The nacl_io Library


nacl_io is a utility library that provides implementations of standard C APIs such as POSIX I/O (stdio.h) and BSD sockets (sys/socket.h). Its primary function is to allow code that uses these standard APIs to be compiled and used in a Native Client module. The library is included as part of Native Client SDK and is implemented in on top of Pepper API.

Since Native Client modules cannot access the host machine’s file system directly, nacl_io provides several alternative filesystem types which can be used by the application. For example, the Chrome browser supports the HTML5 File System API which provides access to a protected area of the local file system. This filesystem can be accessed by an HTML page using JavaScript commands, and also by a Native Client module using the Pepper File IO API.

With nacl_io a Native Client application can mount an HTML5 filesystem and access it via standard POSIX I/O function such as fopen, fseek, fread, fwrite, and fclose, or their low level UNIX counterparts open, lseek, read, write and close. As well as the HTML5 file system, nacl_io provides several other file system types which are described in the table below:

File System Description
memfs An in-memory file system
html5fs An HTML5 local file system, which can be persistent or temporary
http Maps files on a remote webserver into the local filesystem.
dev A file system containing special files (e.g.: /dev/null)

Using nacl_io

Using nacl_io is mostly just a matter of using the standard POSIX C library functions. However, there are some steps required to initialize the library and setup the filesystem mounts. In general the following steps will be needed to use nacl_io in a NaCl application:

  1. Link the application with the nacl_io library (-lnacl_io)
  2. Initialize nacl_io at startup using the nacl_io_init_ppapi or nacl_io_init functions.
  3. Mount any desired filesystems using the mount function. The arguments to mount for the different filesystem types are detailed in include/nacl_io/nacl_io.h.
  4. If you are going to mount an HTML5 file system, be sure to allocate space for it. You can either set the unlimitedStorage permission in the app’s Web Store manifest file, or call the HTML5 QuotaManagement API. These options are explained in the File IO documentation.
  5. Make sure that file and socket API calls are all made from the background thread. This is because the main Pepper thread does not support the blocking behavior needed by the POSIX I/O operations.

Logging in nacl_io

Unlike most input/output for nacl_io, internal logging writes directly to the stderr stream of the NaCl process. It deliberately bypasses the standard library functions implemented in nacl_io to avoid circular calls to itself.

The nacl_io demo

Building and running the demo

The demo application launches a Native Client module that mounts three file systems and displays a set of controls that let you work with them:


Follow these steps to build and run the demo:

  • Open a terminal in the demo directory:

    $ cd $NACL_SDK_ROOT/examples/demo/nacl_io_demo
  • run the demo:

    $ make run

Once the demo is running, try these operations:

  1. select the fopen command (when you select a command the fields in the line below will change according to the command)
  2. type in the filename /persistent/test
  3. check the write checkbox and press the fopen button
  4. select the fwrite command and select the file /persistent/test in the menu that appears below on the left
  5. enter some data and press the fwrite button
  6. select the fclose command, be sure the file /persistent/test is selected in the menu, and press the fclose button
  7. select the fopen command
  8. type in the filename /persistent/test
  9. check the fread checkbox and press the fopen button
  10. select the fread command, be sure the file /persistent/test is selected in the menu, enter a byte count, and press the fread button

A look at the code

The demo is written C and comprises three files.


This is the demo’s main file. The code here creates and initializes the Native Client module instance. The Pepper function Instance_DidCreate initializes nacl_io and mounts an HTML5 filesystem at /persistent.

static PP_Bool Instance_DidCreate(PP_Instance instance,
                                  uint32_t argc,
                                  const char* argn[],
                                  const char* argv[]) {
  g_instance = instance;
  nacl_io_init_ppapi(instance, get_browser_interface);
      "",  /* source */
      "/persistent",  /* target */
      "html5fs",  /* filesystemtype */
      0,  /* mountflags */
      "type=PERSISTENT,expected_size=1048576");  /* data specific to the html5fs type */

  pthread_create(&g_handle_message_thread, NULL, &HandleMessageThread, NULL);

  return PP_TRUE;

Space is allocated to the /persistent file system after the module is initialized. This is accomplished by the domContentLoaded function in the file example.js. This script is included in the module’s html page (see examples/demo/index.html):

function domContentLoaded(name, tc, config, width, height) {
  navigator.webkitPersistentStorage.requestQuota(window.PERSISTENT, 1024 * 1024,
      function(bytes) {
            'Allocated ' + bytes + ' bytes of persistant storage.');
        common.createNaClModule(name, tc, config, width, height);
      function(e) { alert('Failed to allocate space') });

The Instance_DidCreate function also creates a worker thread that receives messages sent from the html page and performs the specified file system operations. The logic for the worker thread is encoded in the other two files, described below.


This file implements a circular queue that is used to receive messages from the browser UI to the Native Client module. The file system commands in the enqueued messages are executed on the worker thread. This keeps blocking calls (like fread) off the main Native Client thread, which is a good thing. The queue is initialized in nacl_io_demo.c Instance_DidCreate.


This file implements the stdio calls associated with the commands sent from the browser. There is a separate Handle* function for each command: fopen, fclose, fseek, fread, fwrite. The handlers are called from the HandleMessage function in nacl_io_demo.c, which runs in the worker thread managing the message queue. The code for the fwrite handler appears below. Notice that it does not contain any PPAPI calls and looks like “ordinary” C code.

int HandleFwrite(int num_params, char** params, char** output) {
  FILE* file;
  const char* file_index_string;
  const char* data;
  size_t data_len;
  size_t bytes_written;

  if (num_params != 2) {
    *output = PrintfToNewString("Error: fwrite takes 2 parameters.");
    return 1;

  file_index_string = params[0];
  file = GetFileFromIndexString(file_index_string, NULL);
  data = params[1];
  data_len = strlen(data);

  if (!file) {
    *output = PrintfToNewString("Error: Unknown file handle %s.",
    return 2;

  bytes_written = fwrite(data, 1, data_len, file);

  *output = PrintfToNewString("fwrite\1%s\1%d", file_index_string,
  return 0;

Reference Information

The example discussed here is included in the SDK in the directory examples/demo/nacl_io_demo.

The nacl_io library is included in the SDK toolchain and is not a part of the Pepper API. For reference information related to the nacl_io interface see its header file in the SDK directory, located at include/nacl_io/nacl_io.h.

For more about the HTML5 file system read the specification.