Creating shim files for porting Linux system calls to QNX

When porting libraries, it’s common to encounter Linux-specific system calls such as eventfd(), signalfd(), and timerfd(). Since QNX doesn't provide direct equivalents for some of these Linux APIs, it requires shim files to emulate their behavior and ensure compatibility. A shim is a piece of code used to modify or extend the behavior of existing code, typically by introducing a new API that provides a workaround for missing functionality. In QNX's case, shim files help port Linux-specific system calls, such as eventfd(), signalfd(), and timerfd(), to QNX by emulating their behavior. This allows Linux applications to function on QNX without requiring significant modifications to their original code. FreeBSD has done similar things regarding this matter.

For example, the eventfd() object doesn't work on QNX because of fundamental differences in kernel structure between QNX and Linux. In Linux, eventfd() is implemented as a kernel-managed event notification mechanism. When a process creates an eventfd(), the eventfd() object can be used by user-space applications and this object contains unsigned 64-bit integer counter that is maintained by the kernel. QNX, on the other hand, follows a microkernel architecture, where most system services are implemented in user space rather than in the kernel. To bridge this gap, you need to implement a shim file that provides similar functionality in QNX.

The advantage of using shim files is that they allow the original library to be ported with minimal changes, making it easier to maintain and increasing the likelihood of being upstreamed to the original repository. However, the disadvantage is that depending on the project, shim files may introduce inefficiencies by not fully leveraging the target operating system’s native mechanisms. Additionally, they may act as informal workarounds rather than proper solutions, potentially introducing new bugs and unintended behavior depending on the implementation.

You can find a simple implementation of an eventfd() shim file here.