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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.
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".
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:
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:
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:
Uninitialized global variables (including pointer variables)
Uninitialized constant global variables.
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:
Initialized global variables (including pointer variables)
Initialized constant global variables.
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
Local variables of the function (including pointer variables)
Arguments passed to function
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
Corrected your wrong sentences
constant data types -----> code //wrong
local constant variables -----> stack
initialized global constant variable -----> data segment
uninitialized global constant variable -----> bss
variables declared and defined in main function -----> heap //wrong
variables declared and defined in main function -----> stack
pointers(ex:char *arr,int *arr) -------> heap //wrong
dynamically allocated space(using malloc,calloc) --------> stack //wrong
pointers(ex:char *arr,int *arr) -------> size of that pointer variable will be in stack.
Consider that you are allocating memory of n bytes (using malloc or calloc) dynamically and then making pointer variable to point it. Now that n bytes of memory are in heap and the pointer variable requries 4 bytes (if 64 bit machine 8 bytes) which will be in stack to store the starting pointer of the n bytes of memory chunk.
Note : Pointer variables can point the memory of any segment.
int x = 10;
void func()
{
int a = 0;
int *p = &a: //Now its pointing the memory of stack
int *p2 = &x; //Now its pointing the memory of data segment
chat *name = "ashok" //Now its pointing the constant string literal
//which is actually present in text segment.
char *name2 = malloc(10); //Now its pointing memory in heap
...
}
dynamically allocated space(using malloc,calloc) --------> heap
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.
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.
One thing one needs to keep in mind about the storage is the as-if rule. The compiler is not required to put a variable in a specific place - instead it can place it wherever it pleases for as long as the compiled program behaves as if it were run in the abstract C machine according to the rules of the abstract C machine. This applies to all storage durations. For example:
a variable that is not accessed all can be eliminated completely - it has no storage... anywhere. Example - see how there is 42 in the generated assembly code but no sign of 404.
a variable with automatic storage duration that does not have its address taken need not be stored in memory at all. An example would be a loop variable.
a variable that is const or effectively const need not be in memory. Example - the compiler can prove that foo is effectively const and inlines its use into the code. bar has external linkage and the compiler cannot prove that it would not be changed outside the current module, hence it is not inlined.
an object allocated with malloc need not reside in memory allocated from heap! Example - notice how the code does not have a call to malloc and neither is the value 42 ever stored in memory, it is kept in a register!
thus an object that has been allocated by malloc and the reference is lost without deallocating the object with free need not leak memory...
the object allocated by malloc need not be within the heap below the program break (sbrk(0)) on Unixen...
pointers(ex:char *arr,int *arr) -------> heap
Nope, they can be on the stack or in the data segment. They can point anywhere.
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
Linux minimal runnable examples with disassembly analysis
Since this is an implementation detail not specified by standards, let's just have a look at what the compiler is doing on a particular implementation.
In this answer, I will either link to specific answers that do the analysis, or provide the analysis directly here, and summarize all results here.
All of those are in various Ubuntu / GCC versions, and the outcomes are likely pretty stable across versions, but if we find any variations let's specify more precise versions.
Local variable inside a function
Be it main or any other function:
void f(void) {
int my_local_var;
}
As shown at: What does <value optimized out> mean in gdb?
-O0: stack
-O3: registers if they don't spill, stack otherwise
For motivation on why the stack exists see: What is the function of the push / pop instructions used on registers in x86 assembly?
Global variables and static function variables
/* BSS */
int my_global_implicit;
int my_global_implicit_explicit_0 = 0;
/* DATA */
int my_global_implicit_explicit_1 = 1;
void f(void) {
/* BSS */
static int my_static_local_var_implicit;
static int my_static_local_var_explicit_0 = 0;
/* DATA */
static int my_static_local_var_explicit_1 = 1;
}
if initialized to 0 or not initialized (and therefore implicitly initialized to 0): .bss section, see also: Why is the .bss segment required?
otherwise: .data section
char * and char c[]
As shown at: Where are static variables stored in C and C++?
void f(void) {
/* RODATA / TEXT */
char *a = "abc";
/* Stack. */
char b[] = "abc";
char c[] = {'a', 'b', 'c', '\0'};
}
TODO will very large string literals also be put on the stack? Or .data? Or does compilation fail?
Function arguments
void f(int i, int j);
Must go through the relevant calling convention, e.g.: https://en.wikipedia.org/wiki/X86_calling_conventions for X86, which specifies either specific registers or stack locations for each variable.
Then as shown at What does <value optimized out> mean in gdb?, -O0 then slurps everything into the stack, while -O3 tries to use registers as much as possible.
If the function gets inlined however, they are treated just like regular locals.
const
I believe that it makes no difference because you can typecast it away.
Conversely, if the compiler is able to determine that some data is never written to, it could in theory place it in .rodata even if not const.
TODO analysis.
Pointers
They are variables (that contain addresses, which are numbers), so same as all the rest :-)
malloc
The question does not make much sense for malloc, since malloc is a function, and in:
int *i = malloc(sizeof(int));
*i is a variable that contains an address, so it falls on the above case.
As for how malloc works internally, when you call it the Linux kernel marks certain addresses as writable on its internal data structures, and when they are touched by the program initially, a fault happens and the kernel enables the page tables, which lets the access happen without segfaul: How does x86 paging work?
Note however that this is basically exactly what the exec syscall does under the hood when you try to run an executable: it marks pages it wants to load to, and writes the program there, see also: How does kernel get an executable binary file running under linux? Except that exec has some extra limitations on where to load to (e.g. is the code is not relocatable).
The exact syscall used for malloc is mmap in modern 2020 implementations, and in the past brk was used: Does malloc() use brk() or mmap()?
Dynamic libraries
Basically get mmaped to memory: https://unix.stackexchange.com/questions/226524/what-system-call-is-used-to-load-libraries-in-linux/462710#462710
envinroment variables and main's argv
Above initial stack: https://unix.stackexchange.com/questions/75939/where-is-the-environment-string-actual-stored TODO why not in .data?
This question already has answers here:
Difference between static memory allocation and dynamic memory allocation
(7 answers)
Closed 5 years ago.
I was wondering if someone could explain the differences between the memory allocation for ai and *pai
int ai[10];
int *pai = (int * ) calloc (10, sizeof(int));
I understand the second one is dynamically allocated but im struggling to explain why.
Let's see what is being specified in standard (difference wise)
From 7.22.3.1 (Under Memory management functions)
... The lifetime of an allocated object extends from the allocation
until the deallocation.
So yes, this is for dynamically allocated memory. Their lifetime is different from that of local variables. By calling free they are deallocated. Until then they will be alive. Doesn't depend on the life time of the scope on which they are created.
The first one is having automatic storage duration. This is the primary difference. So in the functions scope where it is declared, when it ends then it's lifetime will be over.
Also some people say that there is a heap and stack - but (un)fortunately C standard doesn't mention it. It is completely implementation of the features expected by the C standard. The implementation can be anything. The differences presented is least bothered about those kind of stuff.
As a conceptual redpill (taken from movie Matrix) pai is of automatic storage duration but the address of the memory it contains is not. The variable pai will be lost when the function where it is defined is executed. But the memory it points to, doesn't.
Well why is it called dynamic allocation?
Know one thing - when in programming we say dynamic in the context of language - it means we are doing something in runtime. Same here, we are allocating some memory when in run time by calling functions like malloc,calloc etc. That's why dynamic allocation.
In the first line, you create a variable of an array type, but the symbol ai is a constant pointer to this variable.
in the second line, you create a pointer type variable. then you allocate an array dynamically with calloc() and you puts it's address in the pointer.
The array ai is allocated on the stack, it implicitly goes out of scope, when the end of the function is reached. The pointer pai points to a memory location, which can be an array or a single element of the type pointed to, the memory is allocated on the heap and must be freed later. The second can be passed back to the function-caller on the end of the function and can even be resized with realloc (realloc does not clear the new memory like calloc does, malloc is like calloc without zeroing out the new memory). The first is for fast array computation and should be in the cache most of the time. The second is for unknown lenght of arrays, when the function is called. When the size is known, many programmers tend to define an array in the caller and pass it to the function, which modifies it. The array is implicitly converted to a pointer when calling the function.
Some library implementations store a pointer to an array in the global section, which can be reallocated. Or they have a fixed length array in global space. These variables are recommended to be thread_local. The user does not have to care about the memorymanagement of the variable of the other library.
library.h
const char* getResourceString(int id);
library.c
thread_local char* string_buf = NULL;
const char* getResourceString(int id) {
int i = getResourceSize(id);
string_buf = realloc(string_buf, i);
// fill the memory
return string_buffer;
};
These are quite different operations:
int ai[10];
declares an array object of 10 ints. If it is declared inside a block, it will have automatic storage duration, meaning that it will vanish at block end (both identifier and data). If it is declared outside any block (at file level) it will have static storage duration and will exist throughout all program.
int *pai = calloc (10, sizeof(int)); // DON'T CAST MALLOC IN C
declares a pointer to an allocated zone of memory that can contains ten integers. You can use pai as a pointer to the first element of an array and do pointer arithmetics on it. But sizeof(pai) is sizeof(int *). The array will have dynamic storage duration meaning that its life will end:
if the allocated block of memory is freed
if it is reused to store other objects
double * pd = pai;
for (int i=1; i<5; i++) { // assuming sizeof(double) == 2 * sizeof(int) here
pd[i] = i; // the allocated memory now contains 5 double
}
So in both case you can use the identifier as pointing to an array of 10 integers, but first one is an integer array object while second one is just a pointer to a block of dynamic memory (memory with no declared type that can take the type of an object that will be copied/created there) .
Gerenally speaking, automatically allocated objects will be on the stack, while dynamically allocated objects will be on the heap. Although this distinction is implementation (not standard) dependent, stack and heap are the most commonly used way to manage memory in C programs. They are basically two distinct regions of memory, the first is dedicated to automatic allocations and the second is dedicated to dynamic allocations. So when you call a function (say, the main function) all the objects declared in the scope of this function will be stacked (automatically allocated in the stack). If some dynamic allocation happens in this function, the memory will be allocated in the heap so that all pointers to this area will be pointing to objects outside the stack. When your function returns, all objects in the stack are also automatically unstacked and virtually don't exist anymore. But all objects in the heap will exist until you deallocate them (or they will be forcefully deallocated by the OS when the program ends). Arrays are structures that can be allocated automatically or dynamically. See this example:
int * automaticFactor() //wrong, see below
{
int x[10];
return &x[0];
}
int * dynamicFactor()
{
int * y = (int *) malloc(sizeof(int) * 10);
return &y[0];
}
int main()
{
//this will not work because &x[0] points to the stack
//and that area will be unstacked after the function return
int * n = automaticFactor();
//this will work because &y[0] points to the heap
//and that area will be in the heap until manual deallocation
int * m = dynamicFactor();
return 0;
}
Note that the pointers themselves are in the stack. What is in the heap is the area they are pointing to. So when you declare a pointer inside a function (such as the y of the example), it will also be unstacked at the end of the function. But since its value (i.e. the address of the allocated area) was returned to a pointer outside the function (i.e. to m), you will not lose track of the area allocated in the heap by the function.
Is it allowed to dynamically allocate memory for static variable like this:
#include <stdio.h>
#include <stdlib.h>
struct person
{
int age;
int number;
};
static struct person* person_p = NULL;
int main()
{
person_p = (struct person*)malloc(10 * sizeof(struct person));
}
The above code built, but is it really allowed to dynamically allocate memory for static variable?
Yes, it's valid and allowed. (Unless you're using the pointer as a placeholder) You can (and need to) dynamically allocate and free() memory to and from the pointer before and after using it.
Rather, please make a note, you do not cast the return value of malloc() and family in C.
I don't see why not. Even though static means there can only be one instance of the object, you still need space for that object. Keep in mind however that anything that is malloc'd needs to be free'd, so you will want to do that at the end of your main() function.
Memory isn't "owned" by pointers to it. You can do the following things:
Dynamically allocate memory
Make a pointer point to that memory
It doesn't really make sense to say "dynamically allocate memory for a pointer".
Any pointer can point to any object (subject to alignment and aliasing restrictions), it makes no difference what the storage duration of the pointer is.
Note that it is the pointer that is static, not the memory it points to.
static means two unrelated things:
give static memory allocation (memory allocated at start of program, and only released at end of program)
give internal linkage (do not allow other compilation units - modules - to access the identifier. Here a variable, could be a function)
In your code, 1. static memory allocation is irrelevant, since the variable is global anyway and as such already has it.
Then, 2. internal linkage does not matter either, because what you are trying to do is inside the module anyway.
In other words, person_p is exactly as a usual global variable within your module, and you can do whatever you want to it.
It's only the pointer that is defined by this line of code, so you can dynamically allocate memory elsewhere, and assign the memory address to person_p if you wish.
I have a struct defined in this way.
typedef struct COUNTRY {
char Code[3];
char Country[30];
int Population;
float Expectancy;
struct Country *Pointer;
} COUNTRY;
I have seen an array of structs allocated like this:
COUNTRY *countries = calloc(128, sizeof(COUNTRY));
or maybe like this:
COUNTRY *countries = malloc(128 * sizeof(COUNTRY));
But what does this do:
COUNTRY countries[128] = {};
Because I am still able to write to each entries' fields in all cases. Is the third option just bad form? It seems better to me because you can put that line up with the rest of your variable declarations outside of main(). Otherwise, you can only calloc() or malloc() inside of main() or other function.
Am I doing something wrong?
This:
COUNTRY countries[128];
simply defines an object whose type is "array of 128 COUNTRY elements".
The = {} is an initializer -- but empty initializers are illegal in C (I think gcc supports them as an extension). A portable alternative is:
COUNTRY countries[128] = { 0 };
which initializes all members of all elements to zero (0 for integers, \0' for characters, 0.0 for floating-point, NULL for pointers, and recursively for sub-elements). But since you specified the number of elements in the array (as 128), the initializer has no effect on how the array object is allocated.
If the declaration occurs inside a function definition, the array object has automatic storage duration, which means that it ceases to exist when execution reaches the end of the enclosing block. Such objects are commonly allocated on "the stack".
If it occurs outside any function definition (at file scope) or if it has the keyword static, then it has static storage duration, which means that it continues to exist for the entire execution of the program.
Objects allocated with malloc or calloc have allocated storage duration, which means that they continue to exist until they're explicitly deallocated by a call to free(). Such objects are commonly allocated on "the heap". (I'm ignoring realloc(), which complicates the description a bit.)
The first two statements will allocate array of structs on the heap, while the last one will initialize the array of structs on the stack.
It is not a bad form, it is just a matter where you want your data to be stored - on the stack ( freed automatically when your variable goes out of the scope, stack usually have significantly smaller size then heap, so you could overflow it if you place big data structures there), or on the heap (lifetime of data is not related to the scope, you need to manually free your memory).
It seems better to me because you can put that line up with the rest of your variable declarations outside of main().
If you need statically allocated object with the lifetime of the program, use this approach, there's nothing wrong with it. Please note that in this particular case, variable is not stored on the stack, but in the .data segment of your program (check this question for more details: How are global variables stored?).
The last form is 'stack allocated' or 'statically allocated'. Like calloc, all of the fields will be zeroed out.
Inside a function it is 'stack allocated' and that memory will go away when the function returns.
Outside any function, at file scope, it is statically allocated and a global piece of memory allocated before main() starts.
malloc/calloc are used when you don't know how many you need at compile time. For example in a linked list, you need to allocate/deallocate nodes on the fly. When you use an array, you the know exactly how many you need at compile time.
What also differs is where the memory is taken from. If you declare an array in a function, the memory will be taken from the stack. In the case of malloc/calloc, the memory is set aside in the heap.
= {};
is GNU C extension and is the same as:
= {0};
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.
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".
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:
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:
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:
Uninitialized global variables (including pointer variables)
Uninitialized constant global variables.
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:
Initialized global variables (including pointer variables)
Initialized constant global variables.
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
Local variables of the function (including pointer variables)
Arguments passed to function
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
Corrected your wrong sentences
constant data types -----> code //wrong
local constant variables -----> stack
initialized global constant variable -----> data segment
uninitialized global constant variable -----> bss
variables declared and defined in main function -----> heap //wrong
variables declared and defined in main function -----> stack
pointers(ex:char *arr,int *arr) -------> heap //wrong
dynamically allocated space(using malloc,calloc) --------> stack //wrong
pointers(ex:char *arr,int *arr) -------> size of that pointer variable will be in stack.
Consider that you are allocating memory of n bytes (using malloc or calloc) dynamically and then making pointer variable to point it. Now that n bytes of memory are in heap and the pointer variable requries 4 bytes (if 64 bit machine 8 bytes) which will be in stack to store the starting pointer of the n bytes of memory chunk.
Note : Pointer variables can point the memory of any segment.
int x = 10;
void func()
{
int a = 0;
int *p = &a: //Now its pointing the memory of stack
int *p2 = &x; //Now its pointing the memory of data segment
chat *name = "ashok" //Now its pointing the constant string literal
//which is actually present in text segment.
char *name2 = malloc(10); //Now its pointing memory in heap
...
}
dynamically allocated space(using malloc,calloc) --------> heap
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.
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.
One thing one needs to keep in mind about the storage is the as-if rule. The compiler is not required to put a variable in a specific place - instead it can place it wherever it pleases for as long as the compiled program behaves as if it were run in the abstract C machine according to the rules of the abstract C machine. This applies to all storage durations. For example:
a variable that is not accessed all can be eliminated completely - it has no storage... anywhere. Example - see how there is 42 in the generated assembly code but no sign of 404.
a variable with automatic storage duration that does not have its address taken need not be stored in memory at all. An example would be a loop variable.
a variable that is const or effectively const need not be in memory. Example - the compiler can prove that foo is effectively const and inlines its use into the code. bar has external linkage and the compiler cannot prove that it would not be changed outside the current module, hence it is not inlined.
an object allocated with malloc need not reside in memory allocated from heap! Example - notice how the code does not have a call to malloc and neither is the value 42 ever stored in memory, it is kept in a register!
thus an object that has been allocated by malloc and the reference is lost without deallocating the object with free need not leak memory...
the object allocated by malloc need not be within the heap below the program break (sbrk(0)) on Unixen...
pointers(ex:char *arr,int *arr) -------> heap
Nope, they can be on the stack or in the data segment. They can point anywhere.
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
Linux minimal runnable examples with disassembly analysis
Since this is an implementation detail not specified by standards, let's just have a look at what the compiler is doing on a particular implementation.
In this answer, I will either link to specific answers that do the analysis, or provide the analysis directly here, and summarize all results here.
All of those are in various Ubuntu / GCC versions, and the outcomes are likely pretty stable across versions, but if we find any variations let's specify more precise versions.
Local variable inside a function
Be it main or any other function:
void f(void) {
int my_local_var;
}
As shown at: What does <value optimized out> mean in gdb?
-O0: stack
-O3: registers if they don't spill, stack otherwise
For motivation on why the stack exists see: What is the function of the push / pop instructions used on registers in x86 assembly?
Global variables and static function variables
/* BSS */
int my_global_implicit;
int my_global_implicit_explicit_0 = 0;
/* DATA */
int my_global_implicit_explicit_1 = 1;
void f(void) {
/* BSS */
static int my_static_local_var_implicit;
static int my_static_local_var_explicit_0 = 0;
/* DATA */
static int my_static_local_var_explicit_1 = 1;
}
if initialized to 0 or not initialized (and therefore implicitly initialized to 0): .bss section, see also: Why is the .bss segment required?
otherwise: .data section
char * and char c[]
As shown at: Where are static variables stored in C and C++?
void f(void) {
/* RODATA / TEXT */
char *a = "abc";
/* Stack. */
char b[] = "abc";
char c[] = {'a', 'b', 'c', '\0'};
}
TODO will very large string literals also be put on the stack? Or .data? Or does compilation fail?
Function arguments
void f(int i, int j);
Must go through the relevant calling convention, e.g.: https://en.wikipedia.org/wiki/X86_calling_conventions for X86, which specifies either specific registers or stack locations for each variable.
Then as shown at What does <value optimized out> mean in gdb?, -O0 then slurps everything into the stack, while -O3 tries to use registers as much as possible.
If the function gets inlined however, they are treated just like regular locals.
const
I believe that it makes no difference because you can typecast it away.
Conversely, if the compiler is able to determine that some data is never written to, it could in theory place it in .rodata even if not const.
TODO analysis.
Pointers
They are variables (that contain addresses, which are numbers), so same as all the rest :-)
malloc
The question does not make much sense for malloc, since malloc is a function, and in:
int *i = malloc(sizeof(int));
*i is a variable that contains an address, so it falls on the above case.
As for how malloc works internally, when you call it the Linux kernel marks certain addresses as writable on its internal data structures, and when they are touched by the program initially, a fault happens and the kernel enables the page tables, which lets the access happen without segfaul: How does x86 paging work?
Note however that this is basically exactly what the exec syscall does under the hood when you try to run an executable: it marks pages it wants to load to, and writes the program there, see also: How does kernel get an executable binary file running under linux? Except that exec has some extra limitations on where to load to (e.g. is the code is not relocatable).
The exact syscall used for malloc is mmap in modern 2020 implementations, and in the past brk was used: Does malloc() use brk() or mmap()?
Dynamic libraries
Basically get mmaped to memory: https://unix.stackexchange.com/questions/226524/what-system-call-is-used-to-load-libraries-in-linux/462710#462710
envinroment variables and main's argv
Above initial stack: https://unix.stackexchange.com/questions/75939/where-is-the-environment-string-actual-stored TODO why not in .data?