Command Options

Some NNG utilities need to parse command line options, and the supplementary function here allows applications that need the same support to benefit from this.

To make use of this, the supplemental header <nng/supplemental/util/options.h> must be included.

Parse Command Line Options

typedef struct nng_optspec {
    const char *o_name;  // Long style name (may be NULL for short only)
    int         o_short; // Short option (no clustering!)
    int         o_val;   // Value stored on a good parse (>0)
    bool        o_arg;   // Option takes an argument if true
} nng_optspec;

int nng_opts_parse(int argc, char *const *argv,
                   const nng_optspec *spec, int *val, char **arg, int *idx);

The nng_opts_parse function is a intended to facilitate parsing command-line arguments. This function exists largely to stand in for getopt from POSIX systems, but it is available everywhere that NNG is, and it includes some capabilities missing from getopt.

The function parses arguments from main¹ (using argc and argv), starting at the index referenced by idx. (New invocations typically set the value pointed to by idx to 1.)

Options are parsed as specified by spec (see Option Specification.) The value of the parsed option will be stored at the address indicated by val, and the value of idx will be incremented to reflect the next option to parse.

If the option had an argument, a pointer to that is returned at the address referenced by arg.

This function should be called repeatedly, until it returns either -1 (indicating the end of options is reached) or a non-zero error code is returned.

This function may return the following errors:

• NNG_EAMBIGUOUS: Parsed option matches more than one specification.
• NNG_ENOARG: Option requires an argument, but one is not present.
• NNG_EINVAL: An invalid (unknown) argument is present.

Option Specification

The calling program must first create an array of nng_optspec structures describing the options to be supported. This structure has the following members:

• o_name:

  The long style name for the option, such as “verbose”. This will be parsed as a long option on the command line when it is prefixed with two dashes. It may be NULL if only a short option is to be supported.

• o_short:

  This is a single letter (at present only ASCII letters are supported). These options appear as just a single letter, and are prefixed with a single dash on the command line. The use of a slash in lieu of the dash is not supported, in order to avoid confusion with path name arguments. This value may be set to 0 if no short option is needed.

• o_val:

  This is a numeric value that is unique to this option. This value is assigned by the application program, and must be non-zero for a valid option. If this is zero, then it indicates the end of the specifications, and the rest of this structure is ignored. The value will be returned to the caller in val by nng_opts_parse when this option is parsed from the command line.

• o_arg:

  This value should be set to true if the option should take an argument.

Long Options

Long options are parsed from the argv array, and are indicated when the element being scanned starts with two dashes. For example, the “verbose” option would be specified as --verbose on the command line. If a long option takes an argument, it can either immediately follow the option as the next element in argv, or it can be appended to the option, separated from the option by an equals sign (=) or a colon (:).

Short Options

Short options appear by themselves in an argv element, prefixed by a dash (-). If the short option takes an argument, it can either be appended in the same element of argv, or may appear in the next argv element.

Prefix Matching

When using long options, the parser will match if it is equal to a prefix of the o_name member of a option specification, provided that it do so unambiguously (meaning it must not match any other option specification.)

Example

The following program fragment demonstrates this function.

    enum { OPT_LOGFILE, OPT_VERBOSE };
    char *logfile; // options to be set
    bool verbose;

    static nng_optspec specs[] = {
        {
            .o_name = "logfile",
            .o_short = 'D',
            .o_val = OPT_LOGFILE,
            .o_arg = true,
        }, {
            .o_name = "verbose",
            .o_short = 'V',
            .o_val = OPT_VERBOSE,
            .o_arg = false,
        }, {
            .o_val = 0; // Terminate array
        }
    };

    for (int idx = 1;;) {
        int rv, opt;
        char *arg;
        rv = nng_opts_parse(argc, argv, specs, &opt, &arg, &idx);
        if (rv != 0) {
            break;
        }
        switch (opt) {
        case OPT_LOGFILE:
            logfile = arg;
            break;
        case OPT_VERBOSE:
            verbose = true;
            break;
        }
    }
    if (rv != -1) {
        printf("Options error: %s\n", nng_strerror(rv));
        exit(1);
    }

1: Parsing argument strings from other sources can be done as well, although usually then idx will be initialized to zero.