How memory allocation of variables or data in a program are done by compiler and OS

Question

Want to get an overview on a few things about how exactly the memory for a variable is allocated.

In C programming, Taking the context of "auto" variables, which are allocated on the stack section, I have the following question:

Does the compiler generate a logical address for the variables? If yes, then how? Won't the compiler need OS permission to generate or assign such addresses? If no, then is there some sort of indication or instruction that the compiler puts in the code segment asking the OS to allocate memory when running the executable?

Now taking the context of heap allocated variables,

Is the heap of the same size for all programs? If not, then does the executable consist of a header or something that tells the OS how much heap space it needs for dynamic allocation?

I'd be grateful if someone provides the answer or shares any related content/links that explains this.

Answer 1

Does the compiler generate a logical address for the variables? If yes, then how?

Yes, but they are related to the stack pointer at function entry time (which is normally saved as a constant base pointer, stored in a cpu register) This is because the function can be recursive, and you can have two calls to the function with different instances for that variable (and related to different copies of the base pointer), the compiler assigns the offset to the base pointer for the variable, but the base pointer can be different, depending on the stack contents at function entry time.

Won't the compiler need OS permission to generate or assign such addresses?

Nope, the compiler just generates an executable in the format and form needed for the operating system to manage process' memory. When the program starts, it is given normally three (or more) segments of memory:

• text segment. A normally read-only (or execute only) segment that gives no write access to the program. This is normally because the text segment is shared between all programs that are using the same executable at the same time. A program can demand exclusive read-write acces to the text (to allow programs that modify their own executable code) but this happens only rarely. This is normally specified to the compiler and the compiler writes an special flag in the text segment to inform the kernel of this requirement.
• Data segment. A read-write segment, that can be grown by means of a system cal (sbrk(2)) This is used for global variables and the heap (while in modern systems, the heap is allocated into a new segment acquired by calling the mmap(2) system call. Sometimes this segment is divided in two. A data segment read-only for constants (so the program receives a signal in case you try to change the value of a constant) and a read-write segment, freely usable by the program. This is where global variables are stored.
• Stack segment. A read-write segment, that is allocated for the process to use as the stack segment. It has the capability of growing in one direction as the process starts using it. When the process accesses the data one memory page below the start of the segment, it generates a page fault trap that results in a new page being appended to the segment, so its workings are transparent to the process. This is the memory we are talking about.
• the process can ask the kernel explicitly to get a new segment if it wants to (let's say it needs to map some file on memory, or if it has to load a shared executable/library) and on some systems, the read only variables (declared as const) are explititly stored in the text segment or in a specific section called .rodata that demands from the system a special data segment that is read-only. The compiler doesn't normally code this kind of resource itself, it is normally encoded in the program being compiled.

The complete memory is limited by system imposed limits, so if you try to overpass them (around 8Mb of stack space, by default, and depending on the operating system) you will get signalled by the system and your program aborted.

As you see, the process memory is owned by the process, and it can make whatever use it is permitted to. The stack memory is read/write, and allocated on demand, you can use up to 8Mb, but there's no provision to check on the use you do about it.

If no, then is there some sort of indication or instruction that the compiler puts in the code segment asking the OS to allocate memory when running the executable?

The system will know the size of the text segment of the process by the size it has on the executable. The data segment is divided into two parts, normally based on the assumption of what are the global initialized variables and what are the ones defaulting to zero (the memory allocated by the kernel to a process is initialized to zeros for security reasons) so the sum of both the initialized/data and the non initialized data sections are added to know how much memory to assign to the data segment. And the stack segment is assigned initialy just one page of memory, but as the process starts running and filling the stack, it grows as the process generates page faults on the so called next page of the stack segment. As you see, there's no nedd for the compiler to embed in the code any instruction to ask for more memory. Everything is read from the executable file.

The compiler runs as a normal program... it only generates all this information and writes it in a file (the executable file) for the kernel to know the resources needed to run the program. But the compiler's communication with the kernel is just to ask it to open files, write on them, read from source code and struggle it's head to achieve its task. :)

In most POSIX systems, the kernel loads a program in memory by means of the exec*(2) system calls. The kernel reads the executable file pointed to in a parameter of the call and creates the segments above mentioned, based on the parameters passed in the file, checks if another instance of the same program is running in the system to avoid loading the instructions from the file and referencing in this process the segment already open by the other. The data segment contents is initialized to zeros, and the contents of the initialization data are read into the segment (so the first part has the .data section of initialized global variables and the .bss section, which has only a size, is used to calculate the total size of the data segment). Then the stack is normally allocated one or more pages, depending on the initial contents that the exec() calls put in the initial stack. The initial stack is filled with:

• a structure of data containing references to the program parameter list that was used on legacy systems to provide the kernel about the command line parameters to show in the ps(1) command output (this is still being generated for legacy purposes, but not used by the kernel for obvious security reasons) Today, a special system call is used to indicate the kernel the command line parameters to be output in the ps(1) output.
• a snippet of machine code to use in the return from a system call to allow the execution (in user mode) of any signal handler that should be executed (this is the reason for the requirement that all signal handlers are called when the kernel returns from kernel mode and switches back again to user mode, and not otherwise)
• the environment of the process.
• the array of pointers to environment strings.
• the command line parameters.
• the array of char pointers that point to the command line parameters.
• the envp array referenct to main().
• the argv array reference to the command line parameters.
• the argc counter of the number of command line parameters.

Once all these data is pushed to the stack, the program jumps to the start address (fixed by the linker, or by the user by a linker option) and is let to start running.

Before the program jumps to main() the executed code is part of the C runtime, that loads a special shared executable (called /lib/ld.so or similar) that is responsible of searching and loading of all the shared libraries that are linked to the program. Not all the programs have this feature (but almost all of them today are dynamically linked) but IMHO this is out of the scope to this question, as the program has already started and is running.

Answer 2

Some background first

In C programming, Taking the context of "auto" variables, which are allocated on the stack section ...

To understand my answer, you first need to know how the stack works.

Imagine you write the following function:

int myFunction()
{
    return function1() + function2() + function3();
}

Unfortunately, you do not use C as programming language but you use a programming language that neither supports local variables nor return values. (This is how most CPUs work internally.)

You may return a value from a function in a global variable:

function1()
{
    result = 1234; // instead of: return 1234;
}

And your program may now look the following way if you use a global variable instead of local ones:

int a;

myFunction()
{
    function1();
    a = result;
    function2();
    a += result;
    function3();
    result += a;
}

Unfortunately, one of the three functions (e.g. function3()) may call myFunction() (so the function is called recursively) and the variable a is overwritten when calling function3().

To solve this problem, you may define an array for local variables (myvars[]) and a variable named mypos. In the example, the elements 0...mypos in myvars[] are used; the elements (mypos+1)...(MAX_LOCALS-1) are free:

int myvars[MAX_LOCALS];
int mypos;
...
myFunction()
{
    function1();
    mypos++;
    myvars[mypos] = result;
    function2();
    myvars[mypos] += result;
    function3();
    result += myvars[mypos];
    mypos--;
}

By changing the value of mypos from 10 to 11 (as an example), your program indicates that the element mypos[11] is now in use and that the functions being called shall store their data in elements mypos[x] with x>=12.

Exactly this is how the stack is working.

Typically, the "variable" mypos is not a variable but a CPU register named "stack pointer". (However, there are a few historic CPUs where an ordinary variable was used for this!)

The actual answers

Does the compiler generate a logical address for the variables?

In the example above, the compiler will perform a mypos+=3 if there are 3 local variables. Let's say they are named a, b and c.

The compiler simply replaces a by myvars[mypos-2], b by myvars[mypos-1] and c by myvars[mypos].

On most CPUs, the stack pointer (named mypos in the example) is not an index into an array but already a pointer (comparable to int * mypos;), so the compiler would replace a by *(mypos-2) instead of myvars[mypos-2] in the example.

For global variables, the compiler simply counts the number of bytes needed for all global variables. In the simplest case, it chooses a range of memory of the same size (e.g. 0x10000...0x10123) and places the variables there.

Won't the compiler need OS permission to generate or assign such addresses?

No.

The "stack" (in the example this is the array myvars[]) is already provided by the OS and the stack pointer (mypos in the example) is also set to a correct value by the OS before the program is started.

Your program knows that the elements myvars[x] with x>mypos can be used.

For global variables, the information about the range used by global variables (e.g. 0x10000...0x10123) is stored in the executable file. The OS must ensure that this memory range can be used by the program. (For example by configuring the MMU accordingly.)

If this is not possible, the OS will simply refuse to start the program with an error message.

... asking the OS to allocate memory when running the executable?

For variables on the stack:

There may be operating systems where this is done.

However, in most cases, the program will simply crash with a "stack overflow" if too much stack is needed. In the example, this would mean: The program crashes if an elements myvars[x] with x>=MAX_LOCALS is accessed.

Now taking the context of heap allocated variables ...

Please first note that global variables are not stored on the heap.

The heap is used for data allocated using malloc() and similar functions (new in C++ for example).

Under many operating systems, malloc() calls an operating system function - so it is actually the operating system that allocates memory on the heap.

... and if there is not enough space, the OS (and malloc()) will return NULL.

Answer 3

A method typically used in operating systems is that, when a program is starting, there is a piece (or collection) of software used that loads the program. The program loader reads the executable file and sets up memory for the program.

Part of the executable file says what size stack should be allocated for it. Most often, this is set by default when linking the program. (It is 8 MiB for macOS, 2 MiB for Linux, and 1 MiB for Windows.) However, it can be changed by asking the linker to set a different size.

The program loader calls operating system routines to request virtual memory be mapped. It does this for the stack and for other parts of the program, such as the code sections (the parts of memory that contain, mostly, the executable instructions of the program), and the initialized and uninitialized data. When it starts the program, it tells the program where the stack starts by putting that address into a designated register (or similar means).

One of the processor registers is used as a stack pointer; it points to address within the memory allocated for the stack that is the current top of stack. When the compiler arranges to use stack space for objects, it generates instructions that adjust the stack pointer. The addresses for the objects are calculated relative to the stack pointer. If a function needs 128 bytes of data, the compiler generates an instruction that subtracts 128 from the stack pointer. (This may occur in multiple steps, such as “call” and “push” instructions that make some changes to the stack pointer plus an additional “subtract” instruction that finishes the changes.) Then the addresses of all the objects in this stack frame are calculated as offsets from the value of the stack pointer. For example, by taking the stack pointer and adding 40, we calculate the address of the object that has been assigned to be 40 bytes higher than the top of the stack.

(There is some confusion about the wording of directions here because stacks commonly grow from high addresses to low addresses. The program loader may allocate some chunk of memory from, say, address 123000000₁₆ to 124000000₁₆. The stack pointer will start at 124000000₁₆. Subtracting 128 will make it 123FFFF80₁₆. Then 123FFFFA8₁₆ is an address that is 40 bytes “higher” than 123FFFF80₁₆ in the address space, but the “top of stack” is below that. That is because the term “top of stack” refers to the model of physically stacking things on top of each other, with the latest thing on top.)

The so-called “heap” is not the same size of all programs. In typical implementations, the memory management routines call system routines to request more virtual memory when they need it.

Note that “heap” is properly a word for a general data structure. Heaps may be used to organize things other than available memory, and the memory management routine keep track of available memory using data structures other than heaps. When referring to memory allocated via the memory management routines, you can call it “dynamically allocated memory.” It may also be shortened to “allocated memory,” but that can be confusing in some situations since all memory that has been reserved for some use is allocated memory.

Answer 4

Stack (most implementations use stack for automatic storage duration objects) and static storage duration objects memory is allocated during the program load and startup.

Does the compiler generate a logical address for the variables? If yes, then how?

I do not know what is the "logical address" but compilers do "calculate" the references to the automatic storage duration objects. How? Simply compiler knows how far from the stack pointer address the automatic storage duration object is located (offset).

Generally the same applies to the static duration objects and the code, the compiler only calculates the offset from the their sections.

Is the heap of the same size for all programs?

It is implementation defined.