added a section on string manipulation

snippets/stringarg/micropython.mk

@@ -0,0 +1,8 @@
+USERMODULES_DIR := $(USERMOD_DIR)
+
+# Add all C files to SRC_USERMOD.
+SRC_USERMOD += $(USERMODULES_DIR)/stringarg.c
+
+# We can add our module folder to include paths if needed
+# This is not actually needed in this example.
+CFLAGS_USERMOD += -I$(USERMODULES_DIR)

snippets/stringarg/stringarg.c

@@ -0,0 +1,43 @@
+/*
+ * This file is part of the micropython-usermod project, 
+ *
+ * https://github.com/v923z/micropython-usermod
+ *
+ * The MIT License (MIT)
+ *
+ * Copyright (c) 2019 Zoltán Vörös
+*/
+    
+#include <string.h>
+#include "py/obj.h"
+#include "py/runtime.h"
+#include "py/objstr.h"
+
+#define byteswap(a,b) char tmp = a; a = b; b = tmp; 
+
+STATIC mp_obj_t stringarg_function(const mp_obj_t o_in) {
+    mp_check_self(mp_obj_is_str_or_bytes(o_in));
+    GET_STR_DATA_LEN(o_in, str, str_len);
+    printf("string length: %lu\n", str_len);
+    char out_str[str_len];
+    strcpy(out_str, (char *)str);
+    for(size_t i=0; i < (str_len-1)/2; i++) {
+        byteswap(out_str[i], out_str[str_len-i-1]);
+    }
+    return mp_obj_new_str(out_str, str_len);
+} 
+
+STATIC MP_DEFINE_CONST_FUN_OBJ_1(stringarg_function_obj, stringarg_function);
+
+STATIC const mp_rom_map_elem_t stringarg_module_globals_table[] = {
+    { MP_ROM_QSTR(MP_QSTR___name__), MP_ROM_QSTR(MP_QSTR_stringarg) },
+    { MP_ROM_QSTR(MP_QSTR_stringarg), MP_ROM_PTR(&stringarg_function_obj) },
+};
+STATIC MP_DEFINE_CONST_DICT(stringarg_module_globals, stringarg_module_globals_table);
+
+const mp_obj_module_t stringarg_user_cmodule = {
+    .base = { &mp_type_module },
+    .globals = (mp_obj_dict_t*)&stringarg_module_globals,
+};
+
+MP_REGISTER_MODULE(MP_QSTR_stringarg, stringarg_user_cmodule, MODULE_STRINGARG_ENABLED);