1# FIDL: Compiler Interface
2
3This document describes the command-line interface to the FIDL compiler.
4
5Information on the internals of that tool lives
6[alongside the source of the tool](../../system/host/fidl/README.md).
7
8See [FIDL: Overview](index.md) for more information about FIDL's overall
9purpose, goals, and requirements, as well as links to related documents.
10
11## Overview
12
13The FIDL compiler is split into a frontend and a number of backends. The
14compiler processes one library at a time. The frontend consumes the FIDL
15declarations for the library (as well as for all transitive dependencies),
16performs semantic analysis, and outputs an intermediate representation of the
17library. The backends consume the intermediate representation and generate
18language-specific bindings for the library.
19
20## Frontend
21
22The frontend is a command-line program named `fidlc`. The `fidlc` compiler takes
23a number of arguments:
24
25 * `--c-header HEADER_PATH`. If present, this flag instructs `fidlc` to output
26   a C header at the given path. In principle, the C header generation could
27   have been implemented as a C backend, but for some practical reasons, we
28   integrated the C header generation directly into the frontend.
29
30 * `--tables TABLES_PATH`. If present, this flag instructs `fidlc` to output
31   coding tables at the given path. The coding tables are required to encode and
32   decode messages from the C and C++ bindings.
33
34 * `--json JSON_PATH`. If present, this flag instructs `fidlc` to output the
35   library's intermediate representation at the given path. The intermediate
36   representation is JSON that conforms to [a particular schema](../../system/host/fidl/schema.json).
37   The intermediate representation is used as input to the various backends.
38
39 * `--name LIBRARY_NAME`. If present, this flag instructs `fidlc` to validate
40   that the library being compiled has the given name. This flag is useful to
41   cross-check between the library's declaration in a build system and the
42   actual contents of the library.
43
44 * `--files [FIDL_FILE...]...`. Each `--file [FIDL_FILE...]` chunk of arguments
45   describes a library, all of which must share the same top-level library name
46   declaration. Libraries must be presented in dependency order, with later
47   libraries able to use declarations from preceding libraries but not vice versa.
48   Output is only generated for the final library, not for each of its dependencies.
49
50All of the arguments can also be provided via a response file, denoted as
51`@responsefile`. The contents of the file at `responsefile` will be interpreted
52as a whitespace-delimited list of arguments. Response files cannot be nested,
53and must be the only argument.
54
55## Backend
56
57The backend is a command-line program named `fidlgen`. The `fidlgen` compiler
58takes a number of arguments:
59
60 * `--json` (required). The path to the intermediate representation of the
61   library. The intermediate representation is JSON that conforms to
62   [a particular schema](../../system/host/fidl/schema.json).
63
64 * `--generators` (required). A comma-separated list of generators to run on the
65   given library. The following generators are currently supported: `cpp`, `go`,
66   `dart`, and `rust`.
67
68 * `--output-base` (required). The base file name for files generated by this
69   generator. The generator will create files by adding extensions to this file
70   name. For example, the `cpp` backend generates two files, one with the `.h`
71   extension and another with the `.cc` extension.
72
73 * `--include-base` (required). The base directory relative to which C and C++
74   `#include` directives should be computed. For example, when the `cpp` backend
75   generates an `#include` directive to reference the `.h` file from the `.cc`
76   file, the backend will create the `#include` path relative to this directory.
77
78## Limitations
79
80For the `cpp` backend, the generated `.h` file must be incluable as
81`#include <fuchsia/cpp/$LIBRARY_NAME.h>`, where `$LIBRARY_NAME` is the name of
82the corresponding FIDL library. Typically, that means that the `--output-base`
83flag will have the value `$INCLUDE_BASE/fuchsia/cpp/$LIBRARY_NAME`, where
84`$INCLUDE_BASE` is the value of the `--include-base` flag.
85