# How to Use Custom Allocators with mulle-buffer

> Unlock custom memory management with mulle-buffer. Learn how to implement custom allocators for efficient buffer operations and tailor memory strategies. Supports custom allocators via struct mulle_allocator.

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

---

**Yes, mulle-buffer fully supports custom allocators through the `struct mulle_allocator` interface, allowing you to specify custom memory management strategies for buffer creation, growth, and destruction operations.**

The mulle-c/mulle-buffer library provides comprehensive support for custom allocators, enabling you to inject specialized memory allocation logic while maintaining automatic fallback to `mulle_default_allocator` when `NULL` is provided.

## The mulle_allocator Interface

Custom allocators must conform to the `struct mulle_allocator` interface, which defines function pointers for `malloc`, `realloc`, and `free` operations. The buffer stores this allocator in its internal `_allocator` field and invokes it for all dynamic memory operations.

When you pass `NULL` as the allocator argument to any buffer function, the library automatically substitutes `mulle_default_allocator`. This ensures that buffers always have a valid allocator while remaining agnostic to whether the allocation strategy is custom or standard.

## Setting Allocators During Buffer Creation

You can specify a custom allocator when creating heap-allocated buffers or initializing stack-based buffer structures.

### Dynamic Buffer Creation

The `mulle_buffer_create` function defined in [`src/mulle-buffer.c`](https://github.com/mulle-c/mulle-buffer/blob/main/src/mulle-buffer.c) accepts an allocator parameter and stores it in the buffer's metadata. According to the implementation in [`src/mulle-buffer.c:28-40`](https://github.com/mulle-c/mulle-buffer/blob/master/src/mulle-buffer.c#L28-L40), the function forwards the allocator to internal initialization helpers.

```c
#include <mulle-buffer/mulle-buffer.h>

int main(void)
{
    struct mulle_allocator *my_allocator = &custom_allocator;
    
    /* Create buffer using custom allocator for all memory operations */
    struct mulle_buffer *buf = mulle_buffer_create(my_allocator);
    if (!buf)
        return 1;
    
    mulle_buffer_add_string(buf, "Hello, custom allocator!");
    mulle_buffer_destroy(buf);
    return 0;
}

```

### Stack-Based Initialization

For stack-allocated `struct mulle_buffer` instances, use `mulle_buffer_init` with the same allocator parameter pattern. This stores the allocator pointer locally for subsequent growth operations.

## Pre-Allocated Memory Patterns

mulle-buffer supports initializing buffers with existing memory blocks while specifying an allocator for future growth. This is useful for stack-backed buffers that may need to spill to heap storage.

### Static Stack Storage

The `mulle_buffer_init_with_static_bytes` function, defined in [`src/mulle-buffer.h:146-156`](https://github.com/mulle-c/mulle-buffer/blob/master/src/mulle-buffer.h#L146-L156), accepts a storage buffer and an allocator. If you pass `NULL` as the allocator, the buffer operates in non-growable mode.

```c
char stack_storage[256];
struct mulle_buffer buf;

/* Use stack storage initially, allow heap growth via default allocator */
mulle_buffer_init_with_static_bytes(&buf, 
                                   stack_storage, 
                                   sizeof(stack_storage), 
                                   NULL);

```

### External Memory Blocks

Similarly, `mulle_buffer_init_with_allocated_bytes` lets you provide pre-allocated heap memory along with a custom allocator for future reallocations. Both initializers store the allocator after setting up the static storage, as shown in the header implementation.

## Runtime Allocator Queries and Updates

You can inspect or modify the allocator associated with an existing buffer at runtime. The `mulle_buffer_set_allocator` function, implemented in [`src/mulle-buffer.h:86-90`](https://github.com/mulle-c/mulle-buffer/blob/master/src/mulle-buffer.h#L86-L90), updates the `_allocator` field or falls back to the default if you pass `NULL`.

```c
/* Query current allocator */
struct mulle_allocator *alloc = mulle_buffer_get_allocator(buf);
if (alloc == &mulle_default_allocator) {
    /* Using default allocator */
}

/* Switch to custom allocator for remaining operations */
mulle_buffer_set_allocator(buf, &my_custom_allocator);

```

## Allocator Usage in Growth Operations

All dynamic memory operations retrieve the stored allocator before invoking memory functions. Functions like `mulle_buffer_grow`, `mulle_buffer_guarantee`, and `mulle_buffer_add_bytes` obtain the allocator via `mulle_buffer_get_allocator` and pass it to the underlying `_mulle__buffer_*` helpers.

As shown in [`src/mulle-buffer.h:572-582`](https://github.com/mulle-c/mulle-buffer/blob/master/src/mulle-buffer.h#L572-L582), the growth logic uses the stored allocator for reallocation operations, ensuring consistent memory management throughout the buffer's lifecycle.

The test suite in [[`test/buffer/allocator.c`](https://github.com/mulle-c/mulle-buffer/blob/main/test/buffer/allocator.c)](https://github.com/mulle-c/mulle-buffer/blob/master/test/buffer/allocator.c) validates this behavior, confirming that `NULL` allocators are correctly replaced by the default and that custom allocators propagate through all growth operations.

## Summary

- **Full custom allocator support** via the `struct mulle_allocator` interface, with automatic fallback to `mulle_default_allocator` when `NULL` is specified.
- **Allocator storage** occurs in the buffer's `_allocator` field during creation via `mulle_buffer_create` or initialization functions (see `src/mulle-buffer.c:28-40`).
- **Flexible initialization** through `mulle_buffer_init_with_static_bytes` and `mulle_buffer_init_with_allocated_bytes`, allowing pre-allocated memory with custom growth allocators (see `src/mulle-buffer.h:146-L156`).
- **Runtime modification** via `mulle_buffer_set_allocator`, which stores the allocator or falls back to default if `NULL` is passed (see `src/mulle-buffer.h:86-90`).
- **Consistent internal usage** ensures all growth operations use the stored allocator retrieved via `mulle_buffer_get_allocator` (see `src/mulle-buffer.h:572-582`).

## Frequently Asked Questions

### Can I use a stack-only buffer without any heap allocator?

Yes. Pass `NULL` as the allocator to `mulle_buffer_init_with_static_bytes`. This creates a non-growable buffer that operates strictly within the provided stack storage. If you attempt to add data beyond the static buffer's capacity, the operation will fail rather than allocate heap memory.

### What happens if I pass NULL as the allocator to mulle_buffer_create?

The function automatically substitutes `mulle_default_allocator` before storing the pointer in the buffer structure. According to the implementation in [`src/mulle-buffer.c`](https://github.com/mulle-c/mulle-buffer/blob/main/src/mulle-buffer.c), all creation and initialization functions perform this NULL-check fallback to ensure the buffer always has a valid allocator for memory operations.

### Can I change the allocator after the buffer is already in use?

Yes. Use `mulle_buffer_set_allocator` to update the allocator pointer at any time. Subsequent growth operations will use the new allocator for reallocations, though existing allocated memory remains managed by the previous allocator until freed. You should ensure allocator consistency when the buffer frees its memory during destruction.

### Does mulle-buffer support arena allocators or memory pools?

Yes. Any allocator conforming to the `struct mulle_allocator` interface—including arena allocators, memory pools, or tracing allocators—can be used with mulle-buffer. The library treats the allocator as an opaque interface, invoking only the standard `malloc`, `realloc`, and `free` function pointers defined in the structure.