[c Language] To reference variables in external programs

Can't you refer to the address of the variable in the external program?

For example. After verifying the address of variable a declared in the executable file call.exe.

Write the address of a directly on the pointer p of the executable file called main.exe Can't you refer to the variable value of a in call.exe?

When I search, I only find out about the outside, so I don't know how to find out, so I post a question.