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C Academy · Lesson

Parsing Options

Handle flags.

Parsing Options is a free C Academy lesson on CoddyKit — lesson 2 of 4. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the C Academy learning path, one of 4 lessons in the course, and your progress syncs across the web and the CoddyKit app.

Options vs positional arguments

Command-line arguments come in two flavors. Positional arguments are values like filenames. Options (also called flags) start with a dash, like -v or --help.

This lesson shows how to detect and handle flags manually.

#include <stdio.h>

int main(int argc, char *argv[]) {
    for (int i = 1; i < argc; i++) {
        if (argv[i][0] == '-')
            printf("option: %s\n", argv[i]);
        else
            printf("value:  %s\n", argv[i]);
    }
    return 0;
}

Detecting a flag

To check for a specific flag, compare each argument with strcmp.

Here we set a boolean when we see -v for verbose mode.

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

int main(int argc, char *argv[]) {
    int verbose = 0;
    for (int i = 1; i < argc; i++) {
        if (strcmp(argv[i], "-v") == 0)
            verbose = 1;
    }
    printf("verbose = %d\n", verbose);
    return 0;
}

Supporting long options

Many tools accept both a short form (-h) and a long form (--help).

Check for either spelling with two comparisons joined by ||.

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

int main(int argc, char *argv[]) {
    for (int i = 1; i < argc; i++) {
        if (strcmp(argv[i], "-h") == 0 ||
            strcmp(argv[i], "--help") == 0) {
            printf("Usage: prog [options]\n");
            return 0;
        }
    }
    printf("No help requested\n");
    return 0;
}

Options that take a value

Some flags need an argument, like -o output.txt. When you see such a flag, the next element of argv is its value.

Advance the loop index and check you did not run past the end.

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

int main(int argc, char *argv[]) {
    const char *output = "default.txt";
    for (int i = 1; i < argc; i++) {
        if (strcmp(argv[i], "-o") == 0 && i + 1 < argc) {
            output = argv[++i];
        }
    }
    printf("Output file: %s\n", output);
    return 0;
}

Handle the missing value

If -o is the last argument, there is no value after it. Reading argv[i+1] would be out of bounds.

Always test i + 1 < argc before consuming the next token, and report an error otherwise.

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

int main(int argc, char *argv[]) {
    for (int i = 1; i < argc; i++) {
        if (strcmp(argv[i], "-n") == 0) {
            if (i + 1 >= argc) {
                printf("Error: -n needs a value\n");
                return 1;
            }
            printf("n = %s\n", argv[++i]);
        }
    }
    return 0;
}

Combining boolean flags and values

A realistic parser tracks several flags at once: some boolean, some with values. Use separate variables for each setting.

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

int main(int argc, char *argv[]) {
    int verbose = 0;
    const char *name = "world";
    for (int i = 1; i < argc; i++) {
        if (strcmp(argv[i], "-v") == 0) verbose = 1;
        else if (strcmp(argv[i], "-name") == 0 && i+1 < argc) name = argv[++i];
    }
    if (verbose) printf("verbose mode\n");
    printf("Hello, %s\n", name);
    return 0;
}

Recognizing the end of options

By convention, a lone -- marks the end of options. Everything after it is treated as a positional argument, even if it starts with a dash.

This lets users pass filenames that begin with -.

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

int main(int argc, char *argv[]) {
    int opts_done = 0;
    for (int i = 1; i < argc; i++) {
        if (!opts_done && strcmp(argv[i], "--") == 0) {
            opts_done = 1;
            continue;
        }
        printf("%s: %s\n", opts_done ? "value" : "token", argv[i]);
    }
    return 0;
}

Collecting positional arguments

Often you want flags plus a list of files. Skip the recognized options and store the rest as positional arguments.

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

int main(int argc, char *argv[]) {
    for (int i = 1; i < argc; i++) {
        if (argv[i][0] == '-') continue;
        printf("file: %s\n", argv[i]);
    }
    return 0;
}

Unknown option handling

A friendly tool reports unknown flags instead of silently ignoring them. Check against your known options and warn otherwise.

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

int main(int argc, char *argv[]) {
    for (int i = 1; i < argc; i++) {
        if (argv[i][0] != '-') continue;
        if (strcmp(argv[i], "-v") != 0)
            printf("Unknown option: %s\n", argv[i]);
    }
    return 0;
}

Converting option values

Option values arrive as strings. Convert numeric ones with strtol, which also reports parse errors via an end pointer.

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

int main(int argc, char *argv[]) {
    int count = 1;
    for (int i = 1; i < argc; i++) {
        if (strcmp(argv[i], "-c") == 0 && i+1 < argc)
            count = (int) strtol(argv[++i], NULL, 10);
    }
    printf("count = %d\n", count);
    return 0;
}

Why a real parser helps

Manual parsing works but grows messy with combined short flags (-vf), = syntax (--name=foo), and ordering rules.

The standard getopt function, covered next, handles these conventions for you.

#include <stdio.h>

int main(int argc, char *argv[]) {
    printf("Parsed %d tokens manually\n", argc - 1);
    return 0;
}

Quick Check

Test your understanding of option parsing pitfalls.

Recap

You learned manual option parsing:

  • Flags start with -; compare them with strcmp.
  • Support short and long forms with ||.
  • Value-taking options consume the next token — guard with i + 1 < argc.
  • -- ends option parsing; report unknown flags; convert numeric values with strtol.

Frequently asked questions

Is the “Parsing Options” lesson free?

Yes — the full text of “Parsing Options” is free to read here on the web, and the C Academy course includes 4 lessons in total. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the C Academy course, upgrade to CoddyKit PRO.

What will I learn in “Parsing Options”?

Handle flags. You practise C Academy with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.

Do I need any experience to start C Academy?

No prior experience is required. C Academy on CoddyKit is structured for beginners through advanced learners; this is — lesson 2 of 4, so you can start here or from the beginning and move at your own pace.

How long does the “Parsing Options” lesson take?

Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.

Can I write and run code in this C Academy lesson?

Yes. Every C Academy lesson includes a built-in code editor, so you write and run real code right in your browser and get instant AI feedback — no local setup required.

All lessons in this course

  1. argc and argv
  2. Parsing Options
  3. getopt
  4. Building a CLI Tool
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