text written to stdout doesn't appear until program completion

Question

In this example, I am simply printing the numbers from 0 to 9 with a delay after each. However, instead of trickling the numbers to the output window, there is a long pause (with a "spinner" icon churning) and then all of the numbers are displayed at once (tested in both Chromium 44.0 and Firefox 40.0.3).

Is there a way to to make writes to stdout display immediately?

#include <stdio.h>

void time_waster( int reps=100 ) {
   static volatile unsigned counter=0;
   for ( int a=0; a<reps; a++ ) {
      for ( int b=0; b<1000*1000; b++ ) counter++;
   }
}

int main() {
  for ( int i=0; i<10; i++ ) {
     fprintf(stdout,"%d\n",i);
     fflush(stdout);
     time_waster();
  }
}

JavaScript and HTML built with:

emcc -Wall -Werror -o temp.html count.c

By the way, the combined size of the generated HTML+JavaScript for this small example is roughly 600KB (14619 lines), so debugging in the browser would be a non-trivial task.

Note: I had the same issue with std::cout in C++ (also with explicit flushing), but decided to write the question in C as a simplification of the problem.

I have discovered that the output is displayed as intended if I run the program with node.js:

node temp.js

So the problem only occurs when running in a browser using the HTML generated by the Emscripten compiler.

Have you tried it with normal C compiler without any exotics? — Eugene Sh., Sep 14 '15 at 20:30
I wouldn't be surprised if `fflush` is simply mapped to a no-op. This is just a wild guess though. — orlp, Sep 14 '15 at 20:31
@EugeneSh.: I hadn't bothered to do that previously because I know this should work. But since you asked, I did -- and it works as expected (a visible delay after each number). — Brent Bradburn, Sep 14 '15 at 20:32
[Here](https://github.com/kripken/emscripten/issues/2770) you go, I guess.. — Eugene Sh., Sep 14 '15 at 20:34
Maybe fix by providing a custom implementation of the [Module object](http://kripken.github.io/emscripten-site/docs/api_reference/module.html). — Brent Bradburn, Sep 15 '15 at 02:28
**Note:** `-Werror` should be `-Wextra` to get all the bang for your buck. — David C. Rankin, Sep 15 '15 at 02:54
[Why does my HTML app hang?](https://kripken.github.io/emscripten-site/docs/getting_started/FAQ.html#why-does-my-html-app-hang) — Brent Bradburn, Sep 15 '15 at 04:24

score 3 · Answer 1 · answered Sep 14 '15 at 20:35

3

From the emscripten source we find that fflush does nothing.

answered Sep 14 '15 at 20:35

orlp

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1

+1 for the good data. However, there seems to be some ambiguity in the linked pages about whether or not the no-op `fflush()` is relevant in the case where the strings include `\n` (as mine do). In other words, `stdout` might/should be "line-buffered" as described [here](http://stackoverflow.com/questions/3723795/is-stdout-line-buffered-unbuffered-or-indeterminate-by-default), and therefore each line should appear immediately, regardless of `fflush()`. I really only added the `fflush()` to prevent commenters from suggesting it. – Brent Bradburn Sep 14 '15 at 21:13

score 2 · Accepted Answer · answered Sep 15 '15 at 09:06

Perhaps that behavior is by design, thus the main loop needs to be replaced by emscripten_set_main_loop or Emterpreter-Async.

https://github.com/kripken/emscripten/wiki/Emterpreter

#include <stdio.h>
#include <emscripten/emscripten.h>

void time_waster( int reps=100 ) {
   static volatile unsigned counter=0;
   for ( int a=0; a<reps; a++ ) {
      for ( int b=0; b<1000*1000; b++ ) counter++;
   }
}

int main() {
  for ( int i=0; i<10; i++ ) {
     fprintf(stdout,"%d\n",i);
     fflush(stdout);
     emscripten_sleep(0);
     time_waster();
  }
}

em++ -Wall -Werror -o temp.html count.cpp -s EMTERPRETIFY=1 -s EMTERPRETIFY_ASYNC=1 -s EMTERPRETIFY_WHITELIST="['_main']" -Oz

This solves the problem. Note: [You need a web server to run it](http://stackoverflow.com/questions/10752055/cross-origin-requests-are-only-supported-for-http-error-when-loading-a-local). — Brent Bradburn, Sep 15 '15 at 16:43

text written to stdout doesn't appear until program completion

2 Answers2