Initial docs on using the C bindings.

mojo/public/c/bindings/README.md

+# C bindings guide
+
+The Mojo C bindings are a way to talk the Mojom protocol, the canonical protocol
+for communication between Mojo programs.  The library under `bindings/` provides
+functionality for encoding, decoding and other computation, so it needs to be
+linked together with C code generated from .mojom files.  These C bindings are
+lower-level than the C++ bindings (or any other language, for that matter),
+are more error-prone, and require some knowledge of the C Mojo API and the
+mojom encoding format. This document assumes the reader knows about (or knows
+how to look up) this relevant information. Consequently, C bindings can also
+be faster; generated bindings are smaller than the C++ equivalent, while
+encoding and decoding is faster.  The intention is to use them only when you
+require speed and flexibility.
+
+## Structs
+
+Lets look at what the generated code looks like for the following struct:

[viettrungluu, 2016/08/11 17:15:38]

Let's

[vardhan, 2016/08/11 21:48:33]

Done.

+
+``` mojom
+module example;
+
+enum Gender { MALE, FEMALE };
+struct Person {
+  uint32 age;
+  string name;
+  Gender gender;
+};
+```
+
+A small snippet of the generated C code for the struct and enum:
+
+```C
+// Generated code for mojom enum 'example.Gender'.
+typedef uint32_t example_Gender;
+enum example_Gender_Enum {
+  examples_Gender_MALE = 0,
+  examples_Gender_MALE = 1,
+};
+
+// Generated code for mojom struct 'example.Person'.
+union example_PersonPtr {
+  struct example_Person* ptr;
+  uint64_t offset;
+};
+struct example_Person {
+  struct MojomStructHeader header_;
+  uint32_t age;
+  example_Gender gender;
+  union MojomStringHeaderPtr name;
+};
+```
+
+The mojom wire format of a struct is comparable to the C memory model of a
+struct, with some restrictions;  we see that the order of the fields is
+different between the mojom and C structs, since the generated C structs are in
+packing order, not ordinal order.  The `example_PersonPtr` union is used to
+represent an offset in the encoded form, or a pointer in the unencoded form.
+Although not applicable in this example, there may be additional fields for
+padding purposes -- since is required since 4-byte datatypes need to be 4-byte
+aligned, so the generated bindings may include some fields not explicitly
+present in the mojom.  It is important to read the generated struct to make sure

[viettrungluu, 2016/08/11 17:15:39]

The sentence beginning "It is important" doesn't make sense to me.

[vardhan, 2016/08/11 21:48:33]

clarified.

+what the fields are, and if possible, use field initializers.
+
+Since mojom objects appear in the order of when they are referenced, we can use

[viettrungluu, 2016/08/11 17:15:38]

"when" doesn't really make sense.

[vardhan, 2016/08/11 21:48:33]

I changed this to "appear in depth-first order relative to their parent object,"
which i think makes sense but probably needs an extra explanation, but I think
that explanation belongs in a different doc (one that explains the mojom
encoding format)

+a `struct MojomBuffer` and calls to `MojomBuffer_Allocate(..)` to linearly
+allocate space. The struct needs to be constructed and provided by the user, and
+it contains 3 fields:  A pointer to the buffer, size of the buffer in bytes, and
+the byte-position of the next allocation, typically set to 0. 
+
+For instance, to allocate space for the `name` parameter of an `example_Person`,
+we can do so this way:
+```C
+char byte_buffer[512] = {0};
+struct MojomBuffer buf = {byte_buffer, sizeof(byte_buffer), 0};
+
+// First allocate space for the exmaple_Person struct:
+struct example_Person* person =
+   (struct example_Person*)MojomBuffer_Allocate(&buf, sizeof(struct example_Person));
+   
+// Allocate enough space for a 10 character string.
+person->name.ptr = (struct MojomStringHeader*)MojomBuffer_Allocate(
+    &buf,
+    sizeof(struct MojomStringHeader) + 10);
+```
+
+We can extract how much buffer space was used by reading `buf.num_byes_used`.
+
+Along with the C struct, there are some functions generated that help encode and
+decode mojom structs, amongst other things.  For the `example.Person` mojom
+struct, the following functions are generated:
+
+```c
+struct example_Person* example_Person_DeepCopy(
+  struct MojomBuffer* in_buffer,
+  struct example_Person* in_data);
+  
+void example_Person_EncodePointersAndHandles(
+  struct example_Person* inout_struct, uint32_t in_struct_size,
+  struct MojomHandleBuffer* inout_handle_buffer);
+  
+void example_Person_DecodePointersAndHandles(
+  struct example_Person* inout_struct, uint32_t in_struct_size,
+  MojomHandle inout_handles[], uin32_t in_num_handles);
+
+MojomValidationResult example_Person_Validate(
+  const struct example_Person* in_struct, uint32_t in_struct_size,
+  uint32_t in_num_handles);
+```
+
+The generated `example_Person_DeepCopy(..)`  function is used to copy over the
+`in_data`  into another buffer, specified by `MojomBuffer`.  The primary purpose
+of this function is "linearize" a given `struct example_Person` and its
+referenced objects into the new buffer.  This essentially recursively copies all
+objects in encoding order. The new returned copy can then be encoded.
+
+Example usage copying a struct example_Person `person` :
+```c
+...
+char byte_buffer[512] = {0};
+struct MojomBuffer buf = {byte_buffer, sizeof(byte_buffer), 0};
+struct example_Person* new_person = example_Person_DeepCopy(&buf, person);
+assert(new_person != NULL);
+...
+```
+
+The generated `example_Person_EncodePointersAndHandles(..)` is used to encode

[viettrungluu, 2016/08/11 17:15:38]

You really love the passive voice, don't you?

[vardhan, 2016/08/11 21:48:33]

Maybe rephrasing this to be "C structs can be encoded using `..`" but i feel
that keeping this common sentence structure while describing the generated
functions makes the text easier to consume *shrug*.

happy to change it if you find it is bothersome though.

[viettrungluu, 2016/08/11 22:02:41]

It is not particularly bothersome to me, but it is recommended by most style
guides to prefer the active voice over the passive.

+the given C struct so that it's in wire-format. This involves translating
+pointers into relative offsets, and extracting handles out of the struct into a
+separate handle array (and replacing the handle values in the struct with
+references into the handle array).  The supplied `struct MojomHandleBuffer`
+needs to be constructed and provided by the user and contains 3 fields: pointer
+to a handles array, the size of the array (number of elements), and the starting
+offset into the array where handles can be moved into (typically set to 0).
+
+The generated `example_Person_DecodePointersAndHandles(..)` does the inverse --
+it translates relative offsets into pointers, and moves handles out of the
+handle array and into the struct (based on the encoded offset into the array).
+In practice, before decoding a mojom struct into a usable C struct, it should be
+validated.
+
+The generated `example_Person_Validate(..)` validates an encoded `struct
+example_Person`. If valid, returns `MOJOM_VALIDATION_ERROR_NONE`, or the
+relevant error (see `bindings/validation.h` for more error codes).
+
+## Interfaces
+
+It isn't enough to talk to other mojo applications by encoding structs and
+referenced objects alone; communication happens via interface calls, so we need
+to frame our structs this way.  The following example describes what's generated
+for interfaces.  Consider an interface `Population` with a method `GetPerson()`
+that returns a `Person` object given their name:
+
+```mojom
+module example;
+
+[ServiceName="example::EmployeeRegistry"]
+interface EmployeeRegistry {
+  GetPerson(string name) => (Person person);
+};
+```
+
+The generated code:
+```C
+#define example_EmployeeRegistry__ServiceName \
+  ((const char*)"example::EmployeeRegistry")
+#define example_EmployeeRegistry__CurrentVersion ((uint32_t)0)
+
+// For message GetPerson:
+#define example_EmployeeRegistry_GetPerson__Ordinal ((uint32_t)0)
+#define example_EmployeeRegistry_GetPerson__MinVersion ((uint32_t)0)
+
+// Request struct for GetPerson():
+struct example_EmployeeRegistry_GetPerson_Request {
+  struct MojomStructHeader header_;
+  struct MojomStringHeaderPtr name;
+};
+
+// Response struct for GetPerson():
+struct example_EmployeeRegistry_GetPerson_Response {
+  struct MojomStructHeader header_;
+  struct example_PersonPtr person;
+};
+```
+
+We see that the parameters (and return values) of the `GetPerson(..)` message
+are just mojom structs.  To send a `GetPerson(..)` request, an interface request
+message needs to be constructed. An interface request message for
+`GetPerson(..)` consists of the following data in the following order:
+
+ 1. `struct MojomMessageWithRequestId`.  This contains:
+         - the message ordinal (generated above) which represents which message it is.
+         - flags that say if it's a request or response.
+         - a request ID, since this message is expecting a response.
+         -       (see `bindings/message.h`)
+ 2. `struct examples_EmployeeRegistry_GetPerson_Request`.  The actual parameters
+    for GetPerson() are represented by a mojom struct.
+
+Since the request parameters are just a mojom struct, all struct methods are
+also generated (see above), e.g, ` void
+examples_EmployeeRegistry_GetPerson_Request_EncodePointersAndHandles()`.  Once
+the request struct has been encoded, the buffer containing the above two structs
+can be written to a message pipe.
+
+On the other hand, when reading message (request or response), the message
+header must first be validated using
+```
+MojomValidationResult MojomMessage_ValidateHeader(const void* in_buf,
+                                                  uint32_t in_buf_size)
+```
+It is now safe to look at the `request_id` in `struct MojomMessage`.  If it is a
+known ordinal (by checking if its any of `example_EmployeeRegistry_*__Ordinal`),
+you can validate that it has a request or a response.  See `bindings/message.h`
+for more functions that help validate.
+
+## Enums and Constants
+
+Example mojom code:
+``` mojom
+module example;
+
+enum MyEnum { Zero, One, Four = 4, Five };
+const uint64 kMyConst = 34;
+```
+
+Generated C code:
+``` C
+typedef uint32_t example_MyEnum;
+enum example_MyEnum_Enum {
+  examples_MyEnum_Zero = 0,
+  examples_MyEnum_One = 1,
+  examples_MyEnum_Four = 4,
+  examples_MyEnum_Five = 5,
+};
+
+#define example_kMyConst ((uint64_t)34)
+```
+
+## Tagged Unions
+
+Example mojom code:
+``` mojom
+module example;
+
+union MyUnion {
+  int8 f0;
+  string f1;
+  MyUnion f2;
+};
+
+struct StructWithUnion {
+  MyUnion u;
+}
+```
+Generated C code:
+```C
+// Generated code for the Tags enum for |MyUnion|.
+typedef uint32_t example_MyUnion_Tag;
+enum example_MyUnion_Tag_Enum {
+  example_MyUnion_Tag_f0 = 0,
+  example_MyUnion_Tag_f1 = 1,
+  example_MyUnion_Tag_f2 = 2,
+  example_MyUnion_Tag__UNKNOWN__ = 0xFFFFFFFF,
+};
+
+// Generated code for |MyUnion|.
+union example_MyUnionPtr {
+  struct example_MyUnion* ptr;
+  uint64_t offset;
+};
+struct example_MyUnion {
+  uint32_t size;
+  example_MyUnion_Tag tag;
+  union {
+        int8_t f_f0;
+        union MojomStringHeaderPtr f_f1;
+        union example_MyUnionPtr f_f2;
+    uint64_t unknown;
+  } data;
+};
+
+// Snippet of generated code for |StructWithUnion|.
+struct example_StructWithUnion {
+  struct MojomStructHeader header_;
+  struct example_MyUnion u;
+};
+```
+
+Note that the `MyUnion` inside the `MyUnion` is a pointer object, whereas the
+`MyUnion` inside `StructWithUnion` is inlined. The only case when unions are
+pointer objects are when they are inside another union, otherwise they are
+inlined. Unions are initialized by setting their size and their tag. The size is
+always 16 bytes if the union is not null (4 for the size field, 4 for the tag,
+and 8 for the data).  The tag must be set to one defined in the generated enum
+of tags. The unknown tag isn't meant to be encoded over the wire, and exists as
+an initial value for a union's tag, but the tag should be set to something else
+before being written to wire. A union whose size is 0 is considered null. There
+are no functions generated for unions like they are for structs, since unions
+aren't ever encoded as a top-level data type that the programmer should have to
+serialize.
+
+## Arrays and Strings
+
+Arrays and strings (which are just arrays of characters) are not top-level data
+types; they can only be defined within a struct, union or interface method.
+Arrays inside structs are pointers to an array object. The array object's byte
+layout is as follow:
+ 1. `struct MojomArrayHeader`.  This contains:
+   - Number of bytes in the array (this includes the header and the data
+     following the array header; see `2.`)
+   - Number of elements in the array.
+   - (see `bindings/array.h` for more details)
+ 2. The contents of the array (the size of this is accounted for in the number
+    of bytes specified in the array header).
+
+Note that if the array contains pointer objects (structs, arrays, maps), the
+array contains only the 8-byte pointers (or offsets in its encoded form) -- the objects' data

[viettrungluu, 2016/08/11 17:15:38]

It'd be nice to wrap text (outside of quoted code, obviously) to 80 cols.

[vardhan, 2016/08/11 21:48:33]

Done.

+follow the array contents, and their size is not accounted for in the array header.
+
+Example of how to allocate and initialize a new array of 5 int32s, and set each one:
+```C
+...
+struct MojomArrayHeader* int32_array = MojomArray_New(&buf, 5, sizeof(int32_t));
+*MOJOM_ARRAY_INDEX(int32_array, int32_t, 0) = 10;
+*MOJOM_ARRAY_INDEX(int32_array, int32_t, 1) = 20;
+*MOJOM_ARRAY_INDEX(int32_array, int32_t, 2) = 30;
+*MOJOM_ARRAY_INDEX(int32_array, int32_t, 3) = 40;
+*MOJOM_ARRAY_INDEX(int32_array, int32_t, 4) = 50;
+```
+
+Here, `MojomArray_New(..)` allocates space for the buffer and initializes the
+header, while the `MOJOM_ARRAY_INDEX(.., i)` macro returns the address of the
+`i`th element.
+
+Since strings are just arrays of 1-byte characters, you can use

[viettrungluu, 2016/08/11 17:15:38]

Strictly speaking, strings are more than that, since they're supposed to be
UTF-8 encoded.

[vardhan, 2016/08/11 21:48:33]

Done.

+`MojomArray_New(&buf, STR_LENGTH, sizeof(uint8_t))` to allocate a string.  Note
+that the mojom strings by convention aren't null terminated (since their length
+is encoded in the array header).
+ 
+## Maps
+
+Maps on the wire are mojom structs with two arrays; one for the keys, and one
+for the values.  The `i`th element in the keys array corresponds to the `i`th
+element in the values array.  As such, both arrays must have the same number
+of elements, and neither arrays can be null.
+
+# Generated Bindings
+
+TODO(vardhan)