Use div or canvas

Standard input and output can be directed to a <div> or to a <canvas> HTML tag. A <div> is used for streamed character input and output, and a <canvas> is used for streaming to windowed input and output. In the windowed mode, the position of characters in a "terminal" style window can be specified.

  • <div id="twr_iodiv"> will be used for stdin and stdout if found.
  • <canvas id="twr_iocanvas"> will be used for stdin and stdout if it exists and no div found.
  • if neither of the above <div> or <canvas> is defined in your HTML, then stdout is sent to the debug console in your browser. And stdin is not available.
  • If you use twrWasmModule options, a fourth null options is available.

Unicode characters and symbols are supported.

The windows console also supports chunky (low res) graphics (each character cell can be used as a 2x3 graphic array).

stderr streams to the browser's debug console.


Name View Live Link Source Link
char in/out with <div> View square demo Source
"terminal" in/out with a <canvas> View mini-term demo Source

IO Console Docs

See IO Consoles for console io_ functions available.

UTF-8 or Windows-1252

Stdin and stdout can support UTF-8 or Windows-1252 (see localization)


In addition to the IO Console functions referenced above, you can use a number of standard C functions to output to div or canvas attached to stdout:

printf, vprintf, puts, putchar, snprintf, sprintf,  vsnprintf, vasprintf

stdout or stderr

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

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

fputc, putc, vfprintf, fprintf, fwrite

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.


You can get characters from the standard C define stdin with these functions:

  • io_mbgets - get a multibyte string from a console using the current locale character encoding
  • twr_mbgets - similar to io_mbgets, except always gets a multibyte locale format string from stdin.
  • io_mbgetc - get a multibyte character from an IOConsole (like stdin) using the current locale character encoding
  • getc (sames as fgetc) - get a single byte from a FILE * (IOConsole) -- returning ASCII or extended ASCII (window-1252 encoding)
  • io_getc32 - gets a 32 bit unicode code point from an IOConsole (which currently needs to be stdin)

Reading from stdin is blocking, and so twrWasmModuleAsync must be used to receive keys from stdin.

JavaScript needed for char input

You should add a line like the following to your JavaScript for stdin to work:

for twr_iodiv


for twr_iocanvas


You likely want a line like this to set the focus to the div or canvas so the user doesn't have to click on it:


You will also need to set the tabindex attribute in your div tag like this:

<div id="twr_iodiv" tabindex="0"></div>

See the stdio-div and stdio-canvas examples.

Note that this section describes blocking input using stdin. As an alternative, you can send events (keyboard, mouse, timer, etc) to a non-blocking C function from JavaScript using callC.