GDB Internals

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## libgdb

### libgdb 1.0

`libgdb` 1.0 was an abortive project of years ago. The theory was to provide an API to GDB's functionality.

### libgdb 2.0

`libgdb` 2.0 is an ongoing effort to update GDB so that is better able to support graphical and other environments.

Since `libgdb` development is on-going, its architecture is still evolving. The following components have so far been identified:

- Observer - `gdb-events.h`.
- Builder - `ui-out.h`
- Event Loop - `event-loop.h`
- Library - `gdb.h`

The model that ties these components together is described below.

### The `libgdb` Model

A client of `libgdb` interacts with the library in two ways.

- As an observer (using `gdb-events`) receiving notifications from `libgdb` of any internal state changes (break point changes, run state, etc).
- As a client querying `libgdb` (using the `ui-out` builder) to obtain various status values from GDB.

Since `libgdb` could have multiple clients (e.g. a GUI supporting the existing GDB CLI), those clients must co-operate when controlling `libgdb`. In particular, a client must ensure that `libgdb` is idle (i.e. no other client is using `libgdb`) before responding to a `gdb-event` by making a query.

### CLI support

At present GDB's CLI is very much entangled in with the core of `libgdb`. Consequently, a client wishing to include the CLI in their interface needs to carefully co-ordinate its own and the CLI's requirements.

It is suggested that the client set `libgdb` up to be bi-modal (alternate between CLI and client query modes). The notes below sketch out the theory:

- The client registers itself as an observer of `libgdb`.
- The client create and install `cli-out` builder using its own versions of the `ui-file` `gdb_stderr`, `gdb_stdtarg` and `gdb_stdout` streams.
- The client creates a separate custom `ui-out` builder that is only used while making direct queries to `libgdb`.

When the client receives input intended for the CLI, it simply passes it along. Since the `cli-out` builder is installed by default, all the CLI output in response to that command is routed (pronounced rooted) through to the client controlled `gdb_stdout` et. al. streams. At the same time, the client is kept abreast of internal changes by virtue of being a `libgdb` observer.

The only restriction on the client is that it must wait until `libgdb` becomes idle before initiating any queries (using the client's custom builder).

### `libgdb` components

#### Observer - `gdb-events.h`

`gdb-events` provides the client with a very raw mechanism that can be used to implement an observer. At present it only allows for one observer and that observer must, internally, handle the need to delay the processing of any event notifications until after `libgdb` has finished the current command.

#### Builder - `ui-out.h`

`ui-out` provides the infrastructure necessary for a client to create a builder. That builder is then passed down to `libgdb` when doing any queries.

#### Event Loop - `event-loop.h`

`event-loop`, currently non-re-entrant, provides a simple event loop. A client would need to either plug its self into this loop or, implement a new event-loop that GDB would use.

The event-loop will eventually be made re-entrant. This is so that {No value for `GDB'} can better handle the problem of some commands blocking instead of returning.

#### Library - `gdb.h`

`libgdb` is the most obvious component of this system. It provides the query interface. Each function is parameterized by a `ui-out` builder. The result of the query is constructed using that builder before the query function returns.