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Consoles with C/C++ WebAssembly
stdio, stderr, and more

This section describes how to use twr-wasm in order to:

  • create input/output consoles for use by C/C++
  • direct stdin/stdout and stderr to a console
  • use addressable display and canvas 2D consoles
  • use multiple consoles at once

Quick Example

Hello World
#include <stdio.h>

void hello() {
    printf("hello world\n");
}
Using twrConsoleDiv
<body>
   <div id="console-tag"></div>

   <script type="module">
      import {twrConsoleDiv, twrWasmModule} from "twr-wasm";

      const tag=document.getElementById("console-tag");
      const streamConsole=new twrConsoleDiv(tag); 
      const mod = new twrWasmModule({stdio: streamConsole});
      await mod.loadWasm("./helloworld.wasm");
      await mod.callC(["hello"]);

   </script>
</body>
Using twr_iodiv Shortcut
<body>
   <div id="twr_iodiv"></div>

   <script type="module">
      import {twrWasmModule} from "twr-wasm";

      const mod = new twrWasmModule();
      await mod.loadWasm("./helloworld.wasm");
      await mod.callC(["hello"]);

   </script>
</body>

Running Examples

Name View Live Link Source Link
stdin and stdout to <div> View square demo Source
simple "terminal" via <canvas> View hello world demo Source
"cli" with a <canvas> stdio View CLI demo using libc++ Source
Multiple Consoles, including Canvas2D View multi-io demo Source

Capabilities

With a Console you can:

  • read character streams (Use C statements like getc or io_mbgets)
  • write character streams (use C statements like printf or cout)
  • position characters, graphics, colors with an addressable display (Use C statements like io_setc32 or io_set_cursor).
  • draw to a Canvas compatible 2D surface (Use C statements like d2d_fillrect).

Consoles are primarily designed for use by twr-wasm C/C++ modules, but they can also be used by JavaScript/TypeScript.

Although it is common to have a single console, an arbitrary number of consoles can be created, and they can be used by an arbitrary number of twr-wasm C/C++ modules.

Unicode characters are supported by consoles (see Character Encoding Support with twr-wasm).

Tag Shortcuts

If you add a <div id="twr_iodiv">, a <canvas id="twr_iocanvas">, or a <canvas id="twr_d2dcanvas"> tag to your HTML, twr-wasm will create the appropriate class for you when you instantiate the class twrWasmModule or twrWasmModuleAsync. Use these tag shortcuts as an aternative to instantiating the console classes in your JavaScript/TypeScript.

  • <div id="twr_iodiv"> will be used to create a twrConsoleDiv as stdio
  • <canvas id="twr_iocanvas"> will be used to create a twrConsoleTerminal as stdio.
  • <canvas id="twr_d2dcanvas"> will be used to create a twrConsoleCanvas as std2d -- the default 2D drawing surface. See 2D drawing APIs.

If neither of the above <div> or <canvas> is defined in your HTML, and if you have not set stdio via the io or stdio module options, then stdout is sent to the debug console in your browser. And stdin is not available.

Console Classes

Consoles are implemented in TypeScript and run in the JavaScript main thread. This allows consoles to be shared by multiple wasm modules.

For simple cases, when you use the tag shortcuts, you won't need to use these console classes directly. For more bespoke cases, they will come in handy. For details on console classes, see the TypeScript/JavaScript API reference

These conosle classes are available in twr-wasm:

Multiple Consoles with Names

When you instantiate a class twrWasmModule or twrWasmModuleAsync, you can pass it the module option io -- a javascript object containing name-console attributes. Your C/C++ code can then retrieve a console by name. This is described in more detail the TypeScript/JavaScript API Reference.

Also see the multi-io example.

Setting stdio and stderr

stdio can be defined automatically if you use a Tag Shortcut. stderr streams to the browser's debug console by default. Both can be set to a specific console with the module io option.

For example, either of these will set stdio to a streaming div console:

const tag=document.getElementById("console-tag");
const streamConsole=new twrConsoleDiv(tag);

const mod = new twrWasmModule({stdio: streamConsole});
const mod = new twrWasmModule({ io: {stdio: streamConsole} });

This option would send stderr and stdio to the same console:

const mod = new twrWasmModule({ io: 
   {stdio: streamConsole, stderr: streamConsole} 
});

UTF-8 or Windows-1252

Consoles can support UTF-8 or Windows-1252 character encodings (see Character Encoding Support with twr-wasm).

C Access To Consoles

io_functions

io_functions are available to operate on all character based Consoles.

d2d_functions

d2d_functions are available to operate on Canvas 2D Consoles.

Reading from a Console

Reading from a console is blocking, and so twrWasmModuleAsync must be used to receive keys. There are some specific requirements to note in the twrWasmModuleAsync API docs.

You can get characters with any of these functions:

  • io_mbgets - get a multibyte string from a console using the current locale character encoding. Console must support IO_TYPE_CHARREAD.
  • twr_mbgets - the same as io_mbgets with the console set to stdin.
  • io_mbgetc - get a multibyte character from an twr_ioconsole_t * (aka FILE *) like stdin using the current locale character encoding
  • getc (same as fgetc) - get a single byte from a FILE * (aka twr_ioconsole_t *) -- returning ASCII or extended ASCII (window-1252 encoding)
  • io_getc32 - gets a 32 bit unicode code point from an twr_ioconsole_t * (which must support IO_TYPE_CHARREAD)

Standard C Library Functions

Many of the common standard C library functions, plus twr-wasm specific functions, are available to stream characters to and from the standard input and output console that supports character streaming (most do).

In C, a console is represented by twr_ioconsole_t. In addition, FILE is the same as a twr_ioconsole_t (typedef twr_ioconsole_t FILE). stdout, stdin, stderr are all consoles.

#include <stdio.h> to access stdout, stdin, stderr, and FILE.

FILE is supported for user input and output, and for stderr. FILE as filesystem I/O is not currently supported.

stdout and stderr functions

You can use these functions to output to the standard library defines stderr or stdout: 

fputc, putc, vfprintf, fprintf, fwrite

These functions go to stdout: 

printf, vprintf, puts, putchar

Note that when characters are sent to the browser console using stderr they will not render to the console until a newline, return, or ASCII 03 (End-of-Text) is sent.

For example: 

#include <stdio.h>

fprintf(stderr, "hello over there in browser debug console land\n");

A more common method to send output to the debug console is to use twr_conlog. See General C API Section.