What happens to a process when it's overlayed with a new process in Windows

When a process is overlaid with a new process in Windows, it means that a new executable image is loaded into the same process space, effectively replacing the previous process. This is known as process replacement or process overlay.

Here's what happens during the process overlay:

1. The current process is terminated: The operating system initiates the termination of the current process, which involves cleaning up resources, closing handles, and deallocating memory.

2. New process image is loaded: The new executable image is loaded into the same process space that was occupied by the terminated process.

3. Initialization of the new process: The initialization routine of the new process is called. This routine sets up the environment, initializes global variables, and performs any necessary preparations to run the new process.

4. Execution of the new process: The new process starts executing from its entry point, and its main code path begins.

5. Context switch: As the new process starts executing, the CPU switches its execution context from the old process to the new process.

The key point to understand is that the old process is replaced entirely by the new process. The new process inherits the same process ID (PID) as the old process, but all the memory, code, and resources from the old process are replaced with those of the new process.

Process overlay is commonly used when creating child processes in some programming scenarios. For example, in the context of shell commands or scripting, a new process may be created to execute a command, and when the command completes, the original process is overlaid with the result of the command's execution.

It's essential to ensure that the process overlay is done correctly, as any data or resources from the original process that are not correctly cleaned up or released may lead to memory leaks or other issues in the new process.