Command Development Primer

This page provides a primer for writing commands by introducing hello module.

The full source code used in this example can be found in hello.c, and the Trying These Example Commands section shows how to use it.

A summary of this information can be found in OpenOCD Command API .

Command Handlers

Defining new commands and their helpers is easy. The following code defines a simple command handler that delegates its argument parsing:

COMMAND_HANDLER(handle_hello_command)
{
    const char *sep, *name;
    int retval = CALL_COMMAND_HANDLER(handle_hello_args);
    if (ERROR_OK == retval)
        command_print(CMD, "Greetings%s%s!", sep, name);
    return retval;
}

Here, the COMMAND_HANDLER macro establishes the function signature, see in command.h by the __COMMAND_HANDLER macro.

The COMMAND_HELPER macro function allows defining functions with an extended version of the base signature. These helper functions can be called (with the appropriate parameters), the CALL_COMMAND_HANDLER macro to pass any e as parameters to the following helper function:

The subsequent blocks of code are a normal C function that can do anything, so only complex commands deserve should use command helper functions. In this respect, this example uses one to demonstrate how – not when – they should be used.

static COMMAND_HELPER(handle_hello_args, const char **sep, const char **name)
{
    if (argc > 1)
    {
        LOG_ERROR("%s: too many arguments", CMD_NAME);
        return ERROR_COMMAND_SYNTAX_ERROR;
    }
    if (1 == CMD_ARGC)
    {
         *sep = ", ";
         *name = CMD_ARGV[0];
    }
    else
         *sep = *name = "";
 
    return ERROR_OK;
}

Of course, you may also call other macros or functions, but that extends beyond the scope of this tutorial on writing commands.

Command Registration

Before this new function can be used, it must be registered somehow. For a new module, registering should be done in a new function for the purpose, which must be called from openocd.c:

static const struct command_registration hello_command_handlers[] = {
    {
        .name = "hello",
        .mode = COMMAND_ANY,
        .handler = handle_hello_command,
        .help = "print a warm greeting",
        .usage = "[name]",
    },
    {
        .chain = foo_command_handlers,
    }
    COMMAND_REGISTRATION_DONE
};
 
int hello_register_commands(struct command_context_s *cmd_ctx)
{
    return register_commands(cmd_ctx, NULL, handle_command_handlers);
}

Note that the "usage" text should use the same EBNF that's found in the User's Guide: literals in 'single quotes', sequences of optional parameters in [square brackets], and alternatives in (parentheses|with|vertical bars), and so forth. No angle brackets.

That's it! The command should now be registered and available to scripts.

Command Chaining

This example also shows how to chain command handler registration, so your modules can "inherit" commands provided by other (sub)modules. Here, the hello module includes the foo commands in the same context that the 'hello' command will be registered.

If the chain field had been put in the 'hello' command, then the foo module commands would be registered under it. Indeed, that technique is used to define the 'foo bar' and 'foo baz' commands, as well as for the example drivers that use these modules.

The code for the 'foo' command handlers can be found in hello.c.

Trying These Example Commands

These commands have been inherited by the dummy interface, faux flash, and testee target drivers. The easiest way to test these is by using the dummy interface.

Once OpenOCD has been built with this example code, the following command demonstrates the abilities that the hello module provides:

openocd -c 'interface dummy' \
    -c 'dummy hello' \
    -c 'dummy hello World' \
    -c 'dummy hello {John Doe}' \
    -c 'dummy hello John Doe'  # error: too many arguments

If saved in hello.cfg, then running openocd -f hello.cfg should produce the following output before displaying the help text and exiting:

Greetings!
Greetings, World!
Greetings, John Doe!
Error: hello: too many arguments
Runtime error, file "openocd.cfg", line 14:
    hello: too many arguments
dummy hello [<name>]
      prints a warm welcome