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diff --git a/gr-howto-write-a-block/lib/square_ff_impl.cc b/gr-howto-write-a-block/lib/square_ff_impl.cc
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+++ b/gr-howto-write-a-block/lib/square_ff_impl.cc
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+/* -*- c++ -*- */
+/*
+ * Copyright 2004,2010,2012 Free Software Foundation, Inc.
+ *
+ * This file is part of GNU Radio
+ *
+ * GNU Radio is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 3, or (at your option)
+ * any later version.
+ *
+ * GNU Radio is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with GNU Radio; see the file COPYING.  If not, write to
+ * the Free Software Foundation, Inc., 51 Franklin Street,
+ * Boston, MA 02110-1301, USA.
+ */
+
+/*
+ * config.h is generated by configure.  It contains the results
+ * of probing for features, options etc.  It should be the first
+ * file included in your .cc file.
+ */
+#ifdef HAVE_CONFIG_H
+#include "config.h"
+#endif
+
+#include "square_ff_impl.h"
+#include <gr_io_signature.h>
+
+namespace gr {
+  namespace howto {
+    
+    /*
+     * Create a new instance of howto_square_ff and return a boost
+     * shared_ptr. This is effectively the public constructor.
+     */
+    square_ff::sptr
+    square_ff::make()
+    {
+      return gnuradio::get_initial_sptr
+	(new square_ff_impl());
+    }
+
+    /*
+     * Specify constraints on number of input and output streams.
+     * This info is used to construct the input and output signatures
+     * (2nd & 3rd args to gr_block's constructor).  The input and
+     * output signatures are used by the runtime system to
+     * check that a valid number and type of inputs and outputs
+     * are connected to this block.  In this case, we accept
+     * only 1 input and 1 output.
+     */
+    static const int MIN_IN = 1;	// mininum number of input streams
+    static const int MAX_IN = 1;	// maximum number of input streams
+    static const int MIN_OUT = 1;	// minimum number of output streams
+    static const int MAX_OUT = 1;	// maximum number of output streams
+
+    /*
+     * The private constructor
+     */
+    square_ff_impl::square_ff_impl()
+      : gr_block("square_ff",
+		 gr_make_io_signature(MIN_IN, MAX_IN, sizeof(float)),
+		 gr_make_io_signature(MIN_OUT, MAX_OUT, sizeof(float)))
+    {
+      // nothing else required in this example
+    }
+
+    /*
+     * Our virtual destructor.
+     */
+    square_ff_impl::~square_ff_impl()
+    {
+      // nothing else required in this example
+    }
+
+    int
+    square_ff_impl::general_work(int noutput_items,
+				 gr_vector_int &ninput_items,
+				 gr_vector_const_void_star &input_items,
+				 gr_vector_void_star &output_items)
+    {
+      const float *in = (const float*)input_items[0];
+      float *out = (float*)output_items[0];
+
+      for(int i = 0; i < noutput_items; i++) {
+	out[i] = in[i] * in[i];
+      }
+
+      // Tell runtime system how many input items we consumed on
+      // each input stream.
+
+      consume_each(noutput_items);
+
+      // Tell runtime system how many output items we produced.
+      return noutput_items;
+    }
+
+  } /* namespace howto */
+} /* namespace gr */