diff --git a/libc/stdio/stdio_ext.cpp b/libc/stdio/stdio_ext.cpp
index 945813e..99a8af7 100644
--- a/libc/stdio/stdio_ext.cpp
+++ b/libc/stdio/stdio_ext.cpp
@@ -67,6 +67,12 @@
   return fp->_p - fp->_bf._base;
 }
 
+size_t __freadahead(FILE* fp) {
+  // Normally _r is the amount of input already available.
+  // When there's ungetc() data, _r counts that and _ur is the previous _r.
+  return fp->_r + (HASUB(fp) ? fp->_ur : 0);
+}
+
 void _flushlbf() {
   // If we flush all streams, we know we've flushed all the line-buffered streams.
   fflush(nullptr);