Where in memory are my variables stored in C?

Question

By considering that the memory is divided into four segments: data, heap, stack, and code, where do global variables, static variables, constant data types, local variables (defined and declared in functions), variables (in main function), pointers, and dynamically allocated space (using malloc and calloc) get stored in memory?

I think they would be allocated as follows:

• Global variables -------> data
• Static variables -------> data
• Constant data types -----> code
• Local variables (declared and defined in functions) --------> stack
• Variables declared and defined in main function -----> heap
• Pointers (for example, char *arr, int *arr) -------> heap
• Dynamically allocated space (using malloc and calloc) --------> stack

I am referring to these variables only from the C perspective.

Please correct me if I am wrong as I am new to C.

Answer 1

pointers(ex:char *arr,int *arr) -------> heap

Nope, they can be on the stack or in the data segment. They can point anywhere.

Answer 2

You got some of these right, but whoever wrote the questions tricked you on at least one question:

• global variables -------> data (correct)
• static variables -------> data (correct)
• constant data types -----> code and/or data. Consider string literals for a situation when a constant itself would be stored in the data segment, and references to it would be embedded in the code
• local variables(declared and defined in functions) --------> stack (correct)
• variables declared and defined in main function -----> heap also stack (the teacher was trying to trick you)
• pointers(ex: char *arr, int *arr) -------> heap data or stack, depending on the context. C lets you declare a global or a static pointer, in which case the pointer itself would end up in the data segment.
• dynamically allocated space(using malloc, calloc, realloc) --------> stack heap

It is worth mentioning that "stack" is officially called "automatic storage class".

Answer 3

A popular desktop architecture divides a process's virtual memory in several segments:

• Text segment: contains the executable code. The instruction pointer takes values in this range.

• Data segment: contains global variables (i.e. objects with static linkage). Subdivided in read-only data (such as string constants) and uninitialized data ("BSS").

• Stack segment: contains the dynamic memory for the program, i.e. the free store ("heap") and the local stack frames for all the threads. Traditionally the C stack and C heap used to grow into the stack segment from opposite ends, but I believe that practice has been abandoned because it is too unsafe.

A C program typically puts objects with static storage duration into the data segment, dynamically allocated objects on the free store, and automatic objects on the call stack of the thread in which it lives.

On other platforms, such as old x86 real mode or on embedded devices, things can obviously be radically different.

Answer 4

I am referring to these variables only from the C perspective.

From the perspective of the C language, all that matters is extent, scope, linkage, and access; exactly how items are mapped to different memory segments is up to the individual implementation, and that will vary. The language standard doesn't talk about memory segments at all. Most modern architectures act mostly the same way; block-scope variables and function arguments will be allocated from the stack, file-scope and static variables will be allocated from a data or code segment, dynamic memory will be allocated from a heap, some constant data will be stored in read-only segments, etc.

Answer 5

For those future visitors who may be interested in knowing about those memory segments, I am writing important points about 5 memory segments in C:

Some heads up:

1. Whenever a C program is executed some memory is allocated in the RAM for the program execution. This memory is used for storing the frequently executed code (binary data), program variables, etc. The below memory segments talks about the same:
2. Typically there are three types of variables:
   • Local variables (also called as automatic variables in C)
   • Global variables
   • Static variables
   • You can have global static or local static variables, but the above three are the parent types.

5 Memory Segments in C:

1. Code Segment

• The code segment, also referred as the text segment, is the area of memory which contains the frequently executed code.
• The code segment is often read-only to avoid risk of getting overridden by programming bugs like buffer-overflow, etc.
• The code segment does not contain program variables like local variable (also called as automatic variables in C), global variables, etc.
• Based on the C implementation, the code segment can also contain read-only string literals. For example, when you do printf("Hello, world") then string "Hello, world" gets created in the code/text segment. You can verify this using size command in Linux OS.
• Further reading

Data Segment

The data segment is divided in the below two parts and typically lies below the heap area or in some implementations above the stack, but the data segment never lies between the heap and stack area.

2. Uninitialized data segment

• This segment is also known as bss.
• This is the portion of memory which contains:
  1. Uninitialized global variables (including pointer variables)
  2. Uninitialized constant global variables.
  3. Uninitialized local static variables.
• Any global or static local variable which is not initialized will be stored in the uninitialized data segment
• For example: global variable int globalVar; or static local variable static int localStatic; will be stored in the uninitialized data segment.
• If you declare a global variable and initialize it as 0 or NULL then still it would go to uninitialized data segment or bss.
• Further reading

3. Initialized data segment

• This segment stores:
  1. Initialized global variables (including pointer variables)
  2. Initialized constant global variables.
  3. Initialized local static variables.
• For example: global variable int globalVar = 1; or static local variable static int localStatic = 1; will be stored in initialized data segment.
• This segment can be further classified into initialized read-only area and initialized read-write area. Initialized constant global variables will go in the initialized read-only area while variables whose values can be modified at runtime will go in the initialized read-write area.
• The size of this segment is determined by the size of the values in the program's source code, and does not change at run time.
• Further reading

4. Stack Segment

• Stack segment is used to store variables which are created inside functions (function could be main function or user-defined function), variable like
  1. Local variables of the function (including pointer variables)
  2. Arguments passed to function
  3. Return address
• Variables stored in the stack will be removed as soon as the function execution finishes.
• Further reading

5. Heap Segment

• This segment is to support dynamic memory allocation. If the programmer wants to allocate some memory dynamically then in C it is done using the malloc, calloc, or realloc methods.
• For example, when int* prt = malloc(sizeof(int) * 2) then eight bytes will be allocated in heap and memory address of that location will be returned and stored in ptr variable. The ptr variable will be on either the stack or data segment depending on the way it is declared/used.
• Further reading

Answer 6

• Variables/automatic variables ---> stack section
• Dynamically allocated variables ---> heap section
• Initialised global variables -> data section
• Uninitialised global variables -> data section (bss)
• Static variables -> data section
• String constants -> text section/code section
• Functions -> text section/code section
• Text code -> text section/code section
• Registers -> CPU registers
• Command line inputs -> environmental/command line section
• Environmental variables -> environmental/command line section