examples: Add example code for user C modules, both C and C++.

Add working example code to provide a starting point for users with files
that they can just copy, and include the modules in the coverage test to
verify the complete user C module build functionality.  The cexample module
uses the code originally found in cmodules.rst, which has been updated to
reflect this and partially rewritten with more complete information.

examples/usercmodule/cexample/examplemodule.c

@@ -0,0 +1,34 @@
+// Include MicroPython API.
+#include "py/runtime.h"
+
+// This is the function which will be called from Python as cexample.add_ints(a, b).
+STATIC mp_obj_t example_add_ints(mp_obj_t a_obj, mp_obj_t b_obj) {
+    // Extract the ints from the micropython input objects.
+    int a = mp_obj_get_int(a_obj);
+    int b = mp_obj_get_int(b_obj);
+
+    // Calculate the addition and convert to MicroPython object.
+    return mp_obj_new_int(a + b);
+}
+// Define a Python reference to the function above.
+STATIC MP_DEFINE_CONST_FUN_OBJ_2(example_add_ints_obj, example_add_ints);
+
+// Define all properties of the module.
+// Table entries are key/value pairs of the attribute name (a string)
+// and the MicroPython object reference.
+// All identifiers and strings are written as MP_QSTR_xxx and will be
+// optimized to word-sized integers by the build system (interned strings).
+STATIC const mp_rom_map_elem_t example_module_globals_table[] = {
+    { MP_ROM_QSTR(MP_QSTR___name__), MP_ROM_QSTR(MP_QSTR_cexample) },
+    { MP_ROM_QSTR(MP_QSTR_add_ints), MP_ROM_PTR(&example_add_ints_obj) },
+};
+STATIC MP_DEFINE_CONST_DICT(example_module_globals, example_module_globals_table);
+
+// Define module object.
+const mp_obj_module_t example_user_cmodule = {
+    .base = { &mp_type_module },
+    .globals = (mp_obj_dict_t *)&example_module_globals,
+};
+
+// Register the module to make it available in Python.
+MP_REGISTER_MODULE(MP_QSTR_cexample, example_user_cmodule, MODULE_CEXAMPLE_ENABLED);

examples/usercmodule/cexample/micropython.mk

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+EXAMPLE_MOD_DIR := $(USERMOD_DIR)
+
+# Add all C files to SRC_USERMOD.
+SRC_USERMOD += $(EXAMPLE_MOD_DIR)/examplemodule.c
+
+# We can add our module folder to include paths if needed
+# This is not actually needed in this example.
+CFLAGS_USERMOD += -I$(EXAMPLE_MOD_DIR)
+CEXAMPLE_MOD_DIR := $(USERMOD_DIR)

examples/usercmodule/cppexample/example.cpp

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+extern "C" {
+#include <examplemodule.h>
+
+// Here we implement the function using C++ code, but since it's
+// declaration has to be compatible with C everything goes in extern "C" scope.
+mp_obj_t cppfunc(mp_obj_t a_obj, mp_obj_t b_obj) {
+    // Prove we have (at least) C++11 features.
+    const auto a = mp_obj_get_int(a_obj);
+    const auto b = mp_obj_get_int(b_obj);
+    const auto sum = [&]() {
+        return mp_obj_new_int(a + b);
+    } ();
+    // Prove we're being scanned for QSTRs.
+    mp_obj_t tup[] = {sum, MP_ROM_QSTR(MP_QSTR_hellocpp)};
+    return mp_obj_new_tuple(2, tup);
+}
+}

examples/usercmodule/cppexample/examplemodule.c

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+#include <examplemodule.h>
+
+// Define a Python reference to the function we'll make available.
+// See example.cpp for the definition.
+STATIC MP_DEFINE_CONST_FUN_OBJ_2(cppfunc_obj, cppfunc);
+
+// Define all properties of the module.
+// Table entries are key/value pairs of the attribute name (a string)
+// and the MicroPython object reference.
+// All identifiers and strings are written as MP_QSTR_xxx and will be
+// optimized to word-sized integers by the build system (interned strings).
+STATIC const mp_rom_map_elem_t cppexample_module_globals_table[] = {
+    { MP_ROM_QSTR(MP_QSTR___name__), MP_ROM_QSTR(MP_QSTR_cppexample) },
+    { MP_ROM_QSTR(MP_QSTR_cppfunc), MP_ROM_PTR(&cppfunc_obj) },
+};
+STATIC MP_DEFINE_CONST_DICT(cppexample_module_globals, cppexample_module_globals_table);
+
+// Define module object.
+const mp_obj_module_t cppexample_user_cmodule = {
+    .base = { &mp_type_module },
+    .globals = (mp_obj_dict_t *)&cppexample_module_globals,
+};
+
+// Register the module to make it available in Python.
+MP_REGISTER_MODULE(MP_QSTR_cppexample, cppexample_user_cmodule, MODULE_CPPEXAMPLE_ENABLED);

examples/usercmodule/cppexample/examplemodule.h

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+// Include MicroPython API.
+#include "py/runtime.h"
+
+// Declare the function we'll make available in Python as cppexample.cppfunc().
+extern mp_obj_t cppfunc(mp_obj_t a_obj, mp_obj_t b_obj);

examples/usercmodule/cppexample/micropython.mk

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+CPPEXAMPLE_MOD_DIR := $(USERMOD_DIR)
+
+# Add our source files to the respective variables.
+SRC_USERMOD += $(CPPEXAMPLE_MOD_DIR)/examplemodule.c
+SRC_USERMOD_CXX += $(CPPEXAMPLE_MOD_DIR)/example.cpp
+
+# Add our module directory to the include path.
+CFLAGS_USERMOD += -I$(CPPEXAMPLE_MOD_DIR)
+CXXFLAGS_USERMOD += -I$(CPPEXAMPLE_MOD_DIR)
+
+# We use C++ features so have to link against the standard library.
+LDFLAGS_USERMOD += -lstdc++