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

> Learn to handle buffer overflow with mulle-buffer. Discover detection, prevention, and recovery strategies to safeguard your code and data. Read now.

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

---

**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`](https://github.com/mulle-c/mulle-buffer/blob/main/src/mulle--buffer.c), checks if the current write position has moved past the allocated boundary:

```c
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:

```c
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`](https://github.com/mulle-c/mulle-buffer/blob/main/src/mulle--buffer.c) handles expansion:

```c
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:

```c
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:

```c
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:

```c
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:

```c
#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:

```c
#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:

```c
#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`](https://github.com/mulle-c/mulle-buffer/blob/main/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`](https://github.com/mulle-c/mulle-buffer/blob/main/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`](https://github.com/mulle-c/mulle-buffer/blob/main/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.