Can some one please explain difference between Code and Data memory. I know code is stored in Flash and Data is stored in RAM but i am confused.
#include <iostream>
using namespace std;
int main()
{
int a =10, b=20;
int c = a+b;
return 0;
}
Here a,b,c are stored in data memory(RAM), but whats get stored in Code memory? Is this entire code is stored in Code memory? if yes, then does this mean we are storing a,b,c in both data and code memory.
In your example, many scenarios based on the optimization level of your compiler.
Constants placed in "code memory"
In the code below:
int a =10, b=20;
int c = a+b;
return 0;
The variables a and b are constants, they don't change. A compiler could optimize this and optimize them to be:
int c = 10 + 20;
So the values 10 and 20 can be placed into code memory, eliminating the variables a and b.
Registers not Memory
The compiler is allowed to assign the variables a and b to registers. Registers are within the processor, so don't take up any RAM or memory space. Registers are not part of the code space either.
(This can happen because there are no statements that require the addresses of a or b).
All code dropped
On higher optimization settings, the compiler can delete all your code and replace with a return 0.
The variables a and b are not changed.
The variable c is changed but not used by any other statements.
Your program has no effect (nothing is printed, there are no external actions like writing to hardware).
Thus your program can be reduced down to return 0;.
Code Memory vs. Data Memory
In general, processor instructions are placed in a segment you will call "code memory". This may actually reside in RAM and not in Flash or ROM. For example, on a PC, your code could be loaded from the hard drive into RAM and executed in RAM. Similarly with Flash, your code could be loaded from Flash into RAM and executed in RAM.
Constants, like numbers, can be placed into a Read-Only segment or in the Code Segment. Many processors can load constants from the Code Segment (see ARM and Intel assembly instructions). The Read-Only segment can live on a Read Only device, (ROM or Flash) or may live in RAM (or on a device like hard drive). All you can guarantee, is that the code will not write to the Read-Only segment.
Data Memory is different. The C++ language has at least 3 areas of "data" memory (where variables live): 1) Local (a.k.a. stack), where short lifetime variables reside; 2) Dynamic memory (a.k.a. heap), allocated by using new or malloc and 3) Automatic/Global variables. These memory areas can be placed anywhere, as long as the memory has read and write capabilities. They don't need to be fast, only read & write (for example, the hard drive can be used as data memory).
Memory organization is more complicated than having Code, Stack and Heap. In the embedded systems world, memory can be place in non-standard locations and there may be a need to have more detailed memory segments so they can be placed in different areas. For example, an embedded system may want to place the constants into Flash so that they can be changed easily (even though they may be more efficiently accessed in the Code Segment). Some code may want to be placed into the Boot Area of the processor (which is programmed by the processor manufacturer). Some embedded systems may have non-volatile memory (e.g. battery backed memory), which can behave like Read-Only memory.
Trust Your Compiler
Trust in your compiler to place code, data and variables in the most efficient areas as possible. Your compiler knows your platform and will make the best decisions for you. If you need to change your compiler's settings, you can, but you should really know what you are doing and why you need to change them. Most PC platforms load code from a hard drive (or SSD) into RAM and execute the code from RAM. Embedded systems are different and depend on the hardware devices. Code may be run from flash because the platform has minimal RAM. Some may store the code compressed in a serial access read-only device and have to decompress into RAM before executing. In these situations, the compilers are configured for these specializations. So, trust in your compiler and let it place the code and data into the correct segments and locations.
A quick oversimplification:
Code memory stores the sequence of machine language instructions compiled from your C++ piece of code (the ROM).
Actual data that is created and manipulated by the program is instead stored in the RAM, which can be understood as made of stack and heap: data is stored in the slower, but larger heap, while its addresses there are retained by pointers in the stack. The stack is hosted in the faster memory registers.
Pointers in the stack retrieve data in the heap when told so by the current instruction line in the ROM, or more in general when needed.
In programming languages like C and C++, people often refer to static and dynamic memory allocation. I understand the concept but the phrase "All memory was allocated (reserved) during compile time" always confuses me.
Compilation, as I understand it, converts high level C/C++ code to machine language and outputs an executable file. How is memory "allocated" in a compiled file ? Isn't memory always allocated in the RAM with all the virtual memory management stuff ?
Isn't memory allocation by definition a runtime concept ?
If I make a 1KB statically allocated variable in my C/C++ code, will that increase the size of the executable by the same amount ?
This is one of the pages where the phrase is used under the heading "Static allocation".
Back To Basics: Memory allocation, a walk down the history
Memory allocated at compile-time means the compiler resolves at compile-time where certain things will be allocated inside the process memory map.
For example, consider a global array:
int array[100];
The compiler knows at compile-time the size of the array and the size of an int, so it knows the entire size of the array at compile-time. Also a global variable has static storage duration by default: it is allocated in the static memory area of the process memory space (.data/.bss section). Given that information, the compiler decides during compilation in what address of that static memory area the array will be.
Of course that memory addresses are virtual addresses. The program assumes that it has its own entire memory space (From 0x00000000 to 0xFFFFFFFF for example). That's why the compiler could do assumptions like "Okay, the array will be at address 0x00A33211". At runtime that addresses are translated to real/hardware addresses by the MMU and OS.
Value initialized static storage things are a bit different. For example:
int array[] = { 1 , 2 , 3 , 4 };
In our first example, the compiler only decided where the array will be allocated, storing that information in the executable.
In the case of value-initialized things, the compiler also injects the initial value of the array into the executable, and adds code which tells the program loader that after the array allocation at program start, the array should be filled with these values.
Here are two examples of the assembly generated by the compiler (GCC4.8.1 with x86 target):
C++ code:
int a[4];
int b[] = { 1 , 2 , 3 , 4 };
int main()
{}
Output assembly:
a:
.zero 16
b:
.long 1
.long 2
.long 3
.long 4
main:
pushq %rbp
movq %rsp, %rbp
movl $0, %eax
popq %rbp
ret
As you can see, the values are directly injected into the assembly. In the array a, the compiler generates a zero initialization of 16 bytes, because the Standard says that static stored things should be initialized to zero by default:
8.5.9 (Initializers) [Note]:
Every object of static storage duration is zero-initialized at
program startup before any other initial- ization takes place. In some
cases, additional initialization is done later.
I always suggest people to disassembly their code to see what the compiler really does with the C++ code. This applies from storage classes/duration (like this question) to advanced compiler optimizations. You could instruct your compiler to generate the assembly, but there are wonderful tools to do this on the Internet in a friendly manner. My favourite is GCC Explorer.
Memory allocated at compile time simply means there will be no further allocation at run time -- no calls to malloc, new, or other dynamic allocation methods. You'll have a fixed amount of memory usage even if you don't need all of that memory all of the time.
Isn't memory allocation by definition a runtime concept?
The memory is not in use prior to run time, but immediately prior to execution starting its allocation is handled by the system.
If I make a 1KB statically allocated variable in my C/C++ code, will that increase the size of the executable by the same amount?
Simply declaring the static will not increase the size of your executable more than a few bytes. Declaring it with an initial value that is non-zero will (in order to hold that initial value). Rather, the linker simply adds this 1KB amount to the memory requirement that the system's loader creates for you immediately prior to execution.
Memory allocated in compile time means that when you load the program, some part of the memory will be immediately allocated and the size and (relative) position of this allocation is determined at compile time.
char a[32];
char b;
char c;
Those 3 variables are "allocated at compile time", it means that the compiler calculates their size (which is fixed) at compile time. The variable a will be an offset in memory, let's say, pointing to address 0, b will point at address 33 and c at 34 (supposing no alignment optimization). So, allocating 1Kb of static data will not increase the size of your code, since it will just change an offset inside it. The actual space will be allocated at load time.
Real memory allocation always happens in run time, because the kernel needs to keep track of it and to update its internal data structures (how much memory is allocated for each process, pages and so on). The difference is that the compiler already knows the size of each data you are going to use and this is allocated as soon as your program is executed.
Remember also that we are talking about relative addresses. The real address where the variable will be located will be different. At load time the kernel will reserve some memory for the process, lets say at address x, and all the hard coded addresses contained in the executable file will be incremented by x bytes, so that variable a in the example will be at address x, b at address x+33 and so on.
Adding variables on the stack that take up N bytes doesn't (necessarily) increase the bin's size by N bytes. It will, in fact, add but a few bytes most of the time.
Let's start off with an example of how adding a 1000 chars to your code will increase the bin's size in a linear fashion.
If the 1k is a string, of a thousand chars, which is declared like so
const char *c_string = "Here goes a thousand chars...999";//implicit \0 at end
and you then were to vim your_compiled_bin, you'd actually be able to see that string in the bin somewhere. In that case, yes: the executable will be 1 k bigger, because it contains the string in full.
If, however you allocate an array of ints, chars or longs on the stack and assign it in a loop, something along these lines
int big_arr[1000];
for (int i=0;i<1000;++i) big_arr[i] = some_computation_func(i);
then, no: it won't increase the bin... by 1000*sizeof(int)
Allocation at compile time means what you've now come to understand it means (based on your comments): the compiled bin contains information the system requires to know how much memory what function/block will need when it gets executed, along with information on the stack size your application requires. That's what the system will allocate when it executes your bin, and your program becomes a process (well, the executing of your bin is the process that... well, you get what I'm saying).
Of course, I'm not painting the full picture here: The bin contains information about how big a stack the bin will actually be needing. Based on this information (among other things), the system will reserve a chunk of memory, called the stack, that the program gets sort of free reign over. Stack memory still is allocated by the system, when the process (the result of your bin being executed) is initiated. The process then manages the stack memory for you. When a function or loop (any type of block) is invoked/gets executed, the variables local to that block are pushed to the stack, and they are removed (the stack memory is "freed" so to speak) to be used by other functions/blocks. So declaring int some_array[100] will only add a few bytes of additional information to the bin, that tells the system that function X will be requiring 100*sizeof(int) + some book-keeping space extra.
On many platforms, all of the global or static allocations within each module will be consolidated by the compiler into three or fewer consolidated allocations (one for uninitialized data (often called "bss"), one for initialized writable data (often called "data"), and one for constant data ("const")), and all of the global or static allocations of each type within a program will be consolidated by the linker into one global for each type. For example, assuming int is four bytes, a module has the following as its only static allocations:
int a;
const int b[6] = {1,2,3,4,5,6};
char c[200];
const int d = 23;
int e[4] = {1,2,3,4};
int f;
it would tell the linker that it needed 208 bytes for bss, 16 bytes for "data", and 28 bytes for "const". Further, any reference to a variable would be replaced with an area selector and offset, so a, b, c, d, and e, would be replaced by bss+0, const+0, bss+4, const+24, data+0, or bss+204, respectively.
When a program is linked, all of the bss areas from all the modules are be concatenated together; likewise the data and const areas. For each module, the address of any bss-relative variables will be increased by the size of all preceding modules' bss areas (again, likewise with data and const). Thus, when the linker is done, any program will have one bss allocation, one data allocation, and one const allocation.
When a program is loaded, one of four things will generally happen depending upon the platform:
The executable will indicate how many bytes it needs for each kind of data and--for the initialized data area, where the initial contents may be found. It will also include a list of all the instructions which use a bss-, data-, or const- relative address. The operating system or loader will allocate the appropriate amount of space for each area and then add the starting address of that area to each instruction which needs it.
The operating system will allocate a chunk of memory to hold all three kinds of data, and give the application a pointer to that chunk of memory. Any code which uses static or global data will dereference it relative to that pointer (in many cases, the pointer will be stored in a register for the lifetime of an application).
The operating system will initially not allocate any memory to the application, except for what holds its binary code, but the first thing the application does will be to request a suitable allocation from the operating system, which it will forevermore keep in a register.
The operating system will initially not allocate space for the application, but the application will request a suitable allocation on startup (as above). The application will include a list of instructions with addresses that need to be updated to reflect where memory was allocated (as with the first style), but rather than having the application patched by the OS loader, the application will include enough code to patch itself.
All four approaches have advantages and disadvantages. In every case, however, the compiler will consolidate an arbitrary number of static variables into a fixed small number of memory requests, and the linker will consolidate all of those into a small number of consolidated allocations. Even though an application will have to receive a chunk of memory from the operating system or loader, it is the compiler and linker which are responsible for allocating individual pieces out of that big chunk to all the individual variables that need it.
The core of your question is this: "How is memory "allocated" in a compiled file? Isn't memory always allocated in the RAM with all the virtual memory management stuff? Isn't memory allocation by definition a runtime concept?"
I think the problem is that there are two different concepts involved in memory allocation. At its basic, memory allocation is the process by which we say "this item of data is stored in this specific chunk of memory". In a modern computer system, this involves a two step process:
Some system is used to decide the virtual address at which the item will be stored
The virtual address is mapped to a physical address
The latter process is purely run time, but the former can be done at compile time, if the data have a known size and a fixed number of them is required. Here's basically how it works:
The compiler sees a source file containing a line that looks a bit like this:
int c;
It produces output for the assembler that instructs it to reserve memory for the variable 'c'. This might look like this:
global _c
section .bss
_c: resb 4
When the assembler runs, it keeps a counter that tracks offsets of each item from the start of a memory 'segment' (or 'section'). This is like the parts of a very large 'struct' that contains everything in the entire file it doesn't have any actual memory allocated to it at this time, and could be anywhere. It notes in a table that _c has a particular offset (say 510 bytes from the start of the segment) and then increments its counter by 4, so the next such variable will be at (e.g.) 514 bytes. For any code that needs the address of _c, it just puts 510 in the output file, and adds a note that the output needs the address of the segment that contains _c adding to it later.
The linker takes all of the assembler's output files, and examines them. It determines an address for each segment so that they won't overlap, and adds the offsets necessary so that instructions still refer to the correct data items. In the case of uninitialized memory like that occupied by c (the assembler was told that the memory would be uninitialized by the fact that the compiler put it in the '.bss' segment, which is a name reserved for uninitialized memory), it includes a header field in its output that tells the operating system how much needs to be reserved. It may be relocated (and usually is) but is usually designed to be loaded more efficiently at one particular memory address, and the OS will try to load it at this address. At this point, we have a pretty good idea what the virtual address is that will be used by c.
The physical address will not actually be determined until the program is running. However, from the programmer's perspective the physical address is actually irrelevant—we'll never even find out what it is, because the OS doesn't usually bother telling anyone, it can change frequently (even while the program is running), and a main purpose of the OS is to abstract this away anyway.
An executable describes what space to allocate for static variables. This allocation is done by the system, when you run the executable. So your 1kB static variable won't increase the size of the executable with 1kB:
static char[1024];
Unless of course you specify an initializer:
static char[1024] = { 1, 2, 3, 4, ... };
So, in addition to 'machine language' (i.e. CPU instructions), an executable contains a description of the required memory layout.
Memory can be allocated in many ways:
in application heap (whole heap is allocated for your app by OS when the program starts)
in operating system heap (so you can grab more and more)
in garbage collector controlled heap (same as both above)
on stack (so you can get a stack overflow)
reserved in code/data segment of your binary (executable)
in remote place (file, network - and you receive a handle not a pointer to that memory)
Now your question is what is "memory allocated at compile time". Definitely it is just an incorrectly phrased saying, which is supposed to refer to either binary segment allocation or stack allocation, or in some cases even to a heap allocation, but in that case the allocation is hidden from programmer eyes by invisible constructor call. Or probably the person who said that just wanted to say that memory is not allocated on heap, but did not know about stack or segment allocations.(Or did not want to go into that kind of detail).
But in most cases person just wants to say that the amount of memory being allocated is known at compile time.
The binary size will only change when the memory is reserved in the code or data segment of your app.
You are right. Memory is actually allocated (paged) at load time, i.e. when the executable file is brought into (virtual) memory. Memory can also be initialized on that moment. The compiler just creates a memory map. [By the way, stack and heap spaces are also allocated at load time !]
I think you need to step back a bit. Memory allocated at compile time.... What can that mean? Can it mean that memory on chips that have not yet been manufactured, for computers that have not yet been designed, is somehow being reserved? No. No, time travel, no compilers that can manipulate the universe.
So, it must mean that the compiler generates instructions to allocate that memory somehow at runtime. But if you look at it in from the right angle, the compiler generates all instructions, so what can be the difference. The difference is that the compiler decides, and at runtime, your code can not change or modify its decisions. If it decided it needed 50 bytes at compile time, at runtime, you can't make it decide to allocate 60 -- that decision has already been made.
If you learn assembly programming, you will see that you have to carve out segments for the data, the stack, and code, etc. The data segment is where your strings and numbers live. The code segment is where your code lives. These segments are built into the executable program. Of course the stack size is important as well... you wouldn't want a stack overflow!
So if your data segment is 500 bytes, your program has a 500 byte area. If you change the data segment to 1500 bytes, the size of the program will be 1000 bytes larger. The data is assembled into the actual program.
This is what is going on when you compile higher level languages. The actual data area is allocated when it is compiled into an executable program, increasing the size of the program. The program can request memory on the fly, as well, and this is dynamic memory. You can request memory from the RAM and the CPU will give it to you to use, you can let go of it, and your garbage collector will release it back to the CPU. It can even be swapped to a hard disk, if necessary, by a good memory manager. These features are what high level languages provide you.
I would like to explain these concepts with the help of few diagrams.
This is true that memory cannot be allocated at compile time, for sure.
But, then what happens in fact at compile time.
Here comes the explanation.
Say, for example a program has four variables x,y,z and k.
Now, at compile time it simply makes a memory map, where the location of these variables with respect to each other is ascertained.
This diagram will illustrate it better.
Now imagine, no program is running in memory.
This I show by a big empty rectangle.
Next, the first instance of this program is executed.
You can visualize it as follows.
This is the time when actually memory is allocated.
When second instance of this program is running, the memory would look like as follows.
And the third ..
So on and so forth.
I hope this visualization explains this concept well.
There is very nice explanation given in the accepted answer. Just in case i will post the link which i have found useful.
https://www.tenouk.com/ModuleW.html
One among the many thing what a compiler does is that create and maintain a SYMTAB(Symbol Table under the section.symtab). This will be purely created and maintained by compilers using any Data Structure(List, Trees...etc) and not for the developers eyes. Any access request made by the developers this is where it will hit first.
Now about the Symbol Table,
We only need to know about the two columns Symbol Name and the Offset.
Symbol Name column will have the variable names and the offset column will have the offset value.
Lets see this with an example:
int a , b , c ;
Now we all know that the register Stack_Pointer(sp) points to the Top of the Stack Memory. Let that be sp = 1000.
Now the Symbol Name column will have three values in it a then b and then c. Reminding you all that variable a will be at the top of the stack memory.
So a's equivalent offset value will be 0.
(Compile Time Offset_Value)
Then b and its equivalent offset value will be 1. (Compile Time Offset_Value)
Then c and its equivalent offset value will be 2. (Compile Time Offset_Value)
Now calculating a's Physical address (or) Runtime Memory Address = (sp + offset_value of a)
= (1000 + 0) = 1000
Now calculating b's Physical address (or) Runtime Memory Address = (sp - offset_value of b)
= (1000 - 1) = 996
Now calculating c's Physical address (or) Runtime Memory Address = (sp - offset_value of c)
= (1000 - 2) = 992
Therefore at the time of the compilation we will only be having the offset values and only during the runtime the actual physical addresses are calculated.
Note:
Stack_Pointer value will be assigned only after the program is loaded. Pointer Arithmetic happens between the Stack_Pointer register and the variables offset to calculate the variables Physical Address.
"POINTERS AND POINTER ARITHMETIC, WAY OF THE PROGRAMMING WORLD"
Share what I learned about this question.
You can understand this issue in two steps:
First, the compilation step: the compiler generates the binary. In Linux system, binary is a file in ELF (Executable and Linkable Format) format. ELF file contains several sections, including .bss and .data
.data
Initialized data, with read/write access rights
.bss
Uninitialized data, with read/write access rights (=WA)
.data and .bss just map to the segments of process's memory layout, which contains static variables.
second, the loading step. When the binary file get executed, the ELF file will be loaded into process's memory. The loader can find static variables' information from ELF file.
Simply speaking, the compiler and the loader follow the same standard to communicate with each other, and the standard is ELF format.
When you run a program on windows, it is loaded into computer memory organized as:
Data Segment
Stack
Code segment
The data segment may contain data which is read only or has read and write access.
E.g:
char *c = "Hello World";
The string Hello World is said to be stored in read only section of memory. Is it stored in physical memory which is sometimes called RAM or is it stored in ROM which is read only? How can it be written if it is read only?
A PC has only one area of memory that really qualifies as ROM, and that is where the BIOS is stored. All of Windows and the programs loaded in Windows will be in RAM.
The x86 processor memory management is able to mark blocks of memory as read only, but the linker and OS have to work together to enable this. It happens after the program is loaded into memory.
It is stored in RAM. The Operating System, in cooperation with the processor itself, is able to protect regions of memory so that any attempt to write to them from user code causes an exception.
In many embedded systems, they have RAM and some type of Read Only Memory, often times called Flash (it can be programmed multiple times without being removed from the Printed Circuit Board).
Simple embedded applications place the executable and read-only data sections in Flash and execute out of Flash. The read/write variables are placed into RAM. Let us consider this model for your example code fragment:
char * c = "Hello World!";
In the above statement, the variable c lives in RAM because the default setting for variables is read & write access. If you specified that the variable was constant, it would live in ROM {Actually, it would represent a location in ROM.}: enter code here
char * const c = "Hello World!"; // A constant pointer that lives in ROM.
The compiler treats the literal text "Hello World!" a little more complex. The actual text lives in ROM, either in the executable area or in a data area; depends on the translator. Many compilers will allocate memory in RAM and copy the literal into RAM and make the variable c point to the copy in RAM. This is because the literal was not specified as constant.
To avoid the copying of the literal into RAM, declare the variable pointing to constant data:
const char * c = "Hello World"; // A pointer to constant data.
The definition above still allows the pointer to point to different things during execution. If you want to refer to one instance of a text literal throughout the program, declare a constant pointer to constant data:
const * char * const c = "Hello World!"; // A constant pointer to constant data
This technique allows executables to load into RAM (for faster execution) and still access read-only data from ROM (which frees up SRAM for true read/write variables).
On most PCs, everything lives either on Non-volatile memory (hard drive, BIOS, etc) or in RAM. Common method is to load programs from ROM (includes hard drive) and execute in RAM. When loading the executable into RAM, the OS usually loads the Read-Only data into RAM also. The read-only data may be guarded by the OS so an exception is generated when the application writes to this area.
It's stored in RAM.
Char pointers which are initialised with a string, like in your example are readonly.
You can cast a const variable so that you can "write" to it but the behaviour of your program will not be predictable.
Readonly regions of RAM are protected by the Operating system.
This probably varies depending on what kind of system you're on. The answer above would be for your standard PC. Embedded systems might actually write the constant data to some kind of non volatile memory.
If it helps, consider that there's two different ways in which memory can be read-only.
At the hardware level, you're referring to ROM. Once it's written to/created, the physical properties of it don't allow it's value to be changed.
At the software level, the OS (or other level higher than the user's program space) prevents the values stored in RAM from being modified.
When you're talking about read-only memory for program values, you're generally talking about the second type. The values are initialized when the program starts, and then the OS doesn't allow them to be modified.
Note: The above is an extremely simplistic explanation, but it does give the general idea.
It is stored int RAM memory. ]:>
#include <iostream>
using namespace std;
int main(void)
{
int *ptr = new int;
cout << "Memory address of ptr:" << ptr << endl;
cin.get();
delete ptr;
return 0;
}
Every time I run this program, I get the same memory address for ptr. Why?
[Note: my answer assumes you're working with a modern OS that uses a virtual memory system.]
Due to virtual memory, each process operates in its own unique address space, which is independent of and unaffected by any other process. The address you get from new is a virtual address, and is generated by whatever your compiler's implementation of new chooses to do.* There's no reason this couldn't be deterministic.
On the other hand, the physical address associated with your virtual memory address will most likely be different every time, and will be affected by all sorts of things. This mapping is controlled by the OS.
* new is probably implemented in terms of malloc.
I'd say it's mostly coincidence. As the memory allocator/OS can give you whatever address it wants.
The addresses you get are obviously not uniformly random (and is highly dependent on other OS factors), so it's often to get the same (virtual) address several times in the row.
So for example, on my machine: Window 7, compiled with VS2010, I get different addresses with different runs:
00134C40
00124C40
00214C40
00034C40
00144C40
001B4C40
This is an artifact of your environment. The cin.get() suggests to me that you are compiling and executing in Visual Studio, which provides an unusually predictable runtime environment. When I compile and run that code on my linux, two executions gave two different addresses.
ETA:
In comments you expressed an expectation that different processes could obtain the same memory address and that this address would be inaccessible to your program. In any modern operating system this is not the case, because the operating system is providing each process with virtual memory address spaces.
Only the operating system sees the true hardware addresses, and maintains virtual memory maps for each program, redirecting virtual addresses to physical addresses. Therefore, an arbitrary number of different processes can hold data in the same virtual address, while the operating system maps that address to a separate physical address for each process.
This guarantees that process A cannot read or write to memory in use by process B without a special provision enabling such access (such as by instructing the OS to map certain virtual memory in certain processes to the same physical memory). It allows the operating system to make different kinds of memory hardware transparent to programs.
It also allows the OS to move a program's data around behind its back to optimize system performance.
Classical example: Moving data that hasn't been used for some time to a special file on the hard disk. This is sometimes called the page file.
Memory maps are typically broken up into pages: Blocks of contiguous memory of a certain size (the page size). Data held within a page of virtual address space is usually also contiguous in physical memory, but if data runs over a page boundary, information that appears contiguous in virtual memory could easily be separated. If a C/C++ program enters undefined behavior, it may attempt to access memory in a page that the OS has not mapped to physical memory. This will cause the OS to generate an error.
I'm trying to make a program that reads the value at a certain address.
I have this:
int _tmain(int argc, _TCHAR* argv[])
{
int *address;
address = (int*)0x00000021;
cout << *address;
return 0;
}
But this gives a read violation error. What am I doing wrong?
Thanks
That reads the value at that address within the process's own space. You'll need to use other methods if you want to read another process's space, or physical memory.
It's open to some question exactly what OlyDbg is showing you. 32-bit (and 64-bit) Windows uses virtual memory, which means the address you use in your program is not the same as the address actually sent over the bus to the memory chips. Instead, Windows (and I should add that other OSes such as Linux, MacOS, *bsd, etc., do roughly the same) sets up some tables that say (in essence) when the program uses an address in this range, use that range of physical addresses.
This mapping is done on a page-by-page basis (where each page is normally 4K bytes, though other sizes are possible). In that table, it can also mark a page as "not present" -- this is what supports paging memory to disk. When you try to read a page that's marked as not present, the CPU generates an exception. The OS then handles that exception by reading the data from the disk into a block of memory, and updating the table to say the data is present at physical address X. Along with not-present, the tables support a few other values, such as read-only, so you can read by not write some addresses.
Windows (again, like the other OSes) sets up the tables for the first part of the address space, but does NOT associate any memory with them. From the viewpoint of a user program, those addresses simply should never be used.
That gets us back to my uncertainty about what OlyDbg is giving you when you ask it to read from address 0x21. That address simply doesn't refer to any real data -- never has and never will.
What others have said is true as well: a debugger will usually use some OS functions (E.g. ReadProcessMemory and WriteProcessMemory, among others under Windows) to get access to things that you can't read or write directly. These will let you read and write memory in another process, which isn't directly accessible by a normal pointer. Neither of those would help in trying to read from address 0x21 though -- that address doesn't refer to any real memory in any process.
You can only use a pointer that points to an actual object.
If you don't have an object at address 0x00000021, this won't work.
If you want to create an object on the free store (the heap), you need to do so using new:
int* address = new int;
*address = 42;
cout << *address;
delete address;
When your program is running on an operating system that provides virtual memory (Windows, *nix, OS X) Not all addresses are backed by memory. CPU's that support virtual memory use something called Page Tables to control which address refer to memory. The size of an individual page is usually 4096 bytes, but that does vary and is likely to be larger in the future.
The API's that you use to query the page tables isn't part of the standard C/C++ runtime, so you will need to use operating system specific functions to know which adresses are OK to read from and which will cause you to fault. On Windows you would use VirtualQuery to find out if a given address can be read, written, executed, or any/none of the above.
You can't just read data from an arbitrary address in memory.