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

> Learn how to create a custom allocator in mulle-allocator by implementing callback functions and passing them to the interface. Take control of your memory allocation.

- Repository: [mulle-c/mulle-allocator](https://github.com/mulle-c/mulle-allocator)
- Tags: how-to-guide
- Published: 2026-03-07

---

**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`](https://github.com/mulle-c/mulle-allocator/blob/main/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`](https://github.com/mulle-c/mulle-allocator/blob/main/src/mulle-allocator-struct.h)**. This plain C struct stores function pointers to the actual memory management operations:

```c
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:

```c
#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`](https://github.com/mulle-c/mulle-allocator/blob/main/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:

```c
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`](https://github.com/mulle-c/mulle-allocator/blob/main/src/mulle-allocator.c)**:

```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`](https://github.com/mulle-c/mulle-allocator/blob/main/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`](https://github.com/mulle-c/mulle-allocator/blob/main/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`](https://github.com/mulle-c/mulle-allocator/blob/main/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`](https://github.com/mulle-c/mulle-allocator/blob/main/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.