the externals-HOWTO (i almost lost the tex-sources, so here they are now)

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+/*
+ * HOWTO write an External for Pure data
+ * (c) 2001-2006 IOhannes m zmölnig zmoelnig[AT]iem.at
+ *
+ * this is the source-code for the second example in the HOWTO
+ * it creates an object that increments and outputs a counter
+ * whenever it gets banged.
+ *
+ * this file is released under Pd's license (BSD-like)
+ */
+
+
+
+/**
+ * include the interface to Pd 
+ */
+#include "m_pd.h"
+
+/**
+ * define a new "class" 
+ */
+static t_class *counter_class;
+
+
+
+/**
+ * this is the dataspace of our new object
+ * the first (mandatory) "t_object"
+ * and a variable that holds the current counter value
+ */
+typedef struct _counter {
+  t_object  x_obj;
+  t_int i_count;
+} t_counter;
+
+
+/**
+ * this method is called whenever a "bang" is sent to the object
+ * a reference to the class-dataspace is given as argument
+ * this enables us to do something with the data (e.g. increment the counter)
+ */
+void counter_bang(t_counter *x)
+{
+  /*
+   * convert the current counter value to floating-point to output it later
+   */
+  t_float f=x->i_count;
+  /* increment the counter */
+  x->i_count++;
+  /* send the old counter-value to the 1st outlet of the object */
+  outlet_float(x->x_obj.ob_outlet, f);
+}
+
+
+/**
+ * this is the "constructor" of the class
+ * we have one argument of type floating-point (as specified below in the counter_setup() routine)
+ */
+void *counter_new(t_floatarg f)
+{
+  t_counter *x = (t_counter *)pd_new(counter_class);
+
+  /* set the counter value to the given argument */
+  x->i_count=f;
+
+  /* create a new outlet for floating-point values */
+  outlet_new(&x->x_obj, &s_float);
+
+  return (void *)x;
+}
+
+
+/**
+ * define the function-space of the class
+ */
+void counter_setup(void) {
+  counter_class = class_new(gensym("counter"),
+			    (t_newmethod)counter_new,
+			    0, 
+			    sizeof(t_counter),
+			    CLASS_DEFAULT,
+			    A_DEFFLOAT, 0); /* the object takes one argument which is a floating-point and defaults to 0 */
+
+  /* call a function when object gets banged */
+  class_addbang(counter_class, counter_bang);
+}