Question

This is a homework assignment first off.

We have to create a “common application-programming model in UNIX/Linux known as the filter”.

I’m stuck on reading the input passed through as arguments (it’s all I ever seem to have trouble on).

For example, the cmd is open and the following line is entered:

program -isomebinaryfile.bin

I need to determine what the first letter is after the hyphen (-) and so on and so forth.

Is scanf what I would be using? My main is set up to be able to accept arguments:

int main (int argc, char *argv[])
{
FILE *inf = NULL;
char *arg = argv[0];
}

Can someone give me a little help?

Answer 1

Unless your assignment is only to handle processing of arguments, you may want to look up getopt – it’s a standard library parser for arguments.

As for the meat of your question, there are a lot of options, and you could use sscanf as part of it, but you don’t have to.

To parse the one argument you mentioned, you need to do the following: check if the argument begins with -i, grab the data out of the argument.

The easiest way to check if the argument begins with -i is:

if (argv[1][0] == '-' && argv[1][1] == 'i')

Alternatively, if you have a lot of argument options, all beginning with ‘-‘, you may want something like:

char * i = NULL;
char * o = NULL;
char * s = NULL;
for (int i = 1; i < argc; ++i) {
    if (argv[i][0] == '-') {
        switch(argv[i][1]) {
            case 'i':
                i = argv[i][2];
                break;
            case 's':
                s = argv[i][2];
                break;
            case 'o':
                o = argv[i][2];
                break;
            default:
                cerr << "Unknown option: " << argv[i][1];
        }
     } else {
        cerr << "Error: all options must begin with '-'";
     }

Note, I’m using argv[1], not 0. argv[0] is always the name of the executable.

The fastest way to extract the rest of the argument is simple pointer arithmetic:

char * filename = argv[1] + 2;  // (Or you could equivalently say = &argv[1][2]

This is most efficient – it reuses the strings that are already in argv. If you’re planning on changing the strings around, you’d do better with strcpy:

char * filename = (char *)malloc(strlen(argv[1]) - 2);
strcpy(filename, argv1 + 2);
// and eventually you'd have to free(filename)...

Play around and experiment with all the string functions. You’ll find them essential to all of your later programs.