Let's Finally Try to Catch C -- Java like exception handling in C
       =================================================================


Preface
-------
Some time ago I was working on large deeply nested C code (a recursive-descent
parser) and wanted to be able to jump away (to outside the parser) on a fatal 
condition.  Instead of using the setjmp()/longjmp()-pair as is for this occa-
sion, I thought it would be nice to have a generic/reusable mechanism like in 
C++ or Java.

I started with simple 'try', 'catch' and 'throw' macros that worked fine for
this application, but did not allow nesting and lacked other useful features.
When I realized that a complete/generic exception handling package might be 
useful for others and myself, I took up the challenge to create such a package
that looks like Java in the best possible way.

It was really exciting and fun to in a way extend the C language and I am
happy to share it with you!  I hope you will enjoy this package, and if you
have comments or find flaws please let me know:

    Cornelis van der Bent
    [email protected]

    April 1999.


Article Introduction
--------------------
Well written C code has to deal with four kinds of anomalies: regular failures 
(e.g., unable to open file), out-of-memory conditions, run-time error signals 
(like SIGSEGV), and failed assertions. Traditionally, each is handled in a
different way: returning error codes, exiting, using setjmp()/longjmp(), and
printing a message followed by abort() respectively. Furthermore, C does not
support centralized exception handling like C++ and Java do. This all results
in functional code getting cluttered with exception handling statements.

While working on a recursive-descent parser in C, I needed a mechanism to skip
any number of stack frames for easy error handling. The simple try, catch and 
throw macro's I created using setjmp()/longjmp() worked fine, but did not allow
nesting and only addressed one kind of errors. Soon after, I took up the
challenge to create a complete centralized exception handling package in plain
ANSI C, that looks like Java (and C++) in the best possible way, is easy to use
and has as little side-effects as possible.

In this article I will show you how to use the package, describing every
feature in detail; see Example 1 and Figure 1 to get a taste. Also, I will
explain some interesting aspects of the implementation. Because the functio-
nality is so very similar to what C++ and Java offer, reading this article
will be of help even if you will never use my package. Generally speaking,
exception handling is an important and yet difficult subject, thinking about
it and applying the right techniques saves time and makes life easier.

Figure 1:
* Catching all four kinds of exceptions: explicitly thrown by application,
  out-of-memory errors (using a simple optional module not being discussed 
  here), failed assertions, and signals (SIGABRT, SIGFPE, SIGILL and SIGSEGV).
* Unlimited user extendable exception class hierarchy. Classes can extend
  classes defined in other (compiled) modules. An exception class can be
  caught with any (in)direct ancestor or itself. Just like Java and C++.
* Can be used in any multi-threading environment. Compile-time configuration.
  Only requires two platform specific functions: one getting the thread ID and
  the other for mutual exclusion locking.
* Unlimited nesting in try, catch and finally blocks.
* Run-time check for duplicate and superfluous catch clauses (DEBUG option).
  A similar check as Java and C++ do at compile-time.
* Exception propagation, overruling and rethrowing identical to Java.
* Always executed finally block, also when using return(), break or continue.
* Looks very similar to Java (or C++). C implementation details are nicely
  hidden. Little side effects.
* Caught exception member functions like getMessage().  No context pointer
  as first argument; gives a real OO look.
* All messages (except those of signals) contain file name and line number.
* Default behavior or a message printed, for not caught exceptions.
* Fully ANSI C compliant code and no platform dependencies.
* Well documented: source code, extensive README and this article.  (Due to 
  limited space the comments were left out from the listings.)
* Free.



Getting Started
---------------
For a single-threaded application you only need to include "Except.h" and link 
the supplied modules.

Example 1: When the critical code throws a MyFault exception or any of its 
subclasses, it is handled.  For any other exception a message containing
its name and information where thrown (file & line) are printed, followed by
a rethrow.  The clean up code is always executed.

#include <Except.h>

ex_class_define(MyFault, Exception);
ex_class_define(AlsoMyFault, MyFault);
ex_class_define(AgainMyFault, MyFault);

void TryMe(void) {
    try {
        /* critical code */
    }
    catch (MyFault, e) {
	/* handle one of my faults */
    }
    catch (Throwable, e) {
        printf("%s\n", e->getMessage());
	throw (e, NULL);
    }
    finally {
        /* clean up code */
    }
}


Catching
--------
At this moment the exception supplies four public member functions that may
be called inside the 'catch' block:

    char *getMessage(void) - returns a pointer to a static string containing a
                             detailed description of the exception
    int   getClass(void)   - returns the exception class (address)
    void *getData(void)    - returns the pointer to (application) data asso-
                             ciated with the exception: the second argument
                             of throw(), for EX_ASSERT it is a pointer to the
                             static string containing the false expression,
                             for the other exceptions it is NULL

The convenience of not having to supply these functions a pointer to the
current exception, comes with a price.  In a multi-threaded application,
a fast hashtable lookup is performed at 
each call; because this is done as part of handling an error condition, it is
not a big deal (for most applications).  In a single-threaded application only
a very fast direct lookup (involving a few if-statements and some pointer
arithmetic) is performed.

It is important to know that the pointer to the caught exception (<e> in most
examples), is only valid inside its 'catch' block.  You may pass it to a
routine (as <Except *> type) as long as you stay outside another 'try'
statement.  Also note that getMessage() returns a pointer to a static string
that will be overwritten or freed.  Finally, it is the responsibility of the
application to free allocated memory passed with throw() and returned by getData().
 <<<Finally you should also
know that the Except pointer and its structure is only valid in the scope of
its catch clause, for as long you stay outside try statements (that may appear
inside a catch clause).>>>

Of course more than one catch() can be specified for a critical code section.
As in Java, the evaluation process is performed from top to bottom and an 
exception is caught by only one (the first matching) 'catch' clause.  It is
possible to create superfluous or duplicate 'catch' clauses:

    try {}
    catch (Exception, e) {}
    catch (FailedAssertion, e) {}  /* superfluous: already caught by Exception*/
    finally {}

or:

    try {}
    catch (FailedAssertion, e) {}
    catch (RuntimeException, e) {}
    catch (FailedAssertion, e) {}	/* duplicate */
    finally {}

When DEBUG is #defined when compiling the application module, a run-time check
for all possible conditions like this, is done as part of executing the 'try' 
statement.  So this check is performed even before the 'try' block is executed 
and consequently also before an exception has occurred!  Java informs you about
conditions like this at compile-time, but this is not possible in C; at least
you are informed and it is done as early as possible.

These checks add a little overhead; fortunately the persistent overhead is zero
when DEBUG is not #defined.

It is allowed to have no 'catch' clauses at all.  But in that case you will
get a warning that 'catch' clause(s) are missing, when DEBUG is #defined.


As some C flow control statements, catch() expects to be followed by a
statement.  So this can be a single statement closed with a semicolon or a
compound statement between braces.  Consequently it is also allowed to write:

    catch (Throwable, e);   /* 'catch' followed by empty statement */

which will only catch any exception and performs no further actions.
This will of course not affect the run-time checking discussed above.

There is a predefined exception class hierarchy which you can extends as
far as you like, there are no limits!

/* this is in Except.h */
ex_class_declare(Exception,           Throwable);
ex_class_declare(OutOfMemoryError,    Exception);
ex_class_declare(FailedAssertion,     Exception);
ex_class_declare(RuntimeException,    Exception);
ex_class_declare(AbnormalTermination, RuntimeException);  /* SIGABRT */
ex_class_declare(ArithmeticException, RuntimeException);  /* SIGFPE */
ex_class_declare(IllegalInstruction,  RuntimeException);  /* SIGILL */
ex_class_declare(SegmentationFault,   RuntimeException);  /* SIGSEGV */

/* this is in Except.c */
ex_class_define(Exception,           Throwable);
ex_class_define(OutOfMemoryError,    Exception);
ex_class_define(FailedAssertion,     Exception);
ex_class_define(RuntimeException,    Exception);
ex_class_define(AbnormalTermination, RuntimeException);  /* SIGABRT */
ex_class_define(ArithmeticException, RuntimeException);  /* SIGFPE */
ex_class_define(IllegalInstruction,  RuntimeException);  /* SIGILL */
ex_class_define(SegmentationFault,   RuntimeException);  /* SIGSEGV */

It's simple, don't you think.

Erik: zo zien de [check] messages er uit:

Level2Exception: file "Test.c", line 379.

Superfluous catch(Level1Exception): file "Test.c", line 370; already caught by Exception at line 366.

Duplicate catch(Level1Exception): file "Test.c", line 419; already caught at line 418.

Warning: No catch clause(s): file "Test.c", line 396.



Throwing
--------
Deliberately throwing an exception is quite simple using throw().  The first
argument is the exception class.  With the
second argument you can pass a void pointer value which can be retrieved in
the 'catch' code with the exceptions' getData() member function (as described
in "Catching").  Do not pass a pointer to a non-static local variable (which
resides on the stack), because it gets out of scope (the stack is unwound) if
the exception is caught in another routine.  This is regular C programming
practice.

Exceptions can be thrown from any scope:

    outside                - it will be reported as being lost
    inside 'try' block     - it will be caught by corresponding 'catch' or will
                             otherwise be propagated (more on this later)
    inside 'catch' block   - it will be propagated (...)
    inside 'finally' block - it will override any pending exception and
                             will be propagated itself (...)

Note that there is no difference if the 'throw' is done directly in the
exception handling block, or if there are any number of routine calls in
between:

    There(void)
    {
        throw (MyException, "How are you?");
    }

    Hi(void)
    {
        There();
    }

    What(void)
    {
        try
        {
            Hi();
        }
        catch (Exception, e)
        {
            printf("%s\n", e->getData());
        }
        finally;
    }

Invoking What() will result in printing "I'm fine, thank you!" (just kidding).



Rethrowing
----------
Inside a 'catch' block you can throw the just caught exception again (to let
it propagate to a higher exception handling level) using rethrow():

    catch (Throwable, e)
    {
        throw (e, 0);
    }

Note that the second argument (where you normally place your data pointer)
is not looked at, so the data of the original exception is passed on.


Returning
---------
As in Java the 'finally' block is always executed even when return() is used:

    try
    {
        throw(MyFault, NULL);
    }
    catch (MyFault, e)
    {
        return(-1);
    }
    finally
    {
        printf("Just before return.\n");
    }

This code fragment would indeed print "Just before return.".  When "Except.h"
is included return() is defined as macro that takes care of everything and
also works as the regular return() when not directly inside nor outside an
exception handling block.


When you use return() inside the 'finally' block, this will override a
pending exception that would otherwise have been propagated.  This pending
exception will be lost and no notification message will be printed.  Java
behaves in exactly the same manner.


Unless you have already redefined it, having return() as a macro is not a 
problem in most cases.  The macro does however give some overhead, so you
can best avoid it in code sections that frequently use return() and/or have
a high efficiency requirement.  Since return() is defined as a macro having
one parameter (always between parentheses), the C preprocessor will do nothing 
when you omit the parentheses, yielding the native C 'return':

    return -1;                     /* uses the native C 'return' */

However, when you return an expression that starts with a parenthesis the C
preprocessor will replace it up to the matching closing parenthesis; this will
often/always result in erroneous code:

    return (int *)pData;     /* erroneous when return() macro defined */

Here the C preprocessor will use "int *" as argument of the return() macro.
There is a simple remedy, by using the C comma operator you can prevent the
expression from starting with a parenthesis:

    return 0,(int *)pData;  /* problem solved - no additional overhead */

You could also use an intermediate variable (which will probably be optimized 
away by modern compilers):

    int *pTmp = (int *)pData;
    return pTmp;

Although this may seem tricky, keep in mind that you may not encounter this
situation often.



Finally
-------
The 'finally' clause is dealt with in other sections.  The only thing to note
is that every 'try' statement must have a (possibly empty) 'finally' clause.



Using 'break' and 'continue'
----------------------------
The C 'break' and 'continue' statements used directly inside 'try', 'catch' or
'finally' (so without enclosing 'while', 'for' or 'swtich' statement) blocks
work differently compared to Java:

    'break    in     try' - start executing 'finally' block
    'continue in     try' - start executing 'finally' block
    'break    in   catch' - start executing 'finally' block
    'continue in   catch' - start executing 'finally' block
    'break    in finally' - leave 'finally' block
    'continue in finally' - leave 'finally' block

When you embed a 'try' statement in for example a 'while' loop in Java, you can
use 'break' to leave the loop (after first executing the 'finally' code) or use
'continue' to skip to the next iteration (in this case the 'finally' code is
also executed first).  But, if you may need this functionality you can use the
following workaround:

    int breaked;

    breaked = 0;
    while (...)
    {
        try
        {
            if (...)
                breaked = 1;
        }
        catch (...) {}
        finally (...) {}

        if (breaked)
            break;
    }

Although 'break' and 'continue' work differently, they can still be used for
what they do do.



Nesting
-------
You can start a critical code section wherever you like.  Nesting can be
done up to unlimited depth.  In this way different exception handling levels
can be created (across routine boundaries).

    try
    {
        try {
            try {}
            catch (...) {}
            finally {}
        }
        catch(...) {}
        finally {}
    }
    catch
    {
        try {}
        catch(...) {}
        finally {}
    }
    finally
    {
        try {}
        catch(...) {}
        finally {}
    }

Not caught exceptions will propagate upwards as expected (see below).



Propagation
-----------
When an exception is not caught at the exception handling level it occurred,
it is propagated upwards.  Propagation can be best understood by thinking that
the exception is 'rethrown' just below its 'finally' block (that belongs to the 
level the exception occurred).  Then the same rules apply as described above
in "Throwing".  The result depends on where this 'rethrow' takes place:

    outside                - it will be reported as being lost
    inside 'try' block     - it will be caught by corresponding 'catch' on this
                             higher level or will otherwise be propagated again
    inside 'catch' block   - it will be propagated to yet the next level
    inside 'finally' block - it will override any pending exception and
                             will be propagated to the next level upwards

Here's an example of this last rule:

    try
    {
        throw(-777, NULL);      /* not caught on this level - pending */
    }
    catch (EX_MEMORY, e) {}
    catch (-666, e) {}
    finally
    {
        try
        {
            throw(-321, NULL);  /* not caught on this level - 'rethrown' */
        }
        catch (-123, e) {}
        finally {}
        /* 'rethrow(-321)' will override 'throw(-777)' */
    }
    /* 'rethrow(-777)' will not take place because of override */
    /* 'rethrow(-321)' however does take place here */

So because 'throw(-777)' is not caught, it would have been 'rethrown' below
its 'finally', but it is overridden by 'throw(-321)' inside its 'finally'.

### In the previous version you had to work with numbers instead of classes
    now.  It costs me too much time to change this example right now.  The
    mechanism has stayed the same however, so when you think classes you
    will still understand ...



Signals
-------
As part of executing the outermost 'try', the handlers of four ANSI C supported
signals (SIGABRT, SIGFPE, SIGILL and SIGSEGV) are saved and replaced by a
handler from this exception handling package.  This handler causes a 'throw'
to occur when one of these four signals is raised.  The resulting exception
is handled as any of the other exceptions.

    void Foo(void)
    {
        *((int *)0) = 0;        /* causes SIGSEGV - may not be portable */
    }

    void Boo(void)
    {
        try
        {
            Foo();
        }
        catch (SegmentationFault, e)
        {
            printf("%s\n", e->getMessage());
        }
        finally;
    }

This will print a segmentation fault message.


As part of the outermost 'finally', the signal handlers stored during the
outermost 'try' will be restored again.  In a multi-threading environment the 
threads/tasks either share the signal handlers (like on Solaris) or each have
their private set (like on VxWorks); with shared handlers, restoration is
delayed until the final thread leaves the exception handling scope (refer to
"Multi-threading" below).

When a signal exception is not caught and the signal handlers are restored,
this signal is sent after all using raise().  When the signal handlers are not
restored yet (may occur in a multi-threading environment with shared handlers), 
a message that the exception was lost is printed.



Assertion Checking
------------------
The header "Assert.h" redefines the ANSI C assert() macro in order to let it
generate a 'throw' on failure.  Failed assertions are caught using EX_ASSERT.  
The exception specific data (retrieved with the member function getData())
points to a static character array containing the failed expression.

Only when the preprocessor macro DEBUG is defined, will assert() expand to
assertion checking code; without DEBUG defined, assert() is an empty macro.
I experienced that, when building fault tolerant code, it can be very handy to
have a macro that works as assert() during the test phase (i.e. when DEBUG is
defined), but returns on failure during field operation.  For this purpose I 
created a macro validate().  Note that validate() always leaves some code
behind so use it sparingly where high performance is an issue.

    #include "Assert.h"

    void * ListRemoveHead(List *pList)
    {
        assert(pList != NULL);
        validate(pList->count > 0, NULL);
        ...
    }

When in this example (taken from "List.c") <pList> is NULL, the caller has
made a structural/severe error.  I chose to use assert() because this kind of
error should be found during tests and I don't want this test to slow down
ListRemoveHead() when the software is released.  On the other hand, trying to
remove the head of an empty list (when pList->count == 0) is a more functional
and less severe user error.  It might for example be caused by a misunderstan-
ding of how to use this list operation, or by an exceptional condition that 
occurred in a complex algorithm.  By using validate(), the programmer will be
notified during tests; but when DEBUG is no longer defined the ListRemoveHead()
will return the reasonable value NULL instead of causing a program crash (some
time later).

When validate() is used in void function you must use NOTHING (defined in
"Assert.h") as return value :

    void FactorStore(int alpha, int beta)
    {
        validate(alpha < beta, NOTHING);
        ...
    }


When you want the functionality of validate() but rather like to throw an
exception instead of returning, you can use the check() macro.


When assert() is used outside an exception handling scope, it behaves as
usual.  In contrary with the ANSI C assert(), the default behavior of assert() 
(which is also used in validate() and check()) outside, is to not invoke the
C library function abort() to terminate the process.  On a failed assertion,
which typically occurs in the test phase, I rather leave the program running
to see what else happens.  But if you do like to use abort(), simply define
ASSERT_ABORT.



Allocating Memory
-----------------
Error checking code is (or should be) often seen around calls of malloc(),
calloc() and realloc():

    if ((pData = malloc(sizeof(DATA))) == NULL)
        return ERROR;

The header file "Alloc.h" defines each of these operations as macro that per-
form this check for you and throw an EX_MEMORY exception when out-of-memory:

    #include "Alloc.h"

    void Nice(void)
    {
        try
        {
            pData = malloc(sizeof(DATA));
        }
        catch (OutOfMemoryError, e)
        {
            printf("%s\n", e->getMessage());
            exit 1;
        }
        finally;
    }

This will print an out-of-memory message when malloc() fails.


For convenience a new() macro is also supplied which uses the C library routine
calloc().  Because you can simply pass the type, it saves you from having to
write sizeof() all the time:

    pData = new(DATA);


These macros add only a small overhead (a function call and an if statement),
so even when speed is an issue you may still want to use them.  Note that the
check is also performed when using these macros outside a critical code
section; so using these macros you will always be notified when a memory allo-
cation has failed.



Multi-threading
---------------
This package allows multiple threads/tasks, having a common address space, to
concurrently use exception handling.  From a user perspective almost every-
thing stays the same compared to a single-threading application.  The internal
exception context that has to be kept for each thread is conveniently hidden
for the user; [s]he is not confronted with obscure handlers as you would 
expext to see in a C implementation.

Internally the thread ID is used as key for exception context lookup.
Everything is done for you now (compared to previous version).  You choose
one of: EXCEPT_MT_SHARED EXCEPT_MT_PRIVATE, by adding a -D.... C-preprocessor
flag.  You also have to supply a function with which to get the current
thread-ID and a function to perform a mutex lock......

As was mentioned above in "Signals": In a multi-threading environment the
threads/tasks either share the signal handlers (like on Solaris) or each have
their private set (like on VxWorks).  Because exception handling has to decide
if it must delay default signal handler restoration until the final thread 
leaves the exception handling scope (when handlers are shared), the user
has to choose... see above.

    #include <vxWorks.h>
    #include <Except.h>
    
    void Task(void)
    {
        try                      /* implicitly creates exception context */
        {
            /* do something */
        }
        catch (Throwable, e)
        {
        }
        finally;
    }        

    void Launcher(void)
    {
        ex_threading_init(taskIdSelf, EX_PRIVATE_HANDLERS);

        taskSpawn("task1", MY_PRIORITY, 0, MY_STACK_SIZE, Task, 1);
        taskSpawn("task2", MY_PRIORITY, 0, MY_STACK_SIZE, Task, 2);
        taskSpawn("task3", MY_PRIORITY, 0, MY_STACK_SIZE, Task, 3);
    }

Each thread has to invoke ex_thread_enter() before its first 'try' statement.
When the thread ID was used earlier by another thread (some OSs recycle thread
IDs), this routine destroys the outdated exception context if there.  Normally
this context is destroyed automatically, but will be left behind when
for example killed by another thread.  Adding 'outermost'
just before the outermost 'try' prevents
a memory leak, and makes sure that the exception handling package keeps track
of the number of active threads, so it can restore shared signal handlers at 
the right moment.

A thread must not use any of the exception handling macros (including return()),
because this implicitly creates a new exception context for the thread and will
probably result in a memory leak.

When a thread kills another thread it should call ex_thread_cleanup() with the
ID of the killed thread.  This will have the same effect as ex_thread_leave().
In a multi-threading environment that recycles IDs, ex_thread_cleanup() may
lead to hazardous situations when a new thread starts using the ID before the
cleanup() has finished.  To prevent this, thread scheduling should be disabled
during killing the thread an the cleanup which follows immediately.  You could
also decide to not use ex_thread_cleanup() at all, because ex_thread_enter()
takes care of the cleanup when the thread ID was used before.

This is all there is to multi-threading, all the rest remains the same!

### (Almost) all ex_thread... calls are not needed any more in the current
    version!



Preprocessor Flags
------------------
This section summarizes the C preprocessor flags and describes their effect
when defined:

    DEBUG        - switches on assertion checking and catch() checking
    ASSERT_ABORT - causes assert macros to invoke abort()
    EXCEPT_DEBUG - switches on printing debug messages in "Except.h"

The EXCEPT_DEBUG flag is only used during development of the exception
package.



Compiler Warnings
-----------------
When you use the 'return' macro inside a 'try', 'catch' or 'finally' block,
your compiler may warn you:

    try
    {
	return(7);
    }
    catch (Throwable, e)		/* line # 77 */
    {
	printf("%s\n", e->getMessage());
    }
    finally;

    --> "Test.c", line 77: warning: end-of-loop code not reached

These warning are about the exception handling macro code and can be safely
ignored.


### A number of these below have been solved/done.

Possible Extensions & Modifications
-----------------------------------
A. Shield non-user definitions in header files with #ifdef _EXCEPT_PRIVATE.

B. Choose other name for global variables that will less likely clash with
   user names.

C. Configurable central error message output, instead of printing to <stderr>.

D. Place last accessed hash table entry at the head of the node list to spead
   lookup in case killed threads are not cleaned up.

E. Supply some mechanism for hierarchical user exception 'classes' (using
   bit masks: <mask> member in Except structure).  In the current implementa-
   tion it is not very nice that you can only either catch a single user 
   exception (with a negative number) or catch them all (with EX_THROW).

F. Investigate if rethrow() can be eliminated in a clean way.  Refer to the
   discussion above why it was introduced.

G. A new() macro for arrays.  The only 'problem' is choosing a nice name.



Known Problems & Caveats and Todos
----------------------------------
A. Integers are assumed to be (at least) 32 bit.

B. The code belonging to this package does not throw exception when out of 
   memory.  This is not a real issue since only small chunks of memory are 
   allocated; when even these are not available, we're in a fatal situation.

C. In multi-threading, the correct operation of this package fully depends on
   the application supplied routine that returns the thread ID.  When it would
   return different IDs for the same thread, chaos will be the result.

D. In multi-threading not invoking ex_thread_leave() or ex_thread_cleanup()
   for an old thread, results in a memory leak.

E. The assert() and validate() macros are used by the package.  Check what
   happens when each of these internal checks fails.  We don't want recursion
   or crashes.  This must be done before any final release.

F. Some OSs mask the handled signal.  Some restore this mask when the signal
   handler returns.  Investigate if restoration takes place on every OS when
   the handler is left using longjmp().

G. A harsh assertion check scheme for ExceptTreadEnter/Leave() in 
   ExceptGetContext().  Let ExceptGetContext() when ex_thread_enter() has not
   been invoked yet.  Currently ExceptGetContext() is very nice and creates
   a new context when not there yet.

H. Maybe clean up the way <pC> and ExceptGetContext() are used in "Except.c".
   Invoking ExceptGetContext() may not be strictly necessary at all places.

I. Perform optimizations.  The current implementation was made with a main
   focus on clearity.

J. Some pointer variables (like <pC->exStack> and <pContextHash>) are misused
   as flag (i.e., they are NULL or not).  Although this might make "Except.c"
   a bit more efficient, it also makes this code harder to understand/maintain.
   Finally, the routine ExceptFinally() requires some clean up.

K. Better testing and code review for especially multi-threading is needed.
   Please have a close look and tell me what you find!

L. Review (style, syntactics and semantics) of this README and the comments
   in the sources would also be appreciated.



The Files
---------
This package consists of the following files (they are formatted with a TAB
width of 8 and use '\n' as end-of-line character (the UNIX way)):

    Alloc.c  - Memory allocation module.  Contains the private routines used
               by the optional memory allocation macros defined in "Alloc.h".  
               Compile and link this file with your application when you use 
               these macros.

    Alloc.h  - Memory allocation module header.  Include when you want to use
               the memory allocation macros.

    Except.c - Exception handling module.  Contains the private routines and
               global variable definitions used by the exception handling
               macros defined in "Except.h".  This file must be compiled and
               linked with your application.

    Except.h - Exception handling module header.  Must be included.

    Hash.c   - Hash table library.  It is used by the exception handling
               package, so this file must be compiled and linked with your
               application.  You can also use this easy library yourself.

    Hash.h   - Hash table library header.  Only needs to be included if you
               want to use this library yourself.

    Lifo.c   - LIFO buffer library.  It is used by the exception handling
               package, so this file must be compiled and linked with your
               application.  You can also use this easy library yourself.

    Lifo.h   - LIFO buffer library header.  Only needs to be included if you
               want to use this library yourself.

    List.c   - Doubly linked list library.  It is used by the exception
               handling package, so this file must be compiled and linked with 
               your application.  You can also use this easy library
               yourself.  It contains a lot more routines than used by this
               package.

    List.h   - Doubly linked list library header.  Only needs to be included
               if you want to use this library yourself.

    Test.c   - The single-threaded test file.  Can be used as a source of
               examples.  (Multi-threading has been tested on Solaris the
               test file is not finished yet and is therefore not included.)

    README   - Last but noy least, this very file.  It describes how to use
               the package.  Operation is explained in the source.



               >>>>>>>>>>  That's all folks!  <<<<<<<<<<