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

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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

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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

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Corrected your wrong sentences  constant data types ----- gt   code   wrong   local constant variables -----  stack   initialized global constant variable -----  data segment  uninitialized global constant variable -----  bss  variables declared and defined in main function  ----- gt   heap   wrong   variables declared and defined in main function -----  stack  pointers ex char  arr int  arr  ------- gt   heap   wrong  dynamically allocated space using malloc calloc  -------- gt  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    amp a    Now its pointing the memory of stack int  p2    amp 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

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pointers ex char  arr int  arr  -------  heap   Nope  they can be on the stack or in the data segment   They can point anywhere

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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

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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

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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  lt value optimized out gt  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  lt value optimized out gt  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

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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

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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

[c] Where in memory are my variables stored in C?

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