Memory allocation for global and local variables

Question

I have learnt that memory for global variables are allocated at program startup whereas memory for local variables are allocated whenever function call is made.

Answer 1

Cases 2, 3

Variables that you define inside functions are allocated on the stack. That means that the associated memory is cleaned up (the stack is "popped") when the function exits.

Case 1

Variables defined in global scope are allocated in a data segment (or, generally, a memory space requested from the operating system) that exists for the lifetime of the process.

Additionally

Memory allocated using malloc is allocated from a heap and remains allocated until explicitly released using free.

Note that a modern OS may well provide address space requested by a program, but not physically back that address space with RAM until the memory (or a portion of the memory often called a page) is physically accessed.