Why is $$ returning the same id as the parent process?

$$ is defined to return the process ID of the parent in a subshell; from the man page under "Special Parameters":

$ Expands to the process ID of the shell. In a () subshell, it expands to the process ID of the current shell, not the subshell.

In bash 4, you can get the process ID of the child with BASHPID.

~ $ echo $$17601~ $ ( echo $$; echo $BASHPID )1760117634

bash shell pid subshell

You can use one of the following.

bash shell pid subshell

Parentheses invoke a subshell in Bash. Since it's only a subshell it might have the same PID - depends on implementation.
The C program you invoke is a separate process, which has its own unique PID - doesn't matter if it's in a subshell or not.
$$ is an alias in Bash to the current script PID. See differences between $$ and $BASHPID here, and right above that the additional variable $BASH_SUBSHELL which contains the nesting level.

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