CException/README.md

CException ![CI](https://github.com/ThrowTheSwitch/CException/workflows/CI/badge.svg)
==========

_This Documentation Is Released Under a Creative Commons 3.0 Attribution Share-Alike License_

CException is simple exception handling in C. It is significantly faster than full-blown C++ exception handling 
but loses some flexibility. It is portable to any platform supporting `setjmp`/`longjmp`.

Getting Started
================

The simplest way to get started is to just grab the code and pull it into your project:

```
git clone https://github.com/throwtheswitch/cexception.git
```

If you want to contribute to this project, you'll also need to have Ruby and Ceedling installed to run the unit tests.

Usage
=====

### So what's it good for?

Mostly error handling. Passing errors down a long chain of function calls gets ugly. Sometimes really ugly. 
So what if you could just specify certain places where you want to handle errors, and all your errors were 
transferred there? Let's try a lame example:

CException uses C standard library functions setjmp and longjmp to operate. As long as the target system 
has these two functions defined, this library should be useable with very little configuration. It even 
supports environments where multiple program flows are in use, such as real-time operating systems... 
we started this project for use in embedded systems... but it obviously can be used for larger systems too.

### Error Handling with CException:

```
void functionC(void) {
  //do some stuff
  if (there_was_a_problem)
    Throw(ERR_BAD_BREATH);
  //this stuff never gets called because of error
}
```

There are about a gajillion exception frameworks using a similar setjmp/longjmp method out there... and there 
will probably be more in the future. Unfortunately, when we started our last embedded project, all those that 
existed either (a) did not support multiple tasks (therefore multiple stacks) or (b) were way more complex 
than we really wanted. CException was born.

Why?
====

### It's ANSI C

...and it beats passing error codes around.

### You want something simple...

CException throws a single id. You can define those ID's to be whatever you like. 
You might even choose which type that number is for your project. But that's as far as it goes. We weren't interested 
in passing objects or structs or strings... just simple error codes. Fast. Easy to Use. Easy to Understand.

### Performance...

CException can be configured for single tasking or multitasking. In single tasking, there is 
very little overhead past the setjmp/longjmp calls (which are already fast). In multitasking, your only additional 
overhead is the time it takes you to determine a unique task id (0 to num_tasks).

How?
====

Code that is to be protected are wrapped in `Try { }` blocks. The code inside the Try block is _protected_, 
meaning that if any Throws occur, program control is directly transferred to the start of the Catch block. 
The Catch block immediately follows the Try block. It's ignored if no errors have occurred.

A numerical exception ID is included with Throw, and is passed into the Catch block. This allows you to handle 
errors differently or to report which error has occurred... or maybe it just makes debugging easier so you 
know where the problem was Thrown.

Throws can occur from anywhere inside the Try block, directly in the function you're testing or even within 
function calls (nested as deeply as you like). There can be as many Throws as you like, just remember that 
execution of the guts of your Try block ends as soon as the first Throw is triggered. Once you throw, you're 
transferred to the Catch block. A silly example:

```
void SillyExampleWhichPrintsZeroThroughFive(void) {
  volatile CEXCEPTION_T e;
  int i;
  while (i = 0; i < 6; i++) {
    Try {
      Throw(i);
      //This spot is never reached
    } 
    Catch(e) {
      printf(“%i “, e);
    }
  }
}
```

Limitations
===========

This library was made to be as fast as possible, and provide basic exception handling. It is not a full-blown 
exception library like C++. Because of this, there are a few limitations that should be observed in order to 
successfully utilize this library:

### Return & Goto

Do not directly `return` from within a `Try` block, nor `goto` into or out of a `Try` block.
The `Try` macro allocates some local memory and alters a global pointer. These are cleaned up at the 
top of the `Catch` macro. Gotos and returns would bypass some of these steps, resulting in memory leaks 
or unpredictable behavior.

### Local Variables

If (a) you change local (stack) variables within your `Try` block, and (b) wish to make use of the updated 
values after an exception is thrown, those variables should be made `volatile`.

Note that this is ONLY for locals and ONLY when you need access to them after a `Throw`.

Compilers optimize (and thank goodness they do). There is no way to guarantee that the actual memory 
location was updated and not just a register unless the variable is marked volatile.

### Memory Management

Memory which is `malloc`'d within a `Try` block is not automatically released when an error is thrown. This 
will sometimes be desirable, and other times may not. It will be the responsibility of the code you put in 
the `Catch` block to perform this kind of cleanup.

There's just no easy way to track `malloc`'d memory, etc., without replacing or wrapping `malloc` 
calls or something like that. This is a lightweight framework, so these options were not desirable.

CException API
==============

### `Try { ... }`

`Try` is a macro which starts a protected block. It MUST be followed by a pair of braces or a single 
protected line (similar to an 'if'), enclosing the data that is to be protected. It MUST be followed by 
a `Catch` block (don't worry, you'll get compiler errors to let you know if you mess any of that up).

The `Try` block is your protected block. It contains your main program flow, where you can ignore errors 
(other than a quick `Throw` call). You may nest multiple `Try` blocks if you want to handle errors at
multiple levels, and you can even rethrow an error from within a nested `Catch`.

### `Catch(e) { }`

`Catch` is a macro which ends the `Try` block and starts the error handling block. The `Catch` block 
is executed if and only if an exception was thrown while within the `Try` block. This error was thrown 
by a `Throw` call somewhere within `Try` (or within a function called within `Try`, or a function called 
by a function called within `Try`... you get the idea.).

`Catch` receives a single id of type `CEXCEPTION_T` which you can ignore or use to handle the error in 
some way. You may throw errors from within Catches, but they will be caught by a `Try` wrapping the `Catch`, 
not the one immediately preceeding.

### `Throw(e)`

`Throw` is the method used to throw an error. `Throw`s should only occur from within a protected 
(`Try`...`Catch`) block, though it may easily be nested many function calls deep without an impact 
on performance or functionality. `Throw` takes a single argument, which is an exception id which will be 
 passed to `Catch` as the reason for the error. If you wish to _re-throw_ an error, this can be done by
calling `Throw(e)` with the error code you just caught. It _IS_ valid to throw from a `Catch` block.

### `ExitTry()`

`ExitTry` is a method used to immediately exit your current Try block but NOT treat this as an error. Don't 
run the Catch. Just start executing from after the Catch as if nothing had happened.

Configuration
=============

CException is a mostly portable library. It has one universal dependency, plus some macros which are required if 
working in a multi-tasking environment.

The standard C library setjmp must be available. Since this is part of the standard library, it's all good.

If working in a multitasking environment, you need a stack frame for each task. Therefore, you must define 
methods for obtaining an index into an array of frames and to get the overall number of id's are required. If 
the OS supports a method to retrieve task ID's, and those tasks are number 0, 1, 2... you are in an ideal 
situation. Otherwise, a more creative mapping function may be required. Note that this function is likely to 
be called twice for each protected block and once during a throw. This is the only added overhead in the system.

You have options for configuring the library, if the defaults aren't good enough for you. You can add defines 
at the command prompt directly. You can always include a configuration file before including `CException.h`. 
You can make sure `CEXCEPTION_USE_CONFIG_FILE` is defined, which will force make CException look for 
`CExceptionConfig.h`, where you can define whatever you like. However you do it, you can override any or 
all of the following:

### `CEXCEPTION_T`

Set this to the type you want your exception id's to be. Defaults to an 'unsigned int'.

### `CEXCEPTION_NONE`

Set this to a number which will never be an exception id in your system. Defaults to `0x5a5a5a5a`.

### `CEXCEPTION_GET_ID`

If in a multi-tasking environment, this should be set to be a call to the function described in #2 above. 
It defaults to just return 0 all the time (good for single tasking environments, not so good otherwise).

### `CEXCEPTION_NUM_ID`

If in a multi-tasking environment, this should be set to the number of ID's required (usually the number 
of tasks in the system). Defaults to 1 (good for single tasking environments or systems where you will only 
use this from one task).

### `CEXCEPTION_NO_CATCH_HANDLER (id)`

This macro can be optionally specified. It allows you to specify code to be called when a Throw is made 
outside of Try...Catch protection. Consider this the emergency fallback plan for when something has gone 
terribly wrong.

### And More!

You may also want to include any header files which will commonly be needed by the rest of your application 
where it uses exception handling here. For example, OS header files or exception codes would be useful.

Finally, there are some hook macros which you can implement to inject your own target-specific code in 
particular places. It is a rare instance where you will need these, but they are here if you need them:

* `CEXCEPTION_HOOK_START_TRY` - called immediately before the Try block
* `CEXCEPTION_HOOK_HAPPY_TRY` - called immediately after the Try block if no exception was thrown
* `CEXCEPTION_HOOK_AFTER_TRY` - called immediately after the Try block OR before an exception is caught
* `CEXCEPTION_HOOK_START_CATCH` - called immediately before the catch

Testing
=======

If you want to validate that CException works with your tools or that it works with your custom 
configuration, you may want to run the included test suite. This is the test suite (along with real 
projects we've used it on) that we use to make sure that things actually work the way we claim.
The test suite makes use of Ceedling, which uses the Unity Test Framework. It will require a native C compiler. 
The example makefile and rakefile both use gcc.