Add memory type handling (micro-ROS#312)

pablogs9 · Acuadros95 · web-flow · commit 0f7b031aa6a1 · 2021-05-13T11:34:12.000+02:00
* Add memory type handling

* Apply suggestions from code review

Co-authored-by: Antonio Cuadros &lt;49162117+Acuadros95@users.noreply.github.com&gt;

Co-authored-by: Antonio Cuadros &lt;49162117+Acuadros95@users.noreply.github.com&gt;
diff --git a/_data/docs.yml b/_data/docs.yml
@@ -57,6 +57,7 @@
    - tutorials/advanced/overview
    - tutorials/advanced/microxrcedds_rmw_configuration
    - tutorials/advanced/create_new_type
+   - tutorials/advanced/handling_type_memory
    - tutorials/advanced/create_dds_entities_by_ref
    - tutorials/advanced/create_custom_transports
    - tutorials/advanced/create_custom_static_library
diff --git a/_docs/tutorials/advanced/handling_type_memory/index.md b/_docs/tutorials/advanced/handling_type_memory/index.md
@@ -0,0 +1,193 @@
+---
+title: Handling messages memory in micro-ROS
+permalink: /docs/tutorials/advanced/handling_type_memory/
+---
+
+This page aims to explain how to handle messages and types memory in micro-ROS.
+
+First of all, since the micro-ROS user is in an embedded C99 environment, it is important to be aware of what messages and ROS 2 types are being used in order to handle memory correctly.
+
+By watching the `.msg` or `.srv` of the types used in a micro-ROS application, you can determine the type of each member. Currently, the following types are supported:
+- Basic type
+- Array type
+- Sequence type
+- Compound type
+
+Let's take an example `.mgs` for clarification:
+
+```
+# MyType.msg
+std_msgs/Header header
+int32[] values
+float64 duration
+int8[10] coefficients
+string name
+```
+
+In this example:
+- the member `duration` is a **basic type member**,.
+- the member `values` is a **sequence type member** because it has a unbounded sequence of `int32`, in this case.
+- the member `coefficients` is an **array type member** because it has a bounded sequence of 10 units of `int8`, in this case.
+- the member `header` is an **compound type member** because it refers to complex type described in the same or other ROS 2 package.
+- the member `name` is an **string type member** and should be understood as a `char[]` (sequence type member).
+
+When dealing with the **micro-ROS typesupport** the developer needs to take into account how this message is going to be handled in the C99 API of micro-ROS. In general, the micro-ROS typesupport will create a C99 struct representation of the message:
+
+```c
+typedef struct mypackage__msg__MyType
+{
+  std_msgs__msg__Header header;
+  rosidl_runtime_c__int32__Sequence values;
+  double duration;
+  int8 coefficients[10];
+  rosidl_runtime_c__String name;  // equal to rosidl_runtime_c__char__Sequence
+} mypackage__msg__MyType;
+```
+
+So when in an application has a variable of this type, for example `mypackage__msg__MyType mymsg;`, we know that:
+- `mymsg.coefficients` has a C array of `int8`
+- `mymsg.duration` is a `double` member
+
+but, what happens with the `...Sequence` and the compound type member?
+
+## Sequence types in micro-ROS
+
+A **sequence type member** is an especial type member that hosts a pointer `data`, a `size` and a `capacity` value. The pointer should have memory for storing up to `capacity` values and `size` member shows how many element are currently in the sequence. Usually in micro-ROS, the user is in charge of assigning memory and values to this sequence members.
+
+In the case of `MyType.msg`, the `values` sequence member is represented in C99 as this struct:
+
+```c
+typedef struct rosidl_runtime_c__int32__Sequence
+{ 
+  int32_t* data;    /* The pointer to an array of int32 */
+  size_t size;      /* The number of valid items in data */
+  size_t capacity;  /* The number of allocated items in data */
+} rosidl_runtime_c__int32__Sequence;
+```
+
+So user need to handle the type like:
+
+```c
+mypackage__msg__MyType mymsg;
+
+// mymsg.values.data is NULL or garbage now
+// mymsg.values.size is 0 or garbage now
+// mymsg.values.capacity is 0 or garbage now
+
+// Assigning dynamic memory to the sequence
+mymsg.values.capacity = 100;
+mymsg.values.data = (int32_t*) malloc(mymsg.values.capacity * sizeof(int32_t));
+mymsg.values.size = 0;
+
+// Assigning static memory to the sequence
+static int32_t memory[100];
+mymsg.values.capacity = 100;
+mymsg.values.data = memory;
+mymsg.values.size = 0;
+
+// Filling some data
+for(int32_t i = 0; i < 3; i++){
+  mymsg.values.data = i;
+  mymsg.values.size++;
+}
+```
+
+## Compound types in micro-ROS
+
+When dealing with a compound type, the user should recursively inspect the types in order to determine how to handle each internal member. 
+
+For example in the `MyType.msg` example, the `header` member has the following structure:
+
+```c
+typedef struct std_msgs__msg__Header
+{
+  builtin_interfaces__msg__Time stamp;
+  rosidl_runtime_c__String frame_id;
+} std_msgs__msg__Header;
+```
+
+Remember that `rosidl_runtime_c__String` is equivalent to `rosidl_runtime_c__char__Sequence`. And `builtin_interfaces__msg__Time` looks like:
+
+```c
+typedef struct builtin_interfaces__msg__Time
+{
+  int32_t sec;
+  uint32_t nanosec;
+} builtin_interfaces__msg__Time;
+```
+
+To initialize the `header` member of `MyType.msg`:
+
+```c
+mypackage__msg__MyType mymsg;
+
+// Assigning dynamic memory to the frame_id char sequence
+mymsg.header.frame_id.capacity = 100;
+mymsg.header.frame_id.data = (char*) malloc(mymsg.values.capacity * sizeof(char));
+mymsg.header.frame_id.size = 0;
+
+// Assigning value to the frame_id char sequence
+strcpy(mymsg.header.frame_id.data, "Hello World");
+mymsg.header.frame_id.size = strlen(mymsg.header.frame_id.data);
+
+// Assigning value to other members
+mymsg.stamp.sec = 10;
+mymsg.stamp.nanosec = 20;
+```
+
+## Sequences of compound types
+
+Users should take into account that **sequence type member** of **compound type member** are also valid ROS 2 type. For example, let's modify the previous example:
+
+```
+# MyComplexType.msg
+std_msgs/Header[] multiheaders
+int32[] values
+float64 duration
+int8[10] coefficients
+string name
+```
+
+In this case, the generated typesupport will be: 
+
+```c
+typedef struct mypackage__msg__MyComplexType
+{
+  std_msgs__msg__Header__Sequence multiheaders;
+  rosidl_runtime_c__int32__Sequence values;
+  double duration;
+  int8 coefficients[10];
+  rosidl_runtime_c__String name;  // equal to rosidl_runtime_c__char__Sequence
+} mypackage__msg__MyComplexType;
+```
+
+Notice that `multiheaders` is a **sequence type member**, so it should be handled properly, but also it is a **compound type member** which needs to be handled recursively, initializing its own members. For example:
+
+```c
+mypackage__msg__MyComplexType mymsg;
+
+// Init the multiheaders sequence
+mymsg.multiheaders.capacity = 10;
+mymsg.multiheaders.data = (std_msgs__msg__Header*) malloc(mymsg.values.capacity * sizeof(std_msgs__msg__Header));
+mymsg.multiheaders.size = 0;
+
+// Filling some data
+for(int32_t i = 0; i < 3; i++){
+  mymsg.values.data = i;
+
+  // Add memory to this sequence element frame_id
+  mymsg.multiheaders.data[i].frame_id.capacity = 100;
+  mymsg.multiheaders.data[i].frame_id.data = (char*) malloc(mymsg.multiheaders.data[i].frame_id.capacity * sizeof(char));
+  mymsg.multiheaders.data[i].frame_id.size = 0;
+
+  // Assigning value to the frame_id char sequence
+  strcpy(mymsg.multiheaders.data[i].frame_id.data, "Hello World");
+  mymsg.multiheaders.data[i].frame_id.size = strlen(mymsg.multiheaders.data[i].frame_id.data);
+
+  // Assigning value to other members
+  mymsg.multiheaders.data[i].stamp.sec = 10;
+  mymsg.multiheaders.data[i].stamp.nanosec = 20;
+
+  mymsg.multiheaders.size++;
+}
+```
