exceptions4c

Bring the power of exceptions to your C applications with this tiny, portable library.

An exception handling framework for C

This library provides you with a simple set of keywords (macros, actually) which map the semantics of exception handling you're probably already used to:

• try
• catch
• finally
• throw

You can use exceptions in C by writing try/catch/finally blocks:

    #include "e4c.h"

    int foobar(){

        int foo;
        void * buffer = malloc(1024);

        if(buffer == NULL){
            throw(NotEnoughMemoryException, "Could not allocate buffer.");
        }

        try{
            foo = get_user_input(buffer, 1024);
        }catch(BadUserInputException){
            foo = 123;
        }finally{
            free(buffer);
        }

        return(foo);
    }

This way you will never have to deal again with boring error codes, or check return values every time you call a function.

Exception Hierarchies

The possible exceptions in a program are organized in a pseudo-hierarchy of exceptions. RuntimeException is the root of the exceptions pseudo-hierarchy. Any exception can be caught by a catch(RuntimeException) block, except AssertionException.

When an exception is thrown, control is transferred to the nearest dynamically-enclosing catch code block that handles the exception. Whether a particular catch block handles an exception is found out by comparing the type (and supertypes) of the actual thrown exception against the specified exception in the catch clause.

A catch block is given an exception as a parameter. This parameter determines the set of exceptions that can be handled by the code block. A block handles an actual exception that was thrown if the specified parameter is either:

• the same type of that exception.
• the same type of any of the supertypes of that exception.

If you write a catch block that handles an exception with no defined subtype, it will only handle that very specific exception. By grouping exceptions in hierarchies, you can design generic catch blocks that deal with several exceptions:

    /*                   Name             Default message   Supertype */
    E4C_DEFINE_EXCEPTION(ColorException, "Colorful error.", RuntimeException);
    E4C_DEFINE_EXCEPTION(RedException,   "Red error.",      ColorException);
    E4C_DEFINE_EXCEPTION(GreenException, "Green error.",    ColorException);
    E4C_DEFINE_EXCEPTION(BlueException,  "Blue error.",     ColorException);

    ...

    try{
        int color = chooseColor();
        if(color == 0xff0000) throw(RedException, "I don't like it.");
        if(color == 0x00ff00) throw(GreenException, NULL);
        if(color == 0x0000ff) throw(BlueException, "It's way too blue.");
        doSomething(color);
    }catch(GreenException){
        printf("You cannot use green.");
    }catch(ColorException){
        const e4c_exception * e = e4c_get_exception();
        printf("You cannot use that color: %s (%s).", e->name, e->message);
    }

When looking for a match, catch blocks are inspected in the order they appear in the code. If you place a handler for a superclass before a subclass handler, the second block will in fact be unreachable.

Dispose Pattern

There are other keywords related to resource handling:

• with... use
• using

They allow you to express the Dispose Pattern in your code:

    /* syntax #1 */
    FOO f;
    with(f, e4c_dispose_FOO) f = e4c_acquire_FOO(foo, bar); use do_something(f);

    /* syntax #2 (relies on 'e4c_acquire_BAR' and 'e4c_dispose_BAR') */
    BAR bar;
    using(BAR, bar, ("BAR", 123) ){
        do_something_else(bar);
    }

    /* syntax #3 (customized to specific resource types) */
    FILE * report;
    e4c_using_file(report, "log.txt", "a"){
        fputs("hello, world!\n", report);
    }

This is a clean and terse way to handle all kinds of resources with implicit acquisition and automatic disposal.

Signal Handling

In addition, signals such as SIGHUP, SIGFPE and SIGSEGV can be handled in an exceptional way. Forget about scary segmentation faults, all you need is to catch BadPointerException:

    int * pointer = NULL;

    try{
        int oops = *pointer;
    }catch(BadPointerException){
        printf("No problem ;-)");
    }

Multithreading

If you are using threads in your program, you must enable the thread-safe version of the library by defining E4C_THREADSAFE at compiler level.

The usage of the framework does not vary between single and multithreaded programs. The same semantics apply. The only caveat is that the behavior of signal handling is undefined in a multithreaded program so use this feature with caution.

Integration

Whether you are developing a standalone application, or an external library that provides services to independent programs, you can integrate exceptions4c in your code very easily.

The system provides a mechanism for implicit initialization and finalization of the exception framework, so that it is safe to use try, catch, throw, etc. from any external function, even if its caller is not exception-aware whatsoever.

Portability

This library should compile in any ANSI C compiler. It uses nothing but standard C functions. In order to use exceptions4c you have to drop the two files (e4c.h and e4c.c) in your project and remember to #include the header file from your code.

In case your application uses threads, exceptions4c relies on pthreads, the POSIX application programming interface for writing multithreaded applications. This API is available for most operating systems and platforms.

Lightweight Version

If you have the feeling that the standard version of exceptions4c may be a bit overkill for your specific needs, there exists a lightweight version, targeted at small projects and embedded systems. Use it when you just want to handle error conditions that may occur in your program through a simple yet powerful exception handling mechanism. It provides the core functionality of exceptions4c in less than 200 source lines of code.

License

This is free software: you can redistribute it and/or modify it under the terms of the GNU Lesser General Public License as published by the Free Software Foundation, either version 3 of the License, or (at your option) any later version.

This software is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU Lesser General Public License for more details.

You should have received a copy of the GNU Lesser General Public License along with this software. If not, see http://www.gnu.org/licenses/.

Required

• License and copyright notice: Include a copy of the license and copyright notice with the code.
• Library usage: The library may be used within a non-open-source application.
• Disclose Source: Source code for this library must be made available when distributing the software.

Permitted

• Commercial Use: This software and derivatives may be used for commercial purposes.
• Modification: This software may be modified.
• Distribution: You may distribute this software.
• Sublicensing: You may grant a sublicense to modify and distribute this software to third parties not included in the license.
• Patent Grant: This license provides an express grant of patent rights from the contributor to the recipient.

Forbidden

• Hold Liable: Software is provided without warranty and the software author/license owner cannot be held liable for damages.

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