mulle_buffer_do_string Convenience Macros: RAII String Building in C

The mulle_buffer_do_string convenience macros provide block-scoped, automatic resource management for temporary string builders, extracting a finished C-string and finalizing the buffer when the code block exits.

The mulle_buffer_do_string convenience macros in the mulle-c/mulle-buffer repository eliminate manual memory management when constructing dynamic C-strings. These helper macros wrap struct mulle_buffer operations in a declarative, for-loop-based scope that guarantees cleanup—even when you exit early with break—making them ideal for safe, temporary string concatenation.

What Are mulle_buffer_do_string Convenience Macros?

mulle_buffer_do_string is a block-scoped helper macro defined in src/mulle-buffer.h that treats a struct mulle_buffer as a temporary string builder with automatic extraction. The macro accepts three parameters: the buffer identifier name, an allocator (or NULL for the default), and the output variable that receives the final C-string.

When execution enters the macro’s code block, it instantiates a struct mulle_buffer either on the stack or using the supplied allocator. Inside the block, you call standard mulle_buffer_* functions such as mulle_buffer_add_string() or mulle_buffer_add_c_string() to append data. When the block terminates—normally or via break—the macro automatically calls mulle_buffer_extract_string() (implemented in src/mulle--buffer.c) to transfer ownership of the constructed string to your variable and finalizes the buffer to prevent leaks.

This pattern implements resource-acquisition-is-initialization (RAII) semantics in C: the macro creates the resource, you use it, and the macro’s hidden cleanup code releases it.

How the Macro Works (Source Implementation)

The macro’s implementation relies on a clever double for-loop construct that ensures single execution and safe cleanup. Here is the actual definition from src/mulle-buffer.h:

#define mulle_buffer_do_string( name, allocator, s)               \
   for( struct mulle_buffer                                       \
           name ## __storage = MULLE_BUFFER_DATA( allocator),     \

           *name = &name ## __storage,                            \

           *name ## __i = NULL;                                   \

        \
        s = (name ## __i)                                         \

               ? mulle_buffer_extract_string( &name ## __storage) \

               : NULL,                                            \
        ! name ## __i;                                            \

        \
        name ## __i = (void *) 0x1                                \

      )                                                           \
   \
   for( int  name ## __j = 0;    /* break protection */        \

        name ## __j < 1;                                       \

        name ## __j++)

Execution flow:

• First for loop – Initializes name ## __storage using MULLE_BUFFER_DATA(allocator), creates a pointer alias name, and declares a sentinel name ## __i. The condition ! name ## __i is true initially, so the body executes once.

• String extraction – After the inner block completes, the first loop’s iteration expression assigns s by calling mulle_buffer_extract_string(&name ## __storage), which also frees the buffer’s internal storage. It then sets name ## __i to a non-NULL sentinel, causing the outer loop to terminate.

• Second for loop – Provides a scoped block that runs exactly once, allowing you to use break safely without skipping the cleanup code in the outer loop’s iteration section.

Related Convenience Macros

The mulle_buffer_do_string macro builds upon a family of buffer-scoping helpers, all located in src/mulle-buffer.h:

Macro	Purpose	Key Difference
mulle_buffer_do	Creates a temporary buffer with the default allocator	No automatic string extraction; you manually finalize
mulle_buffer_do_allocator	Creates a buffer with a custom allocator	Accepts an explicit struct mulle_allocator* parameter
mulle_buffer_do_flexible / mulle_buffer_do_filled	Backs the buffer with user-supplied static storage	Avoids heap allocation for small, fixed-size workloads
mulle_buffer_do_string	Adds automatic C-string extraction	Assigns the result of mulle_buffer_extract_string() to your variable on exit

Practical Code Examples

Basic String Building with Default Allocator

The most common use case passes NULL for the allocator to use the system default. This example is adapted from the test suite in test/buffer/do-string.c:

#include "mulle-buffer.h"

void example(void)
{
    char *result;
    mulle_buffer_do_string(buf, NULL, result) {
        mulle_buffer_add_string(buf, "Mulle is ");
        mulle_buffer_add_c_string(buf, "awesome!");
    }
    printf("%s\n", result);   // prints: Mulle is awesome!
    mulle_free(result);
}

Using Custom Allocators

When you need control over memory allocation strategies, provide a custom struct mulle_allocator*. Remember to free the resulting string with the same allocator:

struct mulle_allocator *myalloc = mulle_allocator_create(...);

char *out;
mulle_buffer_do_string(buf, myalloc, out) {
    mulle_buffer_add_c_string(buf, "custom-allocator ");
    mulle_buffer_add_string(buf, "example");
}
/* out must be freed with the same allocator */
mulle_allocator_free(myalloc, out);
mulle_allocator_destroy(myalloc);

Early Exit with break

The macro’s dual-loop structure safely handles early exits. The cleanup code in the outer loop’s iteration section always runs, ensuring buf is finalized even when you break:

char *txt;
mulle_buffer_do_string(buf, NULL, txt) {
    if (some_condition())
        break;                     // skips further appends, still safe
    mulle_buffer_add_string(buf, "won't be executed");
}
if (txt) {
    printf("%s\n", txt);
    mulle_free(txt);
} else {
    puts("No string generated");
}

Summary

• mulle_buffer_do_string is defined in src/mulle-buffer.h and provides RAII-style string building in C.
• The macro accepts three arguments: the buffer name, an allocator (or NULL), and the output string variable.
• Upon block exit, it automatically calls mulle_buffer_extract_string() (from src/mulle--buffer.c) to transfer the C-string and finalize the buffer.
• The implementation uses a double for-loop to guarantee cleanup even when using break.
• Related macros like mulle_buffer_do and mulle_buffer_do_allocator offer flexibility for non-string or custom-allocation scenarios.

Frequently Asked Questions

What is the difference between mulle_buffer_do and mulle_buffer_do_string?

mulle_buffer_do creates a temporary buffer scope but requires you to manually call mulle_buffer_extract_string() or mulle_buffer_done() to finalize resources. mulle_buffer_do_string wraps this workflow by automatically extracting the C-string into your specified variable when the block ends, saving boilerplate and preventing leaks.

How do I free the string returned by mulle_buffer_do_string?

The macro assigns ownership of the extracted C-string to your variable, which you must free using the same allocator passed to the macro. If you used NULL (the default allocator), call mulle_free(string). If you used a custom allocator, use mulle_allocator_free(allocator, string).

Can I use mulle_buffer_do_string with a custom allocator?

Yes. Pass a pointer to a struct mulle_allocator as the second argument instead of NULL. All internal buffer operations, including the final extraction in mulle_buffer_extract_string(), will use your allocator. Ensure you destroy the allocator only after freeing the resulting string.

What happens if I break out of the macro block early?

The macro’s implementation uses a nested for loop specifically to handle break statements safely. When you break from the inner loop, control returns to the outer loop’s iteration expression, which still executes mulle_buffer_extract_string() and finalizes the buffer. Thus, early exits do not leak memory or leave the buffer in an undefined state.

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