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Design By Contract (DBC)

Design By Contract (DBC), pioneered by Bertrand Meyer, views a software system as a set of components whose collaboration is based on precisely defined specifications of mutual obligations—the contracts.1 The central idea of this method is to inherently embed the contracts in the code and validate them automatically at run time. Doing so consistently has two major benefits:

1. It automatically helps detect bugs (as opposed to "handling" them), and
2. It is one of the best ways to document code.

DBC for Embedded C and C++

You can implement the most important aspects of DBC (the contracts) in C or C++ with assertions. The Standard C Library macro assert() is rarely applicable to embedded systems, however, because its default behavior (when the integer expression passed to the macro evaluates to false) is to print an error message and exit. Neither of these actions makes much sense for most embedded systems, which rarely have a screen to print to and cannot really exit either (at least not in the same sense that a desktop application can). Therefore, in an embedded environment, you usually have to define your own assertions that suit your tools and allow you to customize the error response. However, you should think twice before you go about "enhancing" assertions because a large part of their power derives from their relative simplicity.

This DBC implementation

The DBC implementation in this repository consists of just one header file dbc_assert.h, which has been specifically tailored for embedded systems and has the following properties:

• allows customizing the error response (by implementing the DBC fault handler DBC_fault_handler());

• conserves memory by avoiding proliferation of multiple copies of the filename string (macro DBC_MODULE_NAME());

• provides additional macros for checking and documenting preconditions (DBC_REQUIRE()), postconditions (DBC_ENSURE()), and invariants (DBC_INVARIANT()).

NOTE
The names of the three last macros are a direct loan from Eiffel, the programming language that natively supports DBC.

Example of Use

#include "dbc_assert.h" /* Design By Contract (DBC) assertions */

DBC_MODULE_NAME("sst")  /* for DBC assertions in this module */

/*..........................................................................*/
void SST_Task_post(SST_Task * const me, SST_Evt const * const e) {
    /*! @pre the queue must be sized adequately and cannot overflow */
    DBC_REQUIRE(300, me->nUsed <= me->end);
    ...
}

More DBC Resources

To learn more about DBC for embedded systems, please check out the following resources:

[1] Key Concept: Design by Contract
[2] "Design by Contract for Embedded C"
[3] "A Nail for a Fuse"
[4] "An Exception or a Bug"

How to Help this Project?

Please feel free to clone, fork, and make pull requests to improve DBC for Embedded C. If you like this project, please give it a star (in the upper-right corner of your browser window):