docs: add README

README.md

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+# libcli
+
+A lightweight C library for parsing command-line arguments. Wraps `getopt_long`
+with a descriptor-based API, typed parse utilities, duplicate detection, and
+automatic help formatting.
+
+## Dependencies
+
+- GCC (uses `__attribute__((unused))`)
+- autoconf ≥ 2.69, automake, libtool
+- [Criterion](https://github.com/Snaipe/Criterion) — optional, required for
+    `--enable-tests`
+
+## Build
+
+```sh
+./autogen.sh
+./configure
+make
+make install
+```
+
+To build and run the test suite:
+
+```sh
+./configure --enable-tests
+make check
+```
+
+## Quickstart
+
+```c
+#include "cli.h"
+#include "cli_parse_utils.h"
+#include "compiler.h"
+
+struct config { const char *name; };
+
+static int handle_name(const char *arg, void *cfg) {
+	((struct config *)cfg)->name = arg;
+	return CLI_SUCCESS;
+}
+
+static int handle_help(const char *arg, void *cfg) {
+	printf("Usage: myprog [OPTIONS]\n\n");
+	cli_print_options(g_opts, NB_OPTS, cli_arg_type_to_str);
+	return CLI_EXIT_SUCCESS;
+}
+
+#define OPTIONS_LIST \
+	X('h', "help", no_argument,			handle_help, OPT_ARG_NONE,   "Display this help") \
+	X('n', "name", required_argument,   handle_name, OPT_ARG_STRING, "Your name")
+
+#define X(s, l, a, h, t, d) + 1
+enum { NB_OPTS = 0 OPTIONS_LIST };
+#undef X
+
+#define X(s, l, a, h, t, d) + (1 + (a != no_argument ? 1 : 0))
+enum { OPTSTR_LEN = 1 OPTIONS_LIST };
+#undef X
+
+#define X(s, l, a, h, t, d) { s, l, a, h, t, d },
+static const struct option_descriptor g_opts[] = { OPTIONS_LIST };
+#undef X
+
+int main(int argc, char **argv) {
+	struct config cfg = { .name = "world" };
+	char opt_str[OPTSTR_LEN + 1];
+	struct option long_opts[NB_OPTS + 1];
+
+	cli_set_prog_name(argv[0]);
+	if (CLI_SUCCESS != cli_parse(argc, argv, &cfg, g_opts, NB_OPTS, opt_str, long_opts))
+		return 1;
+
+	printf("Hello, %s!\n", cfg.name);
+	return 0;
+}
+```
+
+See `example/` for a complete working example.
+
+## API
+
+### Parsing
+
+```c
+enum cli_code cli_parse(int argc, char **argv, void *config,
+	const struct option_descriptor *opts, size_t nb_opts,
+	char *opt_str, struct option *long_opts);
+```
+
+Parses `argv` against the descriptor table `opts`. Calls each option's handler
+with the raw argument string and `config`. Returns `CLI_SUCCESS`, `CLI_ERROR`,
+or `CLI_EXIT_SUCCESS` (handler requested early exit, e.g. `--help`).
+
+`opt_str` and `long_opts` are caller-allocated scratch buffers — size them with
+the X-macro pattern shown above.
+
+```c
+void cli_set_prog_name(const char *name);
+```
+
+Sets the program name used in error messages. Call with `argv[0]` before `cli_parse`.
+
+### Option descriptor
+
+```c
+struct option_descriptor {
+	char			 short_opt;		// e.g. 'h'
+	const char		*long_opt;		// e.g. "help"
+	int				 has_arg;		// no_argument / required_argument / optional_argument
+	t_option_handler handler;		// callback: int f(const char *arg, void *config)
+	int				 arg_type;		// display hint for cli_print_options
+	const char		 *description;  // displayed by cli_print_options
+};
+```
+
+### Help
+
+```c
+void cli_print_options(const struct option_descriptor *opts, size_t nb_opts,
+	t_arg_type_to_str to_str);
+```
+
+Prints a formatted options table to stdout. `to_str` maps `arg_type` integers
+to placeholder strings (`"<NUM>"`, `"<STR>"`, etc.).
+
+```c
+const char *cli_arg_type_to_str(int type);
+```
+
+Default `to_str` implementation covering the built-in types. Pass directly to
+`cli_print_options` if you don't need custom types.
+
+### Return codes
+
+| Value | Meaning |
+|---|---|
+| `CLI_SUCCESS` | Continue — option handled successfully |
+| `CLI_ERROR` | Abort — invalid option or argument |
+| `CLI_EXIT_SUCCESS` | Abort — clean exit requested (e.g. after printing help) |
+
+### Parse utilities
+
+```c
+int cli_parse_uint64(const char *s, uint64_t *out); // non-negative integers
+int cli_parse_int64(const char *s, int64_t *out);   // signed integers (use --opt=-N syntax)
+int cli_parse_float(const char *s, float *out);     // floats (use --opt=-1.5 syntax)
+```
+
+All return 0 on success, 1 on failure. They reject NULL, empty strings,
+trailing characters, and out-of-range values.
+
+## Custom arg types
+
+The built-in `CLI_OPT_ARG_TYPES` X-macro covers `OPT_ARG_NONE`, `OPT_ARG_INT`,
+`OPT_ARG_UINT`, `OPT_ARG_FLOAT`, `OPT_ARG_STRING`. To define custom types with
+their own display strings, declare your own list and generate both the enum and
+the `to_str` function from it:
+
+```c
+#define MY_ARG_TYPES \
+	X(OPT_ARG_NONE,	    "") \
+	X(OPT_ARG_UINT,	    "<NUM>") \
+	X(OPT_ARG_SECONDS,  "<SEC>") \
+	X(OPT_ARG_STRING,   "<STR>")
+
+#define X(name, str) name,
+enum { MY_ARG_TYPES };
+#undef X
+
+static const char *my_arg_type_to_str(int type) {
+	static const char *strs[] = {
+		#define X(name, str) str,
+		MY_ARG_TYPES
+		#undef X
+	};
+	if ((size_t)type < COUNT_OF(strs))
+		return strs[type];
+	return "<ARG>";
+}
+```
+
+Then pass `my_arg_type_to_str` to `cli_print_options`.
+
+## Limitations
+
+- Maximum 64 options
+
+## License
+
+GPLv3 — see [LICENSE](LICENSE).