Is a temporary environment variable added before or after the shell creates a child process which will execute a command?

In bash, when running

myvar=val mycommand myargs 

myvar=val will be added to the environment for executing mycommand.

Suppose the the bash process calls fork() to create a child process which will executes mycommand, i.e. mycommand is an external executable file or a script file.

When does adding myvar=val to the environment happen, before or after the bash shell calls fork()? In other words, which of the following two possibilities actually happens?

• the bash process adds myvar=val to its own environment, then calls fork() to create a child process which will calls execve() to execute mycommand, and myvar=val as part of the environment of the bash process is inherited into the environment of the child process. Upon finishing the execution of mycommand and exiting of the child process, the bash process drops myvar=val from its own environment.

• the bash process calls fork() to create a child process which will executes mycommand, and the child process adds myvar=val to its own environment and then calls execve() to execute mycommand.

My question is motivated from Stephen's reply to my earlier post.

In Bash, _ is a special parameter which is set to the value of the last argument every time a command is parsed. It also has the special property of not being exportable, which is enforced every time a command is executed (see bind_lastarg in the Bash source code).

I am wondering that when a bash process executes a command, if bash doesn't add _ to its own environment, why does it need to drop it from its own environment?

Thanks.