How to Implement a Custom Fail Handler in mulle-allocator That Logs and Continues

To implement a custom fail handler that logs and continues in mulle-allocator, define a function matching the mulle_allocator_fail_t signature that logs the error and returns normally, then install it using mulle_allocator_set_fail().

The mulle-c/mulle-allocator library provides a flexible memory allocation framework for C applications. By default, when allocation fails, the library terminates the program via abort(). However, many production environments require a custom fail handler that logs and continues execution, allowing applications to degrade gracefully rather than crashing immediately.

Understanding the Default Allocation Failure Behavior

In src/mulle-allocator.c at lines 44–51, the default handler mulle_allocation_fail prints an error message using perror and immediately invokes abort(). This function carries the _MULLE_C_NO_RETURN attribute, signaling to the compiler that control never returns to the caller.

The failure handler is invoked within every allocation wrapper, including _mulle_allocator_malloc, _mulle_allocator_calloc, _mulle_allocator_realloc, _mulle_allocator_realloc_strict, and _mulle_allocator_strdup. When the default handler executes, the program terminates unconditionally, making it unsuitable for scenarios requiring error recovery or detailed logging.

Designing a Custom Fail Handler That Logs and Continues

To override the abort behavior, you must provide a replacement function conforming to the mulle_allocator_fail_t type defined in src/mulle-allocator.h at lines 175–178.

The Required Function Signature

Your custom handler must match this exact signature:

typedef void mulle_allocator_fail_t( struct mulle_allocator *allocator,
                                     void *block,
                                     size_t size );

Unlike the default implementation, your function should not be marked with _MULLE_C_NO_RETURN. This attribute omission is critical; it allows the compiler to generate code for the normal return path, enabling the allocation wrapper to return NULL to its caller instead of terminating.

Installing Your Handler

Use the inline helper mulle_allocator_set_fail, defined in src/mulle-allocator.h at lines 187–194, to register your handler:

static inline void mulle_allocator_set_fail( struct mulle_allocator *allocator,
                                             mulle_allocator_fail_t *fail);

Pass a pointer to your allocator instance, or NULL to modify the global default allocator. Passing NULL as the second argument restores the default aborting behavior.

Practical Implementation Examples

Example 1: Instance-Specific Logger

This implementation creates a custom allocator based on the standard library allocator but replaces the failure handler to log and continue:

#include <mulle-allocator/mulle-allocator.h>
#include <stdio.h>

static void my_fail_handler( struct mulle_allocator *allocator,
                             void *block,
                             size_t size)
{
    fprintf( stderr,
             "mulle-allocator: allocation of %zu bytes failed (allocator %p)\n",
             size,
             (void *) allocator );
    
    /* Return normally without aborting - execution continues */
    (void) block;
    (void) allocator;
}

int main( void)
{
    struct mulle_allocator my_allocator = mulle_allocator_stdlib;
    mulle_allocator_set_fail( &my_allocator, my_fail_handler );

    /* Force a failure by requesting an impossible amount of memory */
    void *p = mulle_allocator_malloc( &my_allocator, (size_t) -1 );
    
    if( !p)
        printf( "Allocation returned NULL as expected.\n" );

    return 0;
}

Example 2: Global Default Handler

To affect all code using the default allocator without explicit instance passing, install a handler on the global allocator:

#include <mulle-allocator/mulle-allocator.h>
#include <stdio.h>

static void global_fail_logger( struct mulle_allocator *allocator,
                                void *block,
                                size_t size)
{
    fprintf( stderr,
             "[global] allocation of %zu bytes failed (allocator %p)\n",
             size, (void *) allocator );
}

__attribute__((constructor))
static void install_global_logger(void)
{
    /* NULL selects the global default allocator */
    mulle_allocator_set_fail( NULL, global_fail_logger );
}

The constructor attribute ensures registration occurs automatically before main() executes.

Example 3: Test Suite Reference

The file test/coverage/fail-stuff.c provides a minimal working example at lines 33–37. The custom handler fail_fail prints the allocation size and returns normally, demonstrating exactly how to implement a custom fail handler that logs and continues without terminating the application. This serves as the canonical reference for handler implementation patterns.

Handling NULL Returns in Application Code

When your custom handler returns rather than aborts, the allocation wrapper returns NULL to the caller. Your application code must check for these NULL pointers exactly as you would with standard malloc calls. This pattern maintains consistency with C convention while adding robust logging capabilities and allowing the program to attempt recovery or shutdown procedures.

Summary

• The default mulle_allocation_fail in src/mulle-allocator.c terminates the program via abort().
• Define a function matching mulle_allocator_fail_t from src/mulle-allocator.h that logs and returns normally to enable continuation.
• Install the handler using mulle_allocator_set_fail, passing NULL to modify the global allocator or a specific instance pointer.
• Ensure your application checks for NULL returns from allocation calls when using non-aborting handlers.
• Reference test/coverage/fail-stuff.c for a concrete implementation example of logging without termination.

Frequently Asked Questions

What happens if my custom fail handler returns instead of aborting?

When your handler returns, the allocation wrapper (such as _mulle_allocator_malloc) returns NULL to the caller. The program continues execution, and you must handle the NULL pointer appropriately in your application logic to prevent dereferencing errors.

Can I use different fail handlers for different allocators?

Yes. The fail member belongs to individual struct mulle_allocator instances, as shown in src/mulle-allocator-struct.h. You can create multiple allocator instances with distinct handlers using mulle_allocator_set_fail, allowing different logging strategies or failure behaviors for different subsystems or modules.

How do I restore the default aborting behavior?

Call mulle_allocator_set_fail with NULL as the second argument (the handler pointer). This restores the default mulle_allocation_fail function that prints to stderr and calls abort(), effectively reverting to the library's standard failure mode.

Is the fail handler called for every allocation failure?

Yes. According to the source code in src/mulle-allocator.h, the handler is invoked inside every allocation wrapper including _mulle_allocator_malloc, _mulle_allocator_calloc, _mulle_allocator_realloc, _mulle_allocator_realloc_strict, and _mulle_allocator_strdup whenever the underlying system allocation returns NULL.

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