And then setting the structs via a while() loop? However, I see optarg being used. Is this set by getopt_long()? Or is it missing from the example?

And one last issue, I will always have an unnamed required option: filename, would I just use argv[0] to gain access to that? (Since I can assume it’ll be first).

On a side note, this is related to a homework problem, but it has nothing to do with fixing it, its more of a fundamental, have to understand argument passing and parsing in C via command line first.

Answer 1

First off, you probably don’t want 0 for the has_arg field – it must be one of no_argument, required_arguemnt, or optional_argument. In your case, all of them are going to be required_argument. Besides that, you’re not using the flag field correctly – it has to be an integer pointer. If the corresponding flag is set, getopt_long() will fill it in with the integer you passed in via the val field. I don’t think you need this feature at all. Here’s a better (shortened) example for your case:

static struct option long_options[] =
{
    {"title", required_argument, NULL, 't'},
    {"artist", required_argument, NULL, 'a'},
    {NULL, 0, NULL, 0}
};

Then later, you can use it appropriately (straight from the manpage, I added some comments):

// loop over all of the options
while ((ch = getopt_long(argc, argv, "t:a:", long_options, NULL)) != -1)
{
    // check to see if a single character or long option came through
    switch (ch)
    {
         // short option 't'
         case 't':
             field.title = optarg; // or copy it if you want to
             break;
         // short option 'a'
         case 'a':
             field.artist = optarg; // or copy it if you want to
             break;
    }
}

You can extend for your other fields as necessary (and add some error handling, please!). Note – if you want to use -title and -artist like you have in your example, you’ll need to use getopt_long_only(), which doesn’t have short options.

As to your filename option, you’ll get that out as a '?' from the getopt_long() call, so you could handle it at that time. Your other options are to require that it is either the first or the last option and handle it by itself separately.