From 3b1615c0dea75b082b7d27b73fbce65a836a87e2 Mon Sep 17 00:00:00 2001 From: Garrett D'Amore Date: Wed, 23 Oct 2024 17:38:03 -0700 Subject: docs: nng_opts_parse converted to chapter. --- docs/ref/api/cmd_opts.md | 165 +++++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 165 insertions(+) create mode 100644 docs/ref/api/cmd_opts.md (limited to 'docs/ref/api/cmd_opts.md') diff --git a/docs/ref/api/cmd_opts.md b/docs/ref/api/cmd_opts.md new file mode 100644 index 00000000..c28d3f89 --- /dev/null +++ b/docs/ref/api/cmd_opts.md @@ -0,0 +1,165 @@ +# 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 `` +must be included. + +## Parse Command Line Options + +```c +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 {{i:`nng_opts_parse`}} function is a intended to facilitate parsing +{{i:command-line arguments}}. +This function exists largely to stand in for {{i:`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 +`main`{{footnote: Parsing argument strings from other sources can be done as well, +although usually then _idx_ will be initialized to zero.}} +(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](#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. + +> [!TIP] +> For using this to parse command-line like strings that do not include +> the command name itself, set the value referenced by _idx_ to zero instead of one. + +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 {{i:`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](#long-options) on the command line when it is prefixed with two dashes. + It may be `NULL` if only a [short option](#short-options) 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](#short-options) 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. + +> [!NOTE] +> Option clustering, where multiple options can be crammed together in +> a single _argv_ element, is not supported by this function (yet). + +### 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. + +```c + 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); + } +``` + +{{#include ../xref.md}} -- cgit v1.2.3-70-g09d2