How to Create a Custom Allocator with Your Own Memory Allocation Functions in mulle‑allocator

You create a custom allocator by implementing callback functions that match the signatures in struct mulle_allocator, storing them in a struct instance defined in src/mulle-allocator-struct.h, and passing that instance to the allocator interface functions.

The mulle‑allocator library from the mulle-c ecosystem treats memory allocation as a configurable strategy rather than a fixed system dependency. Creating a custom allocator with your own memory allocation functions enables you to add diagnostics, implement memory pools, or integrate with specialized hardware while maintaining compatibility with the wider mulle‑allocator API.

Understanding the Allocator Structure

At the core of the system is struct mulle_allocator, defined in src/mulle-allocator-struct.h. This plain C struct stores function pointers to the actual memory management operations:

struct mulle_allocator
{
    void   *(*calloc)( size_t n, size_t size,
                       struct mulle_allocator *allocator);
    void   *(*realloc)( void *block, size_t size,
                        struct mulle_allocator *allocator);
    void   (*free)( void *block,
                    struct mulle_allocator *allocator);
    void   (*fail)( struct mulle_allocator *allocator,
                    void *block, size_t size) _MULLE_C_NO_RETURN;
    int    (*abafree)( void *aba,
                       void (*free)( void *, void *),
                       void *block, void *owner);
    void   *aba;
};

Each function receives a pointer to the allocator itself as the final argument, allowing you to store per‑allocator state—such as statistics or context pointers—in the aba field.

Implementing Custom Memory Functions

To create a functional allocator, you must provide implementations for calloc, realloc, and free. You may also supply a custom fail handler for out‑of‑memory scenarios and an abafree routine for lock‑free data structures.

The signatures must match exactly:

• void *(*calloc)(size_t n, size_t size, struct mulle_allocator *allocator)
• void *(*realloc)(void *block, size_t size, struct mulle_allocator *allocator)
• void (*free)(void *block, struct mulle_allocator *allocator)
• void (*fail)(struct mulle_allocator *allocator, void *block, size_t size)

These implementations can wrap standard library functions, use mmap for large allocations, or route requests to a memory pool.

Building and Configuring Your Allocator

After implementing the callbacks, instantiate a struct mulle_allocator and populate its fields. The following example demonstrates a logging allocator that wraps stdlib functions and tracks every operation:

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

/* 1. Implement the required callbacks */
static void *
my_calloc( size_t n, size_t size, struct mulle_allocator *alloc )
{
    void *p = calloc( n, size );
    fprintf( stderr, "[my_alloc] calloc %zu×%zu → %p\n", n, size, p );
    (void) alloc;
    return p;
}

static void *
my_realloc( void *block, size_t size, struct mulle_allocator *alloc )
{
    void *p = realloc( block, size );
    fprintf( stderr, "[my_alloc] realloc %p → %p (%zu)\n", block, p, size );
    (void) alloc;
    return p;
}

static void
my_free( void *block, struct mulle_allocator *alloc )
{
    fprintf( stderr, "[my_alloc] free %p\n", block );
    free( block );
    (void) alloc;
}

static void
my_fail( struct mulle_allocator *alloc, void *block, size_t size )
{
    (void) alloc; (void) block;
    fprintf( stderr, "[my_alloc] out‑of‑memory (requested %zu bytes)\n", size );
    abort();
}

/* 2. Create the allocator instance */
static struct mulle_allocator my_allocator = {
    .calloc   = my_calloc,
    .realloc  = my_realloc,
    .free     = my_free,
    .fail     = my_fail,
    .abafree  = mulle_allocator_no_aba_abort,
    .aba      = NULL
};

/* 3. Optional: Configure hooks programmatically */
static void
install_custom_hooks( void )
{
    mulle_allocator_set_fail( &my_allocator, my_fail );
}

The helper function mulle_allocator_set_fail() is defined inline in src/mulle-allocator.h and ensures the allocator never uses a NULL failure handler.

Using Your Custom Allocator

Pass your allocator explicitly to the typed allocation functions, or temporarily swap the global default. The explicit approach provides maximum control:

int main( void )
{
    install_custom_hooks();

    /* Allocate using the custom allocator explicitly */
    char *msg = mulle_allocator_strdup( &my_allocator, "Hello, mulle‑allocator!" );
    printf( "%s\n", msg );

    msg = mulle_allocator_realloc( &my_allocator, msg, 64 );
    strcpy( msg, "Resized message" );
    printf( "%s\n", msg );

    mulle_allocator_free( &my_allocator, msg );
    return 0;
}

To affect all code using the convenience macros (mulle_malloc, mulle_free), temporarily replace mulle_default_allocator, which is instantiated in src/mulle-allocator.c:

struct mulle_allocator *old = mulle_default_allocator;
mulle_default_allocator = &my_allocator;

void *p = mulle_malloc( 128 );
mulle_free( p );

mulle_default_allocator = old;

Summary

• struct mulle_allocator in src/mulle-allocator-struct.h defines the interface through function pointers for calloc, realloc, free, and optional fail and abafree handlers.
• Implement callbacks that match the required signatures, receiving the allocator pointer to access per‑instance state stored in the aba field.
• Configure the allocator by populating a struct instance and optionally using mulle_allocator_set_fail() or mulle_allocator_set_aba() from src/mulle-allocator.h.
• Route allocations through your custom logic by passing the allocator to functions like mulle_allocator_malloc(), or by temporarily assigning your struct to mulle_default_allocator.

Frequently Asked Questions

What is the purpose of the aba field in struct mulle_allocator?

The aba field provides per‑allocator state storage, typically used for context pointers, statistics counters, or lock‑free bookkeeping data. Because every callback receives a pointer to the allocator struct, you can access aba from within your custom calloc, realloc, or free implementations to maintain state across allocation operations.

Can I replace the global default allocator temporarily?

Yes. The global pointer mulle_default_allocator (defined in src/mulle-allocator.c) can be saved and reassigned to your custom struct. All convenience macros like mulle_malloc() and mulle_free() route through this pointer, allowing you to inject custom behavior into legacy code without changing every call site. Always restore the original pointer after use to prevent memory mismanagement.

How does the custom failure handler work?

The fail callback is invoked when an allocation request cannot be satisfied. By default, allocators use an abort wrapper, but you can install a custom handler using mulle_allocator_set_fail(). Your handler receives the allocator, the block being resized (if applicable), and the requested size, allowing you to log diagnostics, attempt cleanup, or trigger a graceful shutdown instead of immediate termination.

Where are the default allocator implementations located?

The standard allocators—mulle_allocator_stdlib, mulle_allocator_default, and mulle_allocator_stdlib_nofree—are instantiated in src/mulle-allocator.c. These provide reference implementations that wrap the standard C library functions, serving as templates for your own custom allocators or as fallbacks when no specialized behavior is required.

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