willemt / cbuffer

A circular buffer written in C using Posix calls to create a contiguously mapped memory space. BSD Licensed.

Circular buffers are tricky. The caller needs to know if the polled item crosses the maximum size of the underlying array.

This circular buffer uses a mmap magic trick to make the caller's life easier.

This is how we do it:

1. mmap is used to mirror the buffer like below:

2. the "mirrored" buffer is then placed beside the buffer. When the user polls the item it doesn't matter if the item crosses the buffer's boundary:

Compatibility

This only works on UNIX systems.

Windows support existed in d10238600a82de32fe1cb01bec8667d3a67e382c but was removed. This is because Windows doesn't actually have mmap and trying to get the same behaviour is painful. Maybe you should check this out instead: https://github.com/willemt/bipbuffer

cbuffer's Issues

Achieving this buffer in Python

Hi there Willem, I was taking a look at your repo on github and have been looking to do something similar in python. 

However, I cannot seem to figure out how to do this due to not having the pointer arithmetic
available within the mmap call to python. 
https://github.com/willemt/cbuffer/blob/master/cbuffer.c#L52

Line 52 is what I cannot mimic, because the offset param here does not help us with the window. 
Any help would be greatly appreciated!   

Regards 

Update me->tail with a single assignment operation

In the original code, me->tail is updated through a sum and then a modulo operation. This creates a race condition if another thread reads the value of tail at the same time (for instance in the usedspace function). So doing the assignment in a single instruction should be safer. It's no guarantee though, so using atomic primitives would be safer.

I created pull request #4.