Parse a required long-option value

To parse a long option that requires a value, such as --file=output.txt, you define the option in a struct optparse_long array and specify that its argument is mandatory.

The argtype field in the struct optparse_long definition controls this behavior. By setting this field to OPTPARSE_REQUIRED, you instruct the optparse_long function to expect a value for the option. When a matching option is found in the arguments, its value is stored in the optarg field of your struct optparse instance.

The following program demonstrates how to configure and parse a single long option --version that requires a value. It declares a local enum optparse_argtype variable, initializes a parser with optparse_init, defines the --version option using that variable, and then calls optparse_long to process the arguments. After the call, it asserts that the function returned the correct short option character 'v' and that the optarg field contains the expected value, "1.0".

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

int main(void)
{
    enum optparse_argtype arg_required = OPTPARSE_REQUIRED;
    struct optparse_long longopts[] = {
        {"version", 'v', arg_required},
        {0}
    };

    char *argv[] = {
        "program",
        "--version=1.0",
        NULL
    };

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

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

    assert(opt == 'v');
    assert(options.optarg != NULL);
    assert(strcmp(options.optarg, "1.0") == 0);

    return 0;
}

Parsing is driven by the longopts table. Each entry defines a long option string, a corresponding short option character to return upon a match, and an optparse_argtype value. The table must be terminated by an entry where all fields are zero, which is concisely written as {0}.

The optparse_init function prepares the options struct for parsing by associating it with your argv array. The optparse_long function then attempts to parse one option, returning the short option identifier ('v' in this case) on success. The argument to the option is then available at options.optarg.