How mulle_alloca_do Handles Nested Allocations and Stack Size Limits

The mulle_alloca_do macro family in mulle-c/mulle-allocator automatically switches from stack to heap allocation when requests exceed the MULLE_ALLOCA_STACKSIZE limit (default 128 bytes) and supports safe nesting through uniquely named hidden variables, though return shortcuts are restricted to single-level blocks.

The mulle-allocator library provides temporary, block-scoped memory allocation through the mulle_alloca_do and mulle_calloca_do macros. These utilities balance performance and safety by preferring fast stack storage while transparently falling back to heap allocation when necessary, even allowing runtime promotion from stack to heap during reallocation.

Understanding the Stack Size Limit

Compile-Time Constant and Default Value

The library defines a compile-time threshold that determines how much data can reside on the stack. In src/mulle-alloca.h at lines 108–110, the default limit is set:

#ifndef MULLE_ALLOCA_STACKSIZE

# define MULLE_ALLOCA_STACKSIZE  128   // bytes, equivalent of double[16]

#endif

This 128-byte default represents a safety margin suitable for small temporary buffers. You can override this value at compile time to adjust the trade-off between stack usage and heap allocation frequency.

Automatic Escalation to Heap Memory

When you invoke mulle_alloca_do(type, name, count), the macro compares your requested size against MULLE_ALLOCA_STACKSIZE. As implemented in src/mulle-alloca.h at lines 86–89, the logic uses a conditional expression:

type *name = ((uintptr_t) name ## __count) <= _mulle_alloca_do_get_size_as_length(name)

               ? name ## __storage

               : mulle_malloc(sizeof(type) * (uintptr_t)(void *)name ## __count);

If the request fits within the limit, the pointer references the stack-allocated array name__storage. If it exceeds the limit, the macro calls mulle_malloc to create a heap buffer. This automatic escalation ensures that large allocations do not risk stack overflow while small allocations remain zero-cost.

Nested Allocation Support and Safety

Variable Name Isolation

Each mulle_alloca_do expansion generates a dedicated set of hidden variables using token concatenation: name__storage, name__count, name__i, name__j, and name__k. Because these identifiers derive from the user-provided name parameter, you can safely nest multiple blocks provided each uses a unique name:

mulle_alloca_do(buf1, char, 64) {          // Uses buf1__storage, etc.
    // Work with buf1 on stack (64 bytes < 128 bytes)

    mulle_alloca_do(buf2, int, 32) {      // Uses buf2__storage, etc.
        // Independent allocation, may be stack or heap depending on size
    }   // buf2 cleanup runs here
}   // buf1 cleanup runs here

The scoping rules of C ensure that each inner block's cleanup code executes before the outer block's, preventing memory leaks or double-frees.

Limitations with Return Shortcuts

The header explicitly warns against using the convenience return macros _mulle_alloca_do_return and _mulle_alloca_do_return_void within nested blocks. As noted at line 140 in src/mulle-alloca.h:

"They won't magically do the right thing in nested mulle_alloca_dos."

These shortcuts are designed only for single-level blocks. In nested scenarios, you must use standard control flow:

• Use break to exit the innermost block, which triggers its cleanup
• Allow the block to complete normally
• Place the return statement after the outer block closes

This restriction exists because the return shortcuts cannot unwind multiple levels of the internal for-loop machinery that manages the allocations.

Runtime Reallocation and Stack-to-Heap Promotion

Buffers initially allocated on the stack can grow beyond the original limit at runtime. The mulle_alloca_do_realloc macro (and its calloca variant) handles this promotion transparently. The implementation at lines 92–102 in src/mulle-alloca.h detects the current storage location:

if (name == name ## __storage) {

    if (_count > _mulle_alloca_do_get_size_as_length(name)) {
        name = mulle_malloc(sizeof(*name) * _count);
        memcpy(name, name ## __storage,

               ((uintptr_t) name ## __count) * sizeof(*name));

    }
} else {
    _count = _count ? _count : 1;
    name = mulle_realloc(name, _count * sizeof(*name));
}
name ## __count = (void *)_count;

When the new count exceeds the stack limit, the macro allocates a heap buffer, copies the existing stack contents, and updates the pointer. Subsequent reallocations use standard mulle_realloc since the buffer now resides on the heap. This promotion strategy maintains data integrity while adapting to growing memory requirements.

Summary

• Stack limit is controlled by the MULLE_ALLOCA_STACKSIZE compile-time constant (default 128 bytes), defined in src/mulle-alloca.h.
• Automatic escalation occurs when allocation requests exceed the stack limit, triggering mulle_malloc instead of stack allocation.
• Nested blocks are safe when each uses a distinct variable name, as the macro generates isolated hidden identifiers via token pasting.
• Return shortcuts (_mulle_alloca_do_return) do not support nested usage; use break or normal block completion instead.
• Stack-to-heap promotion happens automatically during reallocation when the requested size surpasses the original stack capacity.
• Cleanup is handled by the for-loop structure calling _mulle_alloca_do_free, which frees heap buffers and ignores stack storage.

Frequently Asked Questions

What is the default stack size limit for mulle_alloca_do?

The default limit is 128 bytes, defined by the MULLE_ALLOCA_STACKSIZE macro in src/mulle-alloca.h (lines 108–110). You can override this at compile time by defining the macro before including the header. Requests smaller than or equal to this limit receive stack allocation; larger requests automatically use the heap.

Can I safely nest mulle_alloca_do blocks inside each other?

Yes, provided each block uses a unique variable name. The macro generates hidden variables (like name__storage and name__count) using the name you provide, so distinct names prevent identifier collisions. The scoping ensures inner blocks clean up before outer blocks.

What happens if I reallocate a stack buffer beyond MULLE_ALLOCA_STACKSIZE?

The mulle_alloca_do_realloc macro promotes the buffer from stack to heap automatically. It detects that the current pointer equals the stack storage, allocates a new heap buffer using mulle_malloc, copies the existing data with memcpy, and redirects the pointer. Future reallocations then use mulle_realloc on the heap buffer.

Why do the return shortcuts fail in nested mulle_alloca_do blocks?

The _mulle_alloca_do_return and _mulle_alloca_do_return_void macros are implemented as simple flow-control wrappers that assume a single-level block structure. In nested scenarios, they cannot properly unwind the multiple for-loop layers that manage different buffers. The source code at line 140 in src/mulle-alloca.h explicitly warns that these shortcuts "won't magically do the right thing" when nested.

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