getopt_long() -- proper way to use it? - c

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.

Answer 2

If you use the popt library, you will be able to create something smart as you did in your pseudo-code:
#include <stdio.h>
#include "popt.h"
struct _field {
char *title;
char *artist;
/* etc */
} field;
field.title = NULL;
field.artist = NULL;
/* HERE IS WHAT YOU WANTED IN YOUR PSEUDO-CODE */
struct poptOption optionsTable[] = {
{"title", 't', POPT_ARG_STRING, &field.title, 't'
"set the 'title' of the album" },
{"artist", 'a', POPT_ARG_STRING, &field.artist, 'a'
"set the 'artist' of the album" },
POPT_AUTOHELP
POPT_TABLEEND
};
poptContext optCon = poptGetContext(NULL, argc, argv, optionsTable, 0);
poptSetOtherOptionHelp(optCon, "[OPTIONS]");
char c;
while ((c = poptGetNextOpt(optCon)) >= 0) {
switch (c) {
case 't':
/* do extra stuff only if you need */
break;
case 'a':
/* do extra stuff only if you need */
break;
default:
poptPrintUsage(optCon, stderr, 0);
exit(1);
}
}
if (field.title) printf("\nTitle is [%s]", field.title);
if (field.artist) printf("\nArtist is [%s]", field.artist)
Be smart than getopt ;)