libc: Implement ungetc

libc/src/stdio.cpp

@@ -688,9 +688,18 @@ extern "C"
         return f;
     }
 
-    int ungetc(int, FILE*)
+    int ungetc(int c, FILE* stream)
     {
-        fail("FIXME: ungetc: not implemented");
+        if (stream->_buf.index == 0)
+            return EOF; // No data currently in the read buffer, or no data has been read from it.
+
+        if (stream->_buf.mode == _IONBF)
+            return EOF; // FIXME: C doesn't state that ungetc() should only work on buffered streams.
+
+        stream->_buf.index--;
+        stream->_buf.buffer[stream->_buf.index] = (char)c;
+
+        return 0;
     }
 
     int setvbuf(FILE* stream, char* buf, int mode, size_t size)