Parse a required long-option value

To parse a command-line option that must be accompanied by a value, such as --file=output.txt, you need to define a long option and specify that its argument is required.

First, initialize a struct optparse parser by passing it and your argv array to the optparse_init function. This struct will hold the parser's state, including the parsed argument value.

Next, you define the accepted long options in an array of struct optparse_long. Each element in this array links a long option string, like "file", to a corresponding short option character. To enforce that a value must be provided, set the argtype field of the struct to OPTPARSE_REQUIRED. This enum optparse_argtype value ensures that the parser expects a value to follow the option.

Finally, call the optparse_long function. When it successfully parses an option that requires an argument, it returns the associated short option character and places a pointer to the argument's string in the optarg field of your struct optparse.

The following complete program demonstrates this process. It configures a single --file option, parses an argv array containing --file=output.txt, and asserts that the option was correctly identified and its value was captured in options.optarg.

#include <assert.h>
#include <string.h>
#include "optparse.h"

int main(void)
{
    enum optparse_argtype arg_required = OPTPARSE_REQUIRED;
    struct optparse_long longopts[] = {
        {"file", 'f', arg_required},
        {0} /* Terminator */
    };

    char *argv[] = {
        "program",
        "--file=output.txt",
        NULL
    };

    struct optparse options;
    optparse_init(&options, argv);

    int opt = optparse_long(&options, longopts, NULL);

    assert(opt == 'f');
    assert(options.optarg != NULL);
    assert(strcmp(options.optarg, "output.txt") == 0);

    return 0;
}