JerryScript debugger interface

JerryScript provides a remote debugger which allows debugging JavaScript programs. The debugger has two main components: a server which is part of the JerryScript binary and a separate client application. Currently a Python-based debugger client is available in the /jerry-debugger subdirectory. This simple application demonstrates the communication protocol between the client and server, and can be reused by integrated development environments.

Setting up the debugger server

The following arguments must be passed to tools/build.py:

--jerry-debugger=on

The transport layer of the communication protocol is pluggable. At the moment, a WebSocket-based implementation is provided as a JerryScript extension, which transmits messages over TCP/IP networks. If necessary/implemented, any reliable stream or datagram based protocol can be used for transmitting debugger messages.

Debugging JavaScript applications

The debugger client must be connected to the server before the JavaScript application runs. On-the-fly attachment is supported for more than one file, right after the engine initialization (this feature is available with the python client). The debugging information (e.g. line index of each possible breakpoint location) is not preserved by JerryScript. The client is expected to be run on a system with much more resources and it should be capable of storing this information. JerryScript frees all debug information after it is transmitted to the client to save memory.

The following argument makes JerryScript wait for a client connection:

--start-debug-server

The following argument makes JerryScript wait for a client source code:

--debugger-wait-source

It is also recommended to increase the log level to see the Waiting for client connection message:

--log-level 2

The Python client can connect to the server by specifying its IP address on the command line. The address can be localhost if the server and the client are running on the same machine.

After the connection is established the execution can be controlled by the debugger. The debugger always stops at the first possible breakpoint location. The effect is the same as using the stop command. This allows inserting breakpoints right before the meaningful part of the execution starts.

All available commands of the client can be queried by the help command.

Integrating debugger support into applications using JerryScript

When using the extension-provided WebSocket transport layer, the debugger can be enabled by calling jerryx_debugger_after_connect (jerryx_debugger_tcp_create (debug_port) && jerryx_debugger_ws_create ()) after the jerry_init () function. It initializes the debugger and blocks until a client connects. (Custom transport layers may be implemented and initialized similarly.)

The resource name provided to jerry_parse () is used by the client to identify the resource name of the source code. This resource name is usually a file name.

JerryScript debugger C-API interface

The following section describes the debugger functions available to the host application.

JerryScript debugger types

jerry_debugger_wait_for_source_callback_t

Summary

This callback function is called by jerry_debugger_wait_for_client_source when a source code is received successfully.

Prototype

typedef jerry_value_t
(*jerry_debugger_wait_for_source_callback_t) (const jerry_char_t *resource_name_p,
                                              size_t resource_name_size,
                                              const jerry_char_t *source_p,
                                              size_t source_size, void *user_p);
  • resource_name_p - resource (usually a file) name of the source code
  • resource_name_size - size of resource name
  • source_p - source code character data
  • source_size - size of source code
  • user_p - custom pointer passed to jerry_debugger_wait_for_client_source

JerryScript debugger functions

jerry_debugger_is_connected

Summary

Returns true if a remote debugger client is connected.

Prototype

bool
jerry_debugger_is_connected (void);

Example

#include "jerryscript.h"
#include "jerryscript-ext/debugger.h"

int
main (void)
{
  jerry_init (JERRY_INIT_EMPTY);
  jerryx_debugger_after_connect (jerryx_debugger_tcp_create (5001)
                                 && jerryx_debugger_ws_create ());

  if (jerry_debugger_is_connected ())
  {
    printf ("A remote debugger client is connected.");
  }

  jerry_cleanup ();
}

jerry_debugger_stop

Summary

Stops execution at the next available breakpoint if a remote debugger client is connected and the engine is not waiting at a breakpoint. The engine will stop regardless the breakpoint is enabled or not.

Prototype

void
jerry_debugger_stop (void)

Example

#include "jerryscript.h"
#include "jerryscript-ext/debugger.h"

int
main (void)
{
  jerry_init (JERRY_INIT_EMPTY);
  jerryx_debugger_after_connect (jerryx_debugger_tcp_create (5001)
                                 && jerryx_debugger_ws_create ());

  jerry_debugger_stop ();

  jerry_cleanup ();
}

See also

jerry_debugger_continue

Summary

If the engine would stop at the next available breakpoint it cancels this effect. The engine will still stop at enabled breakpoints. This function effectively negates the effect of jerry_debugger_stop () calls or stop requests issued by the debugger client.

Prototype

void
jerry_debugger_continue (void)

Example

#include "jerryscript.h"
#include "jerryscript-ext/debugger.h"

int
main (void)
{
  jerry_init (JERRY_INIT_EMPTY);
  jerryx_debugger_after_connect (jerryx_debugger_tcp_create (5001)
                                 && jerryx_debugger_ws_create ());

  jerry_debugger_continue ();

  jerry_cleanup ();
}

See also

jerry_debugger_stop_at_breakpoint

Summary

Enables or disables stopping at breakpoints. When stopping is disabled all breakpoints are ignored including user enabled breakpoints. This allows hidden execution of ECMAScript code.

Prototype

void
jerry_debugger_stop_at_breakpoint (bool enable_stop_at_breakpoint)
  • enable_stop_at_breakpoint - enable (=true) or disable (=false) stopping at breakpoints

Example

#include "jerryscript.h"
#include "jerryscript-ext/debugger.h"

int
main (void)
{
  jerry_init (JERRY_INIT_EMPTY);
  jerryx_debugger_after_connect (jerryx_debugger_tcp_create (5001)
                                 && jerryx_debugger_ws_create ());

  jerry_debugger_stop_at_breakpoint (true);

  // Protected execution of JavaScript code.
  const jerry_char_t script[] = "42";
  jerry_eval (script, sizeof (script) - 1, JERRY_PARSE_NO_OPTS);

  jerry_debugger_stop_at_breakpoint (false);

  jerry_cleanup ();
}

jerry_debugger_wait_for_client_source

Summary

Asks the client to provide the next source code. The function waits until the whole source code is received. As a reply the the client may request a context reset or notify that no more source is available. These notifications are passed back as the return value of the function.

Prototype

jerry_debugger_wait_for_source_status_t
jerry_debugger_wait_for_client_source (jerry_debugger_wait_for_source_callback_t callback_p,
                                       void *user_p, jerry_value_t *return_value)

Example

#include "jerryscript.h"
#include "jerryscript-ext/debugger.h"

/**
 * Runs the source code received by jerry_debugger_wait_for_client_source.
 */
static jerry_value_t
wait_for_source_callback (const jerry_char_t *resource_name_p, /**< resource name */
                          size_t resource_name_size, /**< size of resource name */
                          const jerry_char_t *source_p, /**< source code */
                          size_t source_size, /**< source code size */
                          void *user_p __attribute__((unused))) /**< user pointer */
{
  jerry_value_t ret_val = jerry_parse (resource_name_p,
                                       resource_name_size,
                                       source_p,
                                       source_size,
                                       JERRY_PARSE_NO_OPTS);

  if (!jerry_value_is_error (ret_val))
  {
    jerry_value_t func_val = ret_val;
    ret_val = jerry_run (func_val);
    jerry_release_value (func_val);
  }

  return ret_val;
} /* wait_for_source_callback */

int
main (void)
{
  jerry_debugger_wait_for_source_status_t receive_status;

  do
  {
    /* Create a new JerryScript instance when a context reset is
     * received. Applications usually registers their core bindings
     * here as well (e.g. print, setTimeout). */
    jerry_init (JERRY_INIT_EMPTY);
    jerryx_debugger_after_connect (jerryx_debugger_tcp_create (5001)
                                   && jerryx_debugger_ws_create ());

    do
    {
      jerry_value_t run_result;

      receive_status = jerry_debugger_wait_for_client_source (wait_for_source_callback,
                                                              NULL,
                                                              &run_result);

      jerry_release_value (run_result);
    }
    while (receive_status == JERRY_DEBUGGER_SOURCE_RECEIVED);

    jerry_cleanup ();
  }
  while (receive_status == JERRY_DEBUGGER_CONTEXT_RESET_RECEIVED);

  if (receive_status == JERRY_DEBUGGER_SOURCE_RECEIVE_FAILED)
  {
    // Handle the failure (e.g. display an error).
  }
  return 0;
}

jerry_debugger_send_output

Summary

Sends the program’s output to the debugger client.

Prototype

void
jerry_debugger_send_output (const jerry_char_t *buffer, jerry_size_t string_size)

Example

#include "jerryscript.h"
#include "jerryscript-ext/debugger.h"

int
main (void)
{
  jerry_init (JERRY_INIT_EMPTY);
  jerryx_debugger_after_connect (jerryx_debugger_tcp_create (5001)
                                 && jerryx_debugger_ws_create ());

  jerry_char_t my_output[] = "Hey, this should be sent too!";
  jerry_size_t my_output_size = sizeof (my_output);

  jerry_debugger_send_output (my_output, my_output_size);

  jerry_cleanup ();
}

jerry_debugger_send_log

Summary

Sends the program’s log to the debugger client.

Prototype

void
jerry_debugger_send_log (jerry_log_level_t level, const jerry_char_t *buffer, jerry_size_t string_size)

Example

#include "jerryscript.h"
#include "jerryscript-ext/debugger.h"

int
main (void)
{
  jerry_init (JERRY_INIT_EMPTY);
  jerryx_debugger_after_connect (jerryx_debugger_tcp_create (5001)
                                 && jerryx_debugger_ws_create ());

  jerry_char_t my_log[] = "Custom diagnostics";
  jerry_size_t my_log_size = sizeof (my_log);

  jerry_debugger_send_log (JERRY_LOG_LEVEL_DEBUG, my_log, my_log_size);

  jerry_cleanup ();
}