Parsing command-line arguments in C?

Question

I\'m trying to write a program that can compare two files line by line, word by word, or character by character in C. It has to be able to read in command line options

Answer 1

To my knowledge, the three most popular ways how to parse command line arguments in C are:

• Getopt (#include from the POSIX C Library), which can solve simple argument parsing tasks. If you're a bit familiar with bash, the getopt built-in of bash is based on Getopt from the GNU libc.
• Argp (#include from the GNU C Library), which can solve more complex tasks and takes care of stuff like, for example:
  • -?, --help for help message, including email address
  • -V, --version for version information
  • --usage for usage message
• Doing it yourself, which I don't recommend for programs that would be given to somebody else, as there is too much that could go wrong or lower quality. The popular mistake of forgetting about '--' to stop option parsing is just one example.

The GNU C Library documentation has some nice examples for Getopt and Argp.

• http://www.gnu.org/software/libc/manual/html_node/Getopt.html
• http://www.gnu.org/software/libc/manual/html_node/Argp.html

Example for using Getopt

#include 
#include 
#include 
#include 

int main(int argc, char *argv[])
{
    bool isCaseInsensitive = false;
    int opt;
    enum { CHARACTER_MODE, WORD_MODE, LINE_MODE } mode = CHARACTER_MODE;

    while ((opt = getopt(argc, argv, "ilw")) != -1) {
        switch (opt) {
        case 'i': isCaseInsensitive = true; break;
        case 'l': mode = LINE_MODE; break;
        case 'w': mode = WORD_MODE; break;
        default:
            fprintf(stderr, "Usage: %s [-ilw] [file...]\n", argv[0]);
            exit(EXIT_FAILURE);
        }
    }

    // Now optind (declared extern int by ) is the index of the first non-option argument.
    // If it is >= argc, there were no non-option arguments.

    // ...
}

Example for using Argp

#include 
#include 

const char *argp_program_version = "programname programversion";
const char *argp_program_bug_address = "";
static char doc[] = "Your program description.";
static char args_doc[] = "[FILENAME]...";
static struct argp_option options[] = { 
    { "line", 'l', 0, 0, "Compare lines instead of characters."},
    { "word", 'w', 0, 0, "Compare words instead of characters."},
    { "nocase", 'i', 0, 0, "Compare case insensitive instead of case sensitive."},
    { 0 } 
};

struct arguments {
    enum { CHARACTER_MODE, WORD_MODE, LINE_MODE } mode;
    bool isCaseInsensitive;
};

static error_t parse_opt(int key, char *arg, struct argp_state *state) {
    struct arguments *arguments = state->input;
    switch (key) {
    case 'l': arguments->mode = LINE_MODE; break;
    case 'w': arguments->mode = WORD_MODE; break;
    case 'i': arguments->isCaseInsensitive = true; break;
    case ARGP_KEY_ARG: return 0;
    default: return ARGP_ERR_UNKNOWN;
    }   
    return 0;
}

static struct argp argp = { options, parse_opt, args_doc, doc, 0, 0, 0 };

int main(int argc, char *argv[])
{
    struct arguments arguments;

    arguments.mode = CHARACTER_MODE;
    arguments.isCaseInsensitive = false;

    argp_parse(&argp, argc, argv, 0, 0, &arguments);

    // ...
}

Example for Doing it Yourself

#include 
#include 
#include 

int main(int argc, char *argv[])
{   
    bool isCaseInsensitive = false;
    enum { CHARACTER_MODE, WORD_MODE, LINE_MODE } mode = CHARACTER_MODE;
    size_t optind;
    for (optind = 1; optind < argc && argv[optind][0] == '-'; optind++) {
        switch (argv[optind][1]) {
        case 'i': isCaseInsensitive = true; break;
        case 'l': mode = LINE_MODE; break;
        case 'w': mode = WORD_MODE; break;
        default:
            fprintf(stderr, "Usage: %s [-ilw] [file...]\n", argv[0]);
            exit(EXIT_FAILURE);
        }   
    }   

    // *argv points to the remaining non-option arguments.
    // If *argv is NULL, there were no non-option arguments.

    // ...
}   

Disclaimer: I am new to Argp, the example might contain errors.