How to Use the mulle_buffer_do Macro for Flexible Buffer Creation in C

The mulle_buffer_do_flexible macro creates a scoped buffer on the stack that automatically grows to the heap when capacity is exceeded and guarantees cleanup via mulle_buffer_done when the block exits, even if you break early.

The mulle_buffer_do macro family in the mulle-c/mulle-buffer repository provides a deterministic, scope-bound approach to dynamic memory management in C. These macros let you create flexible buffers that start as stack allocations for optimal performance but transparently escalate to heap storage using a specified allocator when data exceeds the initial capacity.

What Makes a Buffer "Flexible"

A flexible buffer begins life with MULLE_BUFFER_FLEXIBLE_DATA, using a fixed-size stack allocation of MULLE_BUFFER_DEFAULT_CAPACITY bytes. If your write operations exceed this initial storage, the buffer automatically allocates memory from the provided allocator (or the default allocator if NULL is passed). This hybrid approach delivers stack-speed for small payloads while handling arbitrarily large data without manual reallocation logic.

Macro Anatomy and Expansion

When you invoke mulle_buffer_do_flexible( name, allocator ), the preprocessor expands this into a nested for-loop structure that declares three critical local variables:

• name ## __storage – The actual struct mulle_buffer instance initialized with flexible storage semantics.

• *name – A pointer to the storage that you manipulate inside the block (e.g., mulle_buffer_add_string( name, "data" )).

• *name ## __i – A sentinel integer ensuring the cleanup phase runs exactly once after the body completes.

The outer loop condition !name ## __i ensures that mulle_buffer_done( &name ## __storage ) executes exactly once when the scope terminates, releasing any heap memory that was allocated during growth.

Safety Guarantees and Control Flow

The macro's design guarantees cleanup execution even if you exit the block early using break. Because the cleanup code resides in the increment section of the outer for loop, it runs regardless of how the body terminates, provided you do not use return.

If you need to return from the enclosing function inside the buffer block, use the companion macro mulle_buffer_return instead of a bare return statement. A standard return bypasses the macro's cleanup logic, potentially leaking heap memory.

Practical Code Examples

The following examples demonstrate common usage patterns for flexible buffer creation in src/mulle-buffer.h.

Basic String Construction with Default Allocator

{
    char *result;

    mulle_buffer_do_flexible( buf, NULL )          // NULL → use default allocator
    {
        mulle_buffer_add_string( buf, "Hello, " );
        mulle_buffer_add_string( buf, "world!" );

        if ( some_condition )
            break;                                 // Cleanup still runs
    }
    result = mulle_buffer_extract_string( &buf );  // Extract before scope closes
    printf("%s\n", result);
    mulle_free( result );
}

Using a Custom Allocator

{
    struct mulle_allocator *alloc = &mulle_default_allocator;
    char *out;

    mulle_buffer_do_flexible( buf, alloc )
    {
        mulle_buffer_add_uint32( buf, 0xDEADBEEF );
        /* Additional writes... */
    }
    out = mulle_buffer_extract_string( &buf );
    puts( out );
    mulle_free( out );
}

Pre-filled Flexible Buffer

For buffers that start with existing data but may need to grow:

{
    unsigned char data[32] = "pre-filled payload";
    size_t len = strlen( (char *)data );

    mulle_buffer_do_flexible_filled( buf, data, len )
    {
        mulle_buffer_add_string( buf, " – more data" );
    }
    /* Buffer can still grow beyond the original 32 bytes if needed */
}

Source Code Reference

The macro definitions reside in src/mulle-buffer.h:

• General mulle_buffer_do infrastructure begins at line 2100
• The flexible variant mulle_buffer_do_flexible starts at line 2164

Additional implementation details appear in src/mulle--buffer.h, which contains the low-level _mulle__buffer_* operations invoked by the macros. For reference test cases, see test/buffer/flexible.c.

Summary

• Stack-to-heap escalation: Buffers begin with MULLE_BUFFER_DEFAULT_CAPACITY bytes on the stack and grow via the specified allocator when full.
• Automatic cleanup: The macro expands to for-loops that guarantee mulle_buffer_done executes exactly once, even on break.
• Scoped pointers: The macro provides a local pointer name to the struct mulle_buffer instance for type-safe operations.
• Safe extraction: Call mulle_buffer_extract_string or similar inside the block to transfer ownership of data to an outer scope variable before automatic destruction.
• Return handling: Use mulle_buffer_return instead of bare return to ensure cleanup when exiting the function from within the block.

Frequently Asked Questions

What happens if I write more data than the initial stack capacity?

The buffer automatically reallocates using the allocator you provided (or the default allocator if NULL). The growth is transparent to your code; you simply continue calling mulle_buffer_add_* functions without checking capacity.

Can I use return inside the mulle_buffer_do_flexible block?

A bare return bypasses the macro's cleanup logic and leaks any heap memory the buffer allocated. Use the companion macro mulle_buffer_return instead, which performs cleanup before returning from the function.

How do I retrieve data after the buffer scope ends?

You must extract data before the closing brace of the block. Functions like mulle_buffer_extract_string return heap-allocated copies of the buffer contents. Store this pointer in a variable declared outside the macro block, then free it manually (e.g., with mulle_free) when done.

What is the difference between mulle_buffer_do_flexible and mulle_buffer_do_flexible_filled?

mulle_buffer_do_flexible starts with empty storage, while mulle_buffer_do_flexible_filled accepts initial data and length parameters to pre-populate the buffer. Both variants maintain the ability to grow beyond their initial allocation if subsequent writes exceed capacity.

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  "mcpServers": {
    "instagit": {
      "command": "npx",
      "args": ["-y", "instagit@latest"]
    }
  }
}