How to Handle Buffer Overflow with mulle-buffer: Detection, Prevention, and Recovery

mulle-buffer detects buffer overflow when the write cursor (_curr) exceeds the sentinel pointer (_sentinel), at which point it marks the buffer by advancing _curr one byte past the boundary and preserving the pre-overflow length in _size for later inspection.

The mulle-c/mulle-buffer library provides a low-level, growable byte buffer implementation for C applications built around struct mulle__buffer. Understanding how to properly handle buffer overflow with mulle-buffer ensures your applications can gracefully manage memory constraints, whether using auto-growing flexible buffers or fixed-size inflexible storage.

How Overflow Detection Works Internally

Overflow detection in mulle-buffer relies on a simple pointer comparison within the core data structure. The internal predicate function _mulle__buffer_has_overflown(), located in src/mulle--buffer.c, checks if the current write position has moved past the allocated boundary:

static inline int _mulle__buffer_has_overflown( struct mulle__buffer *buffer )
{
    return( buffer->_curr > buffer->_sentinel );
}

When this condition evaluates to true, the buffer enters an overflowed state. Subsequent write operations will silently discard data rather than corrupt memory. All public API functions (mulle_buffer_add_*, mulle_buffer_set_length, etc.) ultimately delegate to these internal primitives, ensuring consistent overflow handling across the entire library.

Marking and Storing the Overflow State

Once overflow is detected, the library invokes _mulle__buffer_set_overflown() to record the failure state. This function captures the current length before the overflow occurred and explicitly sets _curr beyond the sentinel to create a persistent flag:

void _mulle__buffer_set_overflown( struct mulle__buffer *buffer )
{
    assert( ! _mulle__buffer_has_overflown( buffer ) );

    buffer->_size = _mulle__buffer_get_length( buffer );
    buffer->_curr = buffer->_sentinel + 1;   // “overflowed”
}

After this state is set, _mulle__buffer_get_length() returns the preserved buffer->_size rather than calculating the current cursor position, allowing you to determine exactly how much data was successfully written before the truncation occurred.

Buffer Types and Overflow Behavior

The overflow handling strategy depends entirely on the buffer's configuration. The library distinguishes between flexible, inflexible, and flushable variants.

Flexible Buffers (Automatic Growth)

Flexible buffers attempt to grow dynamically when reaching capacity. The central routine _mulle__buffer_grow() in src/mulle--buffer.c handles expansion:

if( _mulle__buffer_is_inflexible( buffer ) )
{
    /* no growth possible → overflow */
    _mulle__buffer_set_overflown( buffer );
    return -1;
}

If allocation succeeds, the buffer continues operation transparently. Overflow only occurs if the underlying allocator fails to provide additional memory.

Inflexible Buffers (Fixed Storage)

Inflexible buffers wrap existing memory (such as stack arrays) and cannot grow. When the fast-path add-byte routine detects a full buffer, it immediately triggers overflow handling:

if( _mulle__buffer_is_full( buffer ) )
    if( _mulle__buffer_grow( buffer, 1, allocator ) )
        return;          // overflow already recorded

For these buffers, you must manually check the overflow flag after write operations to verify data integrity.

Flushable Buffers

Flushable buffers implement the struct mulle_flushablebuffer interface with custom callbacks. If _mulle__buffer_flush() cannot provide sufficient space and the buffer cannot expand, it forces an overflow state:

if( ! _mulle__buffer_is_flushable( buffer ) )
{
    _mulle__buffer_set_overflown( buffer );
    return -1;                // cannot flush → overflow
}

This ensures that temporary I/O failures do not result in undefined behavior.

Detecting Overflow in Application Code

While the internal _mulle__buffer_has_overflown() function is not exposed in the public headers, you can safely cast a struct mulle_buffer * to struct mulle__buffer * to inspect the state:

struct mulle_buffer *buf = mulle_buffer_create( allocator );

/* fill the buffer … */
mulle_buffer_add_bytes( buf, data, data_len, allocator );

/* After any write‑operation you can test the overflow flag */
if( _mulle__buffer_has_overflown( (struct mulle__buffer *)buf ))
{
    fprintf( stderr, "buffer overflow – data truncated at %zu bytes\n",
             mulle_buffer_get_length( buf ) );
    /* optional: reset or recreate the buffer */
    mulle_buffer_reset( buf );
}

Calling mulle_buffer_reset() clears the overflow flag and restores the buffer to a usable state without requiring reallocation.

Practical Code Examples

Example 1: Flexible Buffer with Auto-Growth

Flexible buffers typically require no manual overflow checking because the library handles expansion automatically:

#include "mulle-buffer.h"

int main( void )
{
    struct mulle_buffer *buf = mulle_buffer_create( NULL );   // default allocator

    /* Write 1 MiB – the buffer will grow automatically */
    for( size_t i = 0; i < 1024 * 1024; ++i )
        mulle_buffer_add_byte( buf, (uint8_t)i, NULL );

    printf( "length = %zu\n", mulle_buffer_get_length( buf ) );
    mulle_buffer_destroy( buf, NULL );
}

Example 2: Inflexible Buffer with Manual Overflow Handling

When wrapping fixed stack storage, you must verify the overflow flag to detect truncation:

#include "mulle-buffer.h"

int main( void )
{
    unsigned char storage[256];
    struct mulle_buffer *buf = mulle_buffer_create( NULL );

    /* Turn it into an inflexible buffer that uses the stack storage */
    mulle_buffer_make_inflexible( buf, storage, sizeof storage, NULL );

    /* Attempt to write more than 256 bytes */
    for( size_t i = 0; i < 300; ++i )
        mulle_buffer_add_byte( buf, (uint8_t)i, NULL );

    if( _mulle__buffer_has_overflown( (struct mulle__buffer *)buf ) )
        fprintf( stderr, "overflow after %zu bytes\n",
                 mulle_buffer_get_length( buf ) );

    /* Reset clears the overflow flag */
    mulle_buffer_reset( buf );
    /* … reuse the buffer … */

    mulle_buffer_destroy( buf, NULL );
}

Example 3: Flushable Buffer with Custom Callback

For buffers implementing the flushable interface, overflow occurs when the flush mechanism cannot free sufficient space:

#include "mulle-buffer.h"
#include "mulle-flushablebuffer.h"

/* Simple flush that writes to stdout and clears the buffer */
static int my_flush( struct mulle_flushablebuffer *fb )
{
    write( 1, fb->_storage, fb->_curr - fb->_storage );
    mulle_buffer_reset( (struct mulle_buffer *)fb );
    return 0;
}

int main( void )
{
    struct mulle_flushablebuffer *fb =
        mulle_flushablebuffer_create( NULL );

    fb->_flush = my_flush;   // install callback

    for( size_t i = 0; i < 5000; ++i )
        mulle_buffer_add_byte( (struct mulle_buffer *)fb, (uint8_t)i, NULL );

    /* If the buffer cannot flush enough space it will be marked overflowed */
    if( _mulle__buffer_has_overflown( (struct mulle__buffer *)fb ) )
        fprintf( stderr, "flushable buffer overflowed\n" );

    mulle_flushablebuffer_destroy( fb, NULL );
}

Summary

• Detection: Overflow is detected in src/mulle--buffer.c when _curr > _sentinel, checked via _mulle__buffer_has_overflown().
• State storage: _mulle__buffer_set_overflown() preserves the pre-overflow length in _size and advances _curr to _sentinel + 1.
• Flexible buffers: Automatically grow via _mulle__buffer_grow(); overflow only occurs on allocation failure.
• Inflexible buffers: Immediately overflow when full; require manual checking by casting to struct mulle__buffer *.
• Recovery: Call mulle_buffer_reset() to clear the overflow flag and reuse the buffer.
• Flushable buffers: Enter overflow state when flush callbacks cannot provide space, as implemented in src/mulle-flushablebuffer.c.

Frequently Asked Questions

How do I check if a mulle-buffer has overflowed?

Cast the public struct mulle_buffer * pointer to struct mulle__buffer * and call _mulle__buffer_has_overflown(). This function returns a non-zero value if the buffer's write cursor has exceeded its sentinel pointer. According to the source code in src/mulle--buffer.c, this check compares buffer->_curr > buffer->_sentinel.

What happens to data written after an overflow occurs?

All subsequent write operations silently discard data. The public API functions (mulle_buffer_add_byte, mulle_buffer_add_bytes, etc.) return early when they detect the overflow state, preventing memory corruption while allowing the program to continue execution.

Can I recover a buffer after overflow without reallocating it?

Yes. Call mulle_buffer_reset() on the buffer to clear the overflow flag, reset the cursor to the beginning of storage, and restore the _sentinel to its original position. This operation preserves the underlying memory allocation but clears all content and error states.

Does mulle-buffer prevent overflow automatically?

Only for flexible buffers. If you create a buffer with mulle_buffer_create(), it defaults to flexible mode and attempts to grow via _mulle__buffer_grow() when reaching capacity. However, if you convert the buffer to inflexible mode using mulle_buffer_make_inflexible() or if memory allocation fails, the buffer will overflow and require manual handling.

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