libakerror/README.md

Summary

This library provides a TRY/CATCH style exception handling mechanism for C.

Why?

There is nothing wrong with C as it is. This library does not claim to fix some problem with C.

Instead, this library implements a pragmatic and stylistic choice to assist the programmer in better handling errors in their programs. Vanilla C provides everything you need to do this out of the box, but this library makes it easier to avoid pointing certain guns at your foot, and when you do, it provides better context with those errors to help you more quickly recover.

Why? Because some programmers prefer to have the power of C with just a little bit of help in managing their errors.

Library Architecture

Philosophy of Use

This library has 6 guiding principles:

• Manually checking every possible return code for every possible meaning of that return code is tedious and prone to miss unpredicted failure cases
• Functions should return rich descriptive error contexts, not values
• Uncaught errors should cause program termination with a stacktrace
• Dynamic memory allocation is the source of many errors and should be avoided if possible
• Manipulating the call stack directly is error prone and dangerous
• Declaring, capturing, and reacting to errors should be intuitive and no more difficult than managing return codes

Lifecycle of an error in the AKError library

TL;DR - akerr_ErrorContext objects are filled with error context information and bubbled up through nested control structures until they are handled or reach the top level, where an unhandled error halts program termination with a stack trace

1. At the point where an error occurs, an akerr_ErrorContext object is created and populated with information regarding the failure
2. The akerr_ErrorContext is returned from the scope where the error was detected
3. The akerr_ErrorContext enters a control structure provided by the AKError library through a series of macros that examine akerr_ErrorContext objects as they pass through
4. The control structure checks to see if the akerr_ErrorContext has an error set, and if so, if there are any handlers in the current control structure that can handle it
5. If the current control structure can handle the akerr_ErrorContext, it does so
6. If the current control structure can not handle the akerr_ErrorContext, then the current control structure's cleanup code (if any) is executed, and the akerr_ErrorContext object is passed out of the current control structure to the parent control structure
7. Steps 2-6 are repeated through as many control structures as are necessary to reach the first level of the control structure
8. When the first level of the control structure is reached, if the akerr_ErrorContext has an error set in it, then the stack trace information in the akerr_ErrorContext object is used to print a stack trace using the configured logging function, and program termination is halted

What is in an Error Context

The Error Context object is a simple object which contains a few things:

• A numeric error code
• The name of the file in which the error occurred
• The name of the function in which the error occurred
• The line number in the file at which the error occurred
• A character buffer containing a message about the error in question

The structure also contains housekeeping information for the library which are of no specific interest to the user. See include/akerror.h for more details.

What are the control structures

The library is structured around a series of macros that construct switch statements that perform logic against an akerr_ErrorContext which exists in the current scope and has been initialized. These macros must be assembled in a specific order to produce a syntactically correct switch statement which performs correct operations against the akerr_ErrorContext to attempt operations, detect failures, perform cleanup operations, handle errors, and then exit a given scope in a success or failure state.

Functions and Return Codes

This library can catch errors from any function or expression that returns an integer value, or from functions that return akerr_ErrorContext *.

Any function which uses the PREPARE_ERROR macro should have a return type of akerr_ErrorContext *. The macros within this library, when they detect an unhandled error, will attempt to pass up the unhandled error to the context of the previous function in the call stack. This allows for errors to propagate up through the call stack in the same way as exceptions. (For example, if you use traditional C error handling in a call stack of a() -> b() -> c(), and c() fails because it runs out of memory, b() will likely detect that error and return some error to a(), but it may or may not return the context of what failed and why. With this, you get that context all the way up in a() without knowing anything about c().

Error codes

The library uses integer values to specify error codes inside of its context. These integer return codes are defined in akerror.h in the form of AKERR_xxxxx where xxxxx is the name of the error code in question. See akerror.h for a list of defined errors and their descriptions.

You can define additional error types by defining additional AKERR_xxxxx values. Error values up to 127 are reserved by the library, so begin your error values at 128. Define a human-friendly name for the error with the error_name_for_status method:

error_name_for_status(129, "Some Error Code Description")

When you add additional error codes, you need to define -DAKERR_MAX_ERR_VALUE=n to the compiler, where n is the maximum error code you have defined.

Installation

cmake -S . -B build
cmake --build build
cmake --install build

Dependencies

This library depends upon stdlib. If you don't want to link against stdlib, you must modify the library code to include headers and link against a library that provides the following:

• memset function
• strncpy function
• sprintf function
• exit function
• bool type
• NULL type

... then you can compile it thusly:

cmake -S . -B build -DAKERR_USE_STDLIB=OFF
cmake --build build
cmake --install build

Using the library

Setting up your project

Include it

#include <akerror.h>

Link the library directly, or

cc -lakerror

Using pkg-config, or

pkg-config akerror --cflags
pkg-config akerror --ldflags

Using cmake:

find_package(akerror REQUIRED)
pkg_check_modules(akerror REQUIRED akerror)
target_link_libraries(YOUR_TARGET PRIVATE akerror::akerror)

(Optional) Configuring the logging function

The default logging function (used for logging stack traces on failure) defaults to a wrapper that calls fprintf(stderr, f, ...). If you want to override this behavior, then set the error handler to a function with a printf-style signature:

void my_logger(const char *fmt, ...)
{
	/* ... do something */
}


/* set your custom error handler */
error_log_method = &my_logger;
	
/* proceed to use the library */

Setting Up the Error Context

Before you can use any of these macros you must set up an error context inside of the current scope.

PREPARE_ERROR(errctx);

This will create a akerr_ErrorContext structure inside of the current scope named errctx and initialize it. This structure is used for all operations of the library within the current scope. Attempting to use the library in a given scope before calling this will result in compile-time errors.

Attempting an Operation

ATTEMPT {
	// ... code
} CLEANUP {
} PROCESS(errctx) {
} FINISH(errctx, true)

ATTEMPT { ... } is the block within which you will perform operations which may cause errors that need to be caught. See "Capturing errors", below.

CLEANUP { ... } is the block within which you will perform any code which MUST be executed REGARDLESS of whether or not errors were thrown. Closing open file handles, or releasing memory, for example.

PROCESS(errctx) { ... } is the block within which you will handle any errors that were caught inside of the ATTEMPT block. See "Handling Errors" below.

FINISH(errctx, true) terminates the attempt operation. The FINISH macro takes two arguments: the name of the akerr_ErrorContext, and a boolean regarding whether or not to pass unhandled errors up to the calling function. Unless you have a good reason not to, this should be true.

Capturing errors

Inside of an ATTEMPT block, any operation which could generate or represent an error should be wrapped in one of several macros.

Capturing errors from functions which return akerr_ErrorContext *

For functions that return akerr_ErrorContext *, you should use the CATCH macro.

ATTEMPT {
    CATCH(errctx, errorGeneratingFunction())
} // ...

This will assign the return value of the function in question to the akerr_ErrorContext previously prepared in the current scope. If the function returns an akerr_ErrorContext that indicates any type of error, the ATTEMPT block is immediately exited, and the CLEANUP block begins.

Setting errors from functions or expressions returning integer

For functions that return integer, such as logical comparisons or most standard library functions, use the FAIL_ZERO_BREAK and FAIL_NONZERO_BREAK macros. These macros allow you to capture an integer return code from an expression or function and set an error code in the current context based off that return.

Here is an example of checking for a NULL pointer

ATTEMPT {
    FAIL_ZERO_BREAK(errctx, (somePointer == NULL), AKERR_NULLPOINTER, "Someone gave me a NULL pointer")
} // ...

Here is an example of checking for two strings that are not equal

ATTEMPT {
    FAIL_NONZERO_BREAK(errctx, strcmp("not", "equal"), AKERR_VALUE, "Strings are not equal")
} // ...

When either of these two macros are used, the ATTEMPT block is immediately exited, and the CLEANUP block begins.

Handling errors

Inside of the PROCESS { ... } block, you must handle any errors that occurred during the ATTEMPT { ... } block. You do this with HANDLE, HANDLE_GROUP, and HANDLE_DEFAULT.

Handling a specific error with HANDLE

In order to handle a specific error code, use the HANDLE macro.

} PROCESS(errctx) {
} HANDLE(errctx, AKERR_NULLPOINTER) {
    // Something is complaining about a null pointer error. Do something about it.
} // ...

Handling a group of errors with HANDLE_GROUP

In order to handle a group of related errors that all require the same failure behavior, use HANDLE followed by HANDLE_GROUP. For example, to handle a scenario where an IO error, key error, and index error all need to be handled the same way:

} PROCESS(errctx) {
} HANDLE(errctx, AKERR_IO) {
} HANDLE_GROUP(errctx, AKERR_KEY) {
} HANDLE_GROUP(errctx, AKERR_INDEX) {
    // error handling code goes here
}

This creates a fallthrough mechanism where all 3 errors get the same error handling code. Note that while the cases fall through, you can still (if desired) put some code specific to each error in that error's HANDLE or HANDLE_GROUP block; but this is not required, only the final handler needs to get any code.

The fallthrough behavior stops as soon as another HANDLE macro is encountered. For example, in this example, AKERR_IO, AKERR_KEY and AKERR_INDEX are all handled as a group, but AKERR_RELATIONSHIP is not.

} PROCESS(errctx) {
} HANDLE(errctx, AKERR_IO) {
} HANDLE_GROUP(errctx, AKERR_KEY) {
} HANDLE_GROUP(errctx, AKERR_INDEX) {
    // This code handles 3 error cases
} HANDLE(errctx, AKERR_RELATIONSHIP) {
    // This code handles 1 error case
}

Returning success or failure from functions returning akerr_ErrorContext *

If at all possible, when using this library, your functiions should return akerr_ErrorContext *. When returning from such functions, you should use the SUCCEED_RETURN and FAIL_RETURN macros.

SUCCEED_RETURN

This macro is used when your function has reached the end of its happy code path and is prepared to exit successfully. This sets the akerr_ErrorContext to a successful state and exits the function.

PREPARE_ERROR(errctx);
ATTEMPT {
    // ... stuff
} CLEANUP {
} PROCESS(errctx) {
} FINISH(errctx, true);
SUCCEED_RETURN(errctx);

FAIL_RETURN

If the code path in the current function reaches a state wherein an error must be set and the function must return early, you can use FAIL_RETURN to accomplish this. Note that this should not be used inside of an ATTEMPT { ... } block; this immediately exits the function, preventing a CLEANUP { ... } block from executing. This can be safely used from inside of a CLEANUP or PROCESS block, or from anywhere within the function not inside of an ATTEMPT { ... } block.

The function allows you to provide printf-style variable arguments to provide a meaningful failure message.

PREPARE_ERROR(errctx);
FAIL_RETURN(AKERR_BEHAVIOR, "Something went horribly wrong!")

Conditionally failing and returning

In addition to FAIL_RETURN you can also test for zero or non-zero conditions, set an error, and return from the function immediately. Use the FAIL_ZERO_RETURN and FAIL_NONZERO_RETURN macros for this. These macros can be used anywhere that FAIL_RETURN can be used.

PREPARE_ERROR(errctx);
FAIL_ZERO_RETURN(errctx, (somePointer == NULL), AKERR_NULLPOINTER, "Someone gave me a NULL pointer")

PREPARE_ERROR(errctx);
FAIL_NONZERO_RETURN(errctx, strcmp("not", "equal"), AKERR_VALUE, "Strings are not equal")

Uncaught errors

Ensuring that all error codes are captured

Any function which returns akerr_ErrorContext * should also be marked with ERROR_NOIGNORE.

akerr_ErrorContext ERROR_NOIGNORE *f(...);

This will cause a compile-time error if the return value of such a function is not used. "Used" here means assigned to a variable - it does not necessarily mean that the value is checked. However assuming that such functions are called inside of ATTEMPT { ... } blocks, it is safe to assume that such returns will be caught with CATCH(...); therefore this error is a generally effective safeguard against careless coding where errors are not checked.

Beware that ERROR_NOIGNORE is not a failsafe - it implements the warn_unused_result mechanic. By design users may explicitly ignore an error code from a function marked with warn_unused_result by explicitly casting the return to void.

#define ERROR_NOIGNORE __attribute__((warn_unused_result))

Stack Traces

Whenever an error is captured using the FAIL_* or CATCH methods, and is unhandled such that it manages to propagate all the way to the top of the caller stack without being managed, the last FINISH macro to touch the error will trigger a stack trace and kill the program.

Consider the tests/err_trace.c program which intentionally triggers this behavior. It produces output like this:

tests/err_trace.c:func2:7: 1 (Null Pointer Error) : This is a failure in func2
tests/err_trace.c:func2:10
tests/err_trace.c:func1:18: Detected error 0 from array (refcount 1)
tests/err_trace.c:func1:18
tests/err_trace.c:func1:21
tests/err_trace.c:main:30: Detected error 0 from array (refcount 1)
tests/err_trace.c:main:30
tests/err_trace.c:main:33: Unhandled Error 1 (Null Pointer Error): This is a failure in func2

From bottom to top, we have:

• The last line printed is the FINISH macro call that triggered the stacktrace.
• Above that, the CATCH() inside of main() which caught the exception from func1() but did not handle it
• Above that, a statement that the error was detected in the CATCH() statement at the same line
• Above that, the FINISH() macro in the func1 method which detected the presence of an unhandled error and returned it up the calling stack
• Above that, the CATCH() macro in the func1 method which caught the error coming out of func2()
• Above that, a statement that the error was detected in the CATCH() statement at the same line
• Above that, the FINISH() macro in func2() which detected an unhandled error and passed it out of the function
• Above that, a reference to the line where the FAIL() macro set the error code and provided the message which is printed here