All components of Flipper Zero firmware — services, user applications, and system settings — are developed independently. Each component has a build system manifest file, named `application.fam`, which defines the basic properties of that component and its relations to other parts of the system.
When building firmware, **`fbt`** collects all application manifests and processes their dependencies. Then it builds only those components referenced in the current build configuration. See [fbt docs](./fbt.md#firmware-application-set) for details on build configurations.
* **appid**: string, application id within the build system. Used to specify which applications to include in the build configuration and resolve dependencies and conflicts.
* **entry_point**: C function to be used as the application's entry point. Note that C++ function names are mangled, so you need to wrap them in `extern "C"` to use them as entry points.
* **cdefines**: C preprocessor definitions to declare globally for other apps when the current application is included in the active build configuration.
* **requires**: List of application IDs to include in the build configuration when the current application is referenced in the list of applications to build.
* **conflicts**: List of application IDs that the current application conflicts with. If any of them is found in the constructed application list, **`fbt`** will abort the firmware build process.
* **stack_size**: Stack size, in bytes, to allocate for application on its startup. Note that allocating a stack too small for an app to run will cause a system crash due to stack overflow, and allocating too much stack space will reduce usable heap memory size for apps to process data. *Note: you can use `ps`, and `free` CLI commands to profile your app's memory usage.*
* **icon**: Animated icon name from built-in assets to be used when building the app as a part of the firmware.
* **order**: Order of an application within its group when sorting entries in it. The lower the order is, the closer to the start of the list the item is placed. *Used for ordering startup hooks and menu entries.*
* **targets**: list of strings, target names, which this application is compatible with. If not specified, the application is built for all targets. The default value is `["all"]`.
* **sources**: list of strings, file name masks used for gathering sources within the app folder. The default value of `["*.c*"]` includes C and C++ source files. Applications cannot use the `"lib"` folder for their own source code, as it is reserved for **fap_private_libs**.
* **fap_version**: tuple, 2 numbers in the form of (x,y): application version to be embedded within .fap file. The default value is (0,1), meaning version "0.1".
* **fap_libs**: list of extra libraries to link the application against. Provides access to extra functions that are not exported as a part of main firmware at the expense of increased .fap file size and RAM consumption.
* **fap_category**: string, may be empty. App subcategory, also determines the path of the FAP within apps folder in the file system.
* **fap_icon_assets**: string. If present defines a folder name to be used for gathering image assets for this application. These images will be preprocessed and built alongside the application. See [FAP assets](./AppsOnSDCard.md#fap-assets) for details.
* **fap_extbuild**: provides support for parts of application sources to be built by external tools. Contains a list of `ExtFile(path="file name", command="shell command")` definitions. **`fbt`** will run the specified command for each file in the list.
Note that commands are executed at the firmware root folder's root, and all intermediate files must be placed in an application's temporary build folder. For that, you can use pattern expansion by **`fbt`**: `${FAP_WORK_DIR}` will be replaced with the path to the application's temporary build folder, and `${FAP_SRC_DIR}` will be replaced with the path to the application's source folder. You can also use other variables defined internally by **`fbt`**.
* **fap_private_libs**: a list of additional libraries distributed as sources alongside the application. These libraries will be built as a part of the application build process.
- **fap_include_paths**: list of library's relative paths to add to parent fap's include path list. The default value is `["."]` meaning the library's source root.
- **sources**: list of filename masks to be used for gathering include files for this library. Paths are relative to the library's source root. The default value is `["*.c*"]`.
- **cflags**: list of additional compiler flags to be used for building this library. The default value is `[]`.
- **cdefines**: list of additional preprocessor definitions to be used for building this library. The default value is `[]`.
- **cincludes**: list of additional include paths to be used for building this library. Paths are relative to the application's root. This can be used for providing external search paths for this library's code - for configuration headers. The default value is `[]`.
For that snippet, **`fbt`** will build 2 libraries: one from sources in `lib/mbedtls` folder and another from sources in `lib/loclass` folder. For the `mbedtls` library, **`fbt`** will add `lib/mbedtls/include` to the list of include paths for the application and compile only the files specified in the `sources` list. Additionally, **`fbt`** will enable `MBEDTLS_ERROR_C` preprocessor definition for `mbedtls` sources.
For the `loclass` library, **`fbt`** will add `lib/loclass` to the list of the include paths for the application and build all sources in that folder. Also, **`fbt`** will disable treating compiler warnings as errors for the `loclass` library, which can be useful when compiling large 3rd-party codebases.
