Merge pull request rust-lang#2315 from ferrous-systems/book-custom-ta…

…rget

Add FAQ about custom targets

book/src/faq.md

@@ -7,6 +7,7 @@
 - [Why isn't `bindgen` generating bindings to inline functions?](#why-isnt-bindgen-generating-bindings-to-inline-functions)
 - [Does `bindgen` support the C++ Standard Template Library (STL)?](#does-bindgen-support-the-c-standard-template-library-stl)
 - [How to deal with bindgen generated padding fields?](#how-to-deal-with-bindgen-generated-padding-fields)
+- [How to generate bindings for a custom target?](#how-to-generate-bindings-for-a-custom-target)
 
 <!-- END doctoc generated TOC please keep comment here to allow auto update -->
 
@@ -68,7 +69,7 @@ you're binding to that is pulling in STL headers.
 ### How to deal with bindgen generated padding fields?
 
 Depending the architecture, toolchain versions and source struct, it is
-possible that bindgen will generate padding fields named `__bindgen_padding_N`. 
+possible that bindgen will generate padding fields named `__bindgen_padding_N`.
 As these fields might be present when compiling for one architecture but not
 for an other, you should not initialize these fields manually when initializing
 the struct. Instead, use the `Default` trait. You can either enable this when
@@ -95,3 +96,15 @@ SRC_DATA {
 
 In the case bindgen generates a padding field, then this field will
 be automatically initialized by `..Default::default()`.
+
+### How to generate bindings for a custom target?
+
+To generate bindings for a custom target you only need to pass the `--target`
+argument to `libclang`. For example, if you want to generate bindings for the
+`armv7a-none-eabi` target using the command line, you need to invoke `bindgen`
+like so:
+```bash
+$ bindgen <input_headers> -- --target=armv7a-none-eabi
+```
+If you are using `bindgen` as a library, you should call
+`builder.clang_arg("--target=armv7a-none-eabi")` on your `builder`.