2 files changed, 33 insertions, 2 deletions
diff --git a/src/library.js b/src/library.js
index 9a1f29e5..f7c7a3ba 100644
--- a/src/library.js
+++ b/src/library.js
@@ -3660,8 +3660,23 @@ LibraryManager.library = {
     // http://pubs.opengroup.org/onlinepubs/000095399/functions/scanf.html
     if (FS.streams[stream]) {
       var i = _ftell(stream), SEEK_SET = 0;
-      var get = function () { i++; return _fgetc(stream); };
-      var unget = function () { _fseek(stream, --i, SEEK_SET); };
+      // if the stream does not support seeking backwards (e.g. stdin), buffer it here
+      var buffer = [], bufferIndex = 0;
+      var get = function() {
+        if (bufferIndex < buffer.length) {
+          return buffer[bufferIndex++];
+        }
+        i++;
+        bufferIndex++;
+        var c = _fgetc(stream);
+        buffer.push(c);
+        return c;
+      };
+      var unget = function() {
+        if (_fseek(stream, --i, SEEK_SET) !== 0) {
+          bufferIndex--;
+        }
+      };
       return __scanString(format, get, unget, varargs);
     } else {
       return -1;
diff --git a/tests/runner.py b/tests/runner.py
index effdfb20..77f36cf5 100755
--- a/tests/runner.py
+++ b/tests/runner.py
@@ -11227,6 +11227,22 @@ f.close()
 
       self.assertContained('libf1\nlibf2\n', run_js(os.path.join(self.get_dir(), 'a.out.js')))
 
+    def test_stdin(self):
+      open('main.cpp', 'w').write(r'''
+#include <stdio.h>
+int main(int argc, char const *argv[])
+{
+    char str[10] = {0};
+    scanf("%10s", str);
+    printf("%s\n", str);
+    return 0;
+}
+''')
+      Building.emcc('main.cpp', output_filename='a.out.js')
+      open('in.txt', 'w').write('abc')
+      # node's stdin support is broken
+      self.assertContained('abc', Popen(listify(SPIDERMONKEY_ENGINE) + ['a.out.js'], stdin=open('in.txt'), stdout=PIPE, stderr=PIPE).communicate()[0])
+
     def test_abspaths(self):
       # Includes with absolute paths are generally dangerous, things like -I/usr/.. will get to system local headers, not our portable ones.