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Overview

The bulk of Tock applications are written in C.

Entry Point

Applications written in C that compile against libtock should define a main method with the following signature:

int main(void);

Applications should return 0 from main. Returning non-zero is undefined and the behavior may change in future versions of libtock. Today, main is called from _start and includes an implicit while() loop:

void _start(void* text_start, void* mem_start, void* memory_len, void* app_heap_break) {
  main();
  while (1) {
    yield();
  }
}

Applications should set up a series of event subscriptions in their main method and then return.

Stack and Heap

Applications can specify their required stack and heap sizes by defining the make variables STACK_SIZE and APP_HEAP_SIZE, which default to 2K and 1K respectively as of this writing.

libtock will set the stack pointer during startup. To allow each application to set its own stack size, the linker script expects a symbol STACK_SIZE to be defined. The Tock build system will define this symbol during linking, using the make variable STACK_SIZE. A consequence of this technique is that changing the stack size requires that any source file also be touched so that the app will re-link.

Libraries

Application code does not need to stand alone, libraries are available that can be utilized!

Newlib

Application code written in C has access to most of the C standard library which is implemented by Newlib. Newlib is focused on providing capabilities for embedded systems. It provides interfaces such as printf, malloc, and memcpy. Most, but not all features of the standard library are available to applications. The built configuration of Newlib is specified in build.sh.

libtock

In order to interact with the Tock kernel, application code can use the libtock library. The majority of libtock are wrappers for interacting with Tock drivers through system calls. They provide the user a meaningful function name and arguments and then internally translate these into a command, subscribe, etc. Where it makes sense, the libraries also provide a synchronous interface to a driver using an internal callback and yield_for (example: tmp006_read_sync)

libtock also provides the startup code for applications (crt0.c), an implementation for the system calls (tock.c), and pin definitions for platforms.

libc++

Provides support for C++ apps. See examples/cxx_hello.

libnrfserialization

Provides a pre-compiled library for using the Nordic nRF serialization library for writing BLE apps.

lua53

Provides support for running a lua runtime as a Tock app. See examples/lua-hello.

Style & Format

We try to keep a consistent style in mainline userland code. For C/C++, we use uncrustify. High level:

  • Two space character indents.
  • Braces on the same line.
  • Spaces around most operators.

For details, see the configuration.

Travis will automatically check formatting. You can format code locally using make format, or check the whole codebase with format_all.sh. Formatting will overwrite files when it runs.