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diff --git a/include/gras/io_signature.hpp b/include/gras/io_signature.hpp
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+++ b/include/gras/io_signature.hpp
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+//
+// Copyright 2012 Josh Blum
+//
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Lesser General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// This program 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 Lesser General Public License for more details.
+//
+// You should have received a copy of the GNU Lesser General Public License
+// along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+#ifndef INCLUDED_GRAS_IO_SIGNATURE_HPP
+#define INCLUDED_GRAS_IO_SIGNATURE_HPP
+
+#include <vector>
+#include <stdexcept>
+
+namespace gras
+{
+
+/*!
+ * An IO signature describes the input or output specifications
+ * for the streaming input or output ports of a block.
+ * Properties are a maximum and minimum number of ports,
+ * and an item size in bytes for each port.
+ */
+struct IOSignature : std::vector<int>
+{
+    static const int IO_INFINITE = -1;
+
+    //! Create an empty signature with infinite IO
+    IOSignature(void)
+    {
+        this->set_min_streams(IO_INFINITE);
+        this->set_max_streams(IO_INFINITE);
+    }
+
+    //! Create a signature with the specified min and max streams
+    IOSignature(const int min_streams, const int max_streams)
+    {
+        if (min_streams > max_streams and max_streams != IO_INFINITE)
+        {
+            throw std::invalid_argument("io signature fail: min_streams > max_streams");
+        }
+        this->set_min_streams(min_streams);
+        this->set_max_streams(max_streams);
+    }
+
+    //! Construct from pointer for backwards compatible shared_ptr usage.
+    IOSignature(const IOSignature *sig)
+    {
+        *this = *sig;
+    }
+
+    //! Overloaded arrow operator for backwards compatible shared_ptr usage.
+    IOSignature* operator->(void)
+    {
+        return this;
+    };
+
+    //! Overloaded arrow operator for backwards compatible shared_ptr usage.
+    const IOSignature* operator->(void) const
+    {
+        return this;
+    };
+
+    void set_min_streams(const int val)
+    {
+        _min_streams = val;
+    }
+
+    void set_max_streams(const int val)
+    {
+        _max_streams = val;
+    }
+
+    int min_streams(void) const
+    {
+        return _min_streams;
+    }
+
+    int max_streams(void) const
+    {
+        return _max_streams;
+    }
+
+    int sizeof_stream_item(const int index) const
+    {
+        if (this->empty()) return 0;
+        if (this->size() > unsigned(index))
+        {
+            return this->at(index);
+        }
+        return this->back();
+    }
+
+    std::vector<int> sizeof_stream_items(void) const
+    {
+        return *this;
+    }
+
+    int _min_streams;
+    int _max_streams;
+};
+
+} //namespace gras
+
+#endif /*INCLUDED_GRAS_IO_SIGNATURE_HPP*/