Optly

Optly is a single-header command line argument parser for C (C99).

It is designed to be small, dependency‑free, and allocation‑free, making it ideal for small CLI tools and embedded-style programs.

Features

• Single-header library
• No dynamic memory allocation
• Portable C99
• Commands and nested subcommands
• Command-specific flags
• Long flags (--verbose)
• Short flags (-v)
• Batched short flags (-abc)
• Inline flag values (--threads=4)
• Separate flag values (--threads 4)
• Typed flag values
• Positional arguments
• Optional and required flags

Installation

Just drop optly.h into your project.

In one C file:

#define OPTLY_IMPLEMENTATION
#include "optly.h"

In other files:

#include "optly.h"

Example

#define OPTLY_IMPLEMENTATION
#include "optly.h"
#include <stdio.h>

static OptlyCommand cmd = {
  .name = "app",

  .flags = optly_flags(
    optly_flag_bool("verbose", 'v', "Enable verbose output", .value.as_bool = false),
    optly_flag_uint32("threads", 't', "Worker threads", .value.as_uint32 = 4)
  ),

  .commands = optly_commands(
    optly_command("run", "Run server",
      .flags = optly_flags(
        optly_flag_uint16("port", 'p', "Server port", .value.as_uint16 = 8080)
      )
    )
  )
};

int main(int argc, char **argv)
{
  optly_parse_args(argc, argv, &cmd);

  printf("threads: %u\n", optly_uint32(&cmd, "threads"));

  if (cmd.next_command) {
    printf("command: %s\n", cmd.next_command->name);
    printf("port: %u\n", optly_uint16(cmd.next_command, "port"));
  }
}

Run:

./app --threads 8 run --port 9000

Error handling

This library reports its errors by itself, no complicated error handling required.

By default reporting is disabled, but Optly provides logcie interface.

Just put logcie somewhre next to optly in your project, or define logging macros

#define OPTLY_LOG_BACKEND(level, ...) fprintf(stderr, ##level ": " __VA_ARGS__);

You can disable or enable certan logging levels in compiile time:

#define OPTLY_LOG_DEBUG // define to nothing

To control logging in runtime check out logcie.

Flags

Supported forms:

--verbose
-v
--threads 4
--threads=4
-t 4

Short flags can be batched:

-abc

Equivalent to:

-a -b -c

(Batched flags must be boolean.)

Commands

Commands are positional tokens:

app run
app build
app run check

Each command may define its own:

• flags
• subcommands
• positional arguments

Positional Arguments

Example:

app build file1 file2

Access them via:

OptlyPositional *p = optly_get_positional(cmd, "files");

Usage Helpers

Print global usage:

optly_usage(argv[0], cmd.commands, cmd.flags);

Print command usage:

optly_command_usage(argv[0], command);

Design Goals

Optly focuses on:

• minimal runtime overhead
• zero allocations
• simple static configuration

This makes it suitable for:

• CLI utilities
• embedded tools
• static binaries
• low-level C projects

License

Optly is dual licensed:

• MIT License
• Public Domain (Unlicense)

Choose whichever works best for your project.