Having a good argument

I’ve seen all sorts of stuff on forums about how to process command line argument in C or C++. What a load of fuss and bother. There’s a standard getopt() function in the ‘C’ library, similar to the shell programming command, but it’s not great.

The main problem with getopt() is that it produces its own error message. Although this saves you the trouble, it can be a bit user unfriendly for the user, especially when things get complex. For example, you might have mutually exclusive arguments and want to print out a suitable message. That said, it’ll work most of the time.

Here’s a page showing a good explanation for the GCC version, whcih is pretty standard.

Example of Getopt (The GNU C Library)

But rolling your own is not hard. Here’s a skeleton I use. It’s pretty self-explanatory. My rules allow single options (e.g. -a -b) or combined options (e.g. -ab), or any mixture. “–” ends options, meaning subsequent arguments are going to be a actual arguments.

If you want to pass something as an option, such as a filename, you can. -fmyfile or -f myfile are both handled in the example.

You can add code to detect a long option by adding “if (!strcmp(p,”longname”)) … just after the char c. But I don’t like long options.

#include <stdio.h>

void process(char *s)
{
    printf("Processing %s\n",s);
}

int main (int cnt, char **a)
{
    int i;
    for (i=1; i<cnt && a[i][0] == '-'; i++)
    {
        char *p = &a[i][1];
        char c;
        if (*p == '-')
        {
            i++;
            break;
        }
        while (c = *p)
            switch (*p++)
            {
            case 'a':
                printf("Option a\n");
                break;
                
            case 'b':
                printf("Option b\n");
                break;

            case 'f':
                if (!*p && i+1 < cnt)
                    printf("Value for f=%s\n", a[++i]);
                else
                {
                    printf("Value for f=%s\n", p);
                    while (*p)
                        p++;
                }
                break;
                
            default:
                printf("Bad switch %c\n",c);

            }
    }
    for (;i<cnt;i++)
        process(a[i]);
}

The above code assumes that options precede arguments. If you want to mix them the following code allows for complete anarchy – but you can end it using the “–” option, which will take any following flags as arguments. As a bonus it shows how to add a long argument.

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

void process(char *s)
{
    printf("Processing %s\n",s);
}

int main (int cnt, char **a)
{
    int i;
    int moreargs=1;

    for (i=1; i<cnt; i++)
    {
            if (moreargs && a[i][0] == '-')
            {
            char *p = &a[i][1];
            char c;
                if (*p == '-')
                {
                    moreargs = 0;
                    continue;
                }
                if (!strcmp(p,"long"))
                {
                    printf("Long argument\n");
                    i++;
                    continue;
                }

                while (c = *p)
                    switch (*p++)
                    {
                    case 'a':
                        printf("Option a\n");
                        break;

                    case 'b':
                        printf("Option b\n");
                        break;

                    case 'f':
                        if (!*p && i+1 < cnt)
                            printf("Value for f=%s\n", a[++i]);
                        else
                        {
                            printf("Value for f=%s\n", p);
                            while (*p)
                                p++;
                        }
                        break;

                        default:
                            printf("Bad switch %c\n",c);

            }
        }
    else
        process(a[i]);
    }
}