How is memory layout shared with other processes/threads?

Question

I'm currently learning memory layout in C. For now I know there exist several sections in C program memory: text, data, bss, heap and stack. They also say heap is shared with other things beyond the program.

My questions are these.

1. What exactly is the heap shared with? One source states that Heap must always be freed in order to make it available for other processes whereas another says The heap area is shared by all threads, shared libraries, and dynamically loaded modules in a process. If it is not shared with other processes, do I really have to free it while my program is running (not at the end of it)?
2. Some sources also single out high addresses (the sixth section) for command line arguments and environment variables. Shall this be considered as another layer and a part of a program memory?
3. Are the other sections shared with anything else beyond a program?

Answer 1

1. The heap is a per-process memory: each process has its own heap, which is shared only within the same process space (like between the process threads, as you said). Why should you free it? Not properly to give space to other processes (at least in modern OS where the process memory is reclaimed by the OS when the process dies), but to prevent heap exhaustion within your process memory: in C, if you don't deallocate the heap memory regions you used, they will be always considered as busy even when they are not used anymore. Thus, to prevent undesired errors, it's a good practice to free the memory in the heap as soon as you don't need it anymore.
2. In a C program the command line variables are stored in the stack as function variables of the main. What happens is that usually the stack is allocated in the highest portion of a process memory, which is mapped to the high addresses (this is probably the reason why some sources point out what you wrote). But, generally speaking, there isn't any sixth memory area.
3. As said by the others, the text area can be shared by processes. This area usually contains the binary code, which would be the same for different processes which share the same binary. For performance reasons, the OS can allow to share such memory area, (think for example when you fork a child process).

Answer 2

On the official aspect:

The terms thread, process, text section, data section, bss, heap and stack are not even defined by the C language standard, and every platform is free to implement these components however "it may like".

Threads and processes are typically implemented at the operating-system layer, while all the different memory sections are typically implemented at the compiler layer.

On the practical aspect:

For every given process, all these memory sections (text section, data section, bss, heap and stack) are shared by all the threads of that process.

Hence, it is under the responsibility of the programmer to ensure mutual-exclusion when accessing these memory sections from different threads.

Typically, this is achieved via synchronization utilities such as semaphores, mutexes and message queues.

In between processes, it is under the responsibility of the operating system to ensure mutual-exclusion.

Typically, this is achieved via virtual-memory abstraction, where each process runs inside its own logical address space, and each logical address space is mapped to a different physical address space.

Disclaimer: some would claim that each thread has its own stack, but technically speaking, those stacks are usually allocated consecutively on the stack of the process, and there's usually no one to prevent a thread from accessing the stacks of other threads, whether intentionally or by mistake (aka stack overflow).

Answer 3

1. Heap is shared with other processes in a sense that all processes use RAM. The more of it you use, the less is available to other programs. Heap sharing with other threads in your own program means that all your threads actually see and access the same heap (same virtual address space, same actual RAM, with some luck also same cache).
2. No.
3. text can be shared with other processes. These days it is marked as read-only, so having several processes share text makes sense. In practice this means that if you are already running top and you run another instance it makes no sense to load text part again. This would waste time and physical RAM. If the OS is smart enough it can map those RAM pages into virtual address space of both top instances, saving time and space.