In static memory allocation (SMA) memory is not allocated at compile time

isn't it about linking, not memory allocation?

When I hit enter on my ".exe" file, then only both statically and dynamically created variables get memory. So how's that different

idk, but here some article about it. https://www.geeksforgeeks.org/difference-between-static-and-dynamic-memory-allocation-in-c/

When I hit enter on my ".exe" file, then only both statically and dynamically created variables get memory. So how's that different

isn't it about linking, not memory allocation?

When I hit enter on my ".exe" file, then only both statically and dynamically created variables get memory. So how's that different

When I hit enter on my ".exe" file, then only both statically and dynamically created variables get memory. So how's that different

What do you mean by "hit enter on exe"?

When I hit enter on my ".exe" file, then only both statically and dynamically created variables get memory. So how's that different

Your executable has a size in bytes. What do you think that size represents? Do you think all of that is code?

When I hit enter on my ".exe" file, then only both statically and dynamically created variables get memory. So how's that different

Try compiling these 2 codes separately (without optimizations) and see what is the size of the executable in each case. Now you will figure out what static memory allocation is #include <iostream> int main(){ std::cout << "Hello World"; } #include <iostream> int arr[10000] = {1}; int main(){ std::cout << "Hello World"; }

Compiler will delete your array

Compiler will delete your array

What do you mean by "hit enter on exe"?

Your executable has a size in bytes. What do you think that size represents? Do you think all of that is code?

Try compiling these 2 codes separately (without optimizations) and see what is the size of the executable in each case. Now you will figure out what static memory allocation is #include <iostream> int main(){ std::cout << "Hello World"; } #include <iostream> int arr[10000] = {1}; int main(){ std::cout << "Hello World"; }

Why is the size different for them? Entire array is allocated at compile time?

Wherever I looked for sma and DMA, they say static or compile time memory allocation doesn't mean the memory is allocated at compile time, it just means that memory requirement is known at compile time.

Memory is not allocated at compile time. It is allocated at load time. Your executable has different sections in it one of which is the .data segment which stores all initialized data. This section will map to some specific address in your process's virtual memory space and will have physical pages allocated for it. All of this happens when your executable is loaded into memory. Only after this is done and the runtime setup is done will your program start executing the first statement in main function

So on the hard disk it is stored that these are the variables in code. And when I run the program it loads onto ram and then these variables gets memory. But in case of dynamic allocation no variables are stored in hard disk they are directly alloted memory on ram?

Static memory allocation refers to memory that is allocated after you click your executable file and before your main function starts executing. These variables all have a fixed initialization value determined during compilation process itself. Runtime memory allocation refers to all allocations done after main starts executing.

Ok then why sma is faster than dma

Who said it is faster? Depending on how much memory you allocate, it can slow the loading time. Or in OSes where pages are mapped in only when they are accessed, it may slow your execution time as well.

And if it's not faster than is it just used because compiler takes care of allocation and deallocation of statically created variables

Static memory allocation refers to memory that is allocated after you click your executable file and before your main function starts executing. These variables all have a fixed initialization value determined during compilation process itself. Runtime memory allocation refers to all allocations done after main starts executing.

Who said it is faster? Depending on how much memory you allocate, it can slow the loading time. Or in OSes where pages are mapped in only when they are accessed, it may slow your execution time as well.

I read it at many places. Even on gfg. Everyone says use sma for your online judge like if the constraints are 10^6 then create an array of the biggest size i.e. a[10e6]

I'm not sure that start of main is a good point to think about, I would say "any of your code" instead. I think it still possible to execute some code before main starts using static objects initialization tricks, and then do some dynamic allocation in that code

What is "page of memory"

Read about OSes and virtual memory

From where?

Google it

I can not understand the last 2-3 lines. And also which one should we use preferably sma or dma?

I mean suggest me some structured place to study about os. There are many resources on google but which one to choose

Read OS - 3 Easy Pieces

And also I read this somewhere. " Int a[100]; Now I can't do this a=(some other address) As a is coming from symbol table But if I do this Int *a = new int[ 100 ]; Now I can make "a" point to some other memory location, as it's not coming from symbol table" This is what I saw in a tutorial. And I couldn't understand it. Can you help me what did it mean?

If you can use sma, I would prefer that. E.g. if you know that your array will always have a specific size (or you know for sure max possible size), then it would be better to statically allocate it. If you don't know or not sure, then it depends: sometimes the cost of dynamically allocating some stuff once or twice is not that big compared to increasing of complexity of handling all possible sizes or extra memory cost of unused memory.

I'm not sure that start of main is a good point to think about, I would say "any of your code" instead. I think it still possible to execute some code before main starts using static objects initialization tricks, and then do some dynamic allocation in that code

int a[100] Here a is an array and it's type is fixed i.e. it is of type int[100]. C/C++ don't allow you to assign to arrays. int* a = new int[100] Here a is just a pointer and you can allocate a different value to it just like you can do with pointers that don't point to dynamically allocated arrays.

Again can you show me how you will do this?

In 1st can I say a is const pointer

Don't hurry me, I'm preparing an example :)

But you once said arrays decay to pointers

Yes. They decay to pointers doesn't mean the array name is a pointer. It is converted to a pointer which is different from the array name

*For statically created array* Does the value inside " a" is decided at compile time?? (The starting address of array)

The starting address will be fixed at linking stage.

Ah alright. I thought you didn't see the message

Something like this https://wandbox.org/permlink/dlx3OPWmBvgiZmxF Note that code that initializes c and d will be executed before main this hack is described in Alexandrescu's Modern C++ Design, but I would not suggest to use it for anything since it's very easy to hit static initialization order fiasco with code like this (e.g. if b, c and d were declared in different *.cpp files then this code would be UB)

But this is not static initialization. This is dynamic initialization. You are confusing initialization of a static with static memory allocation and initialization. Two different concepts. What I had said was that static memory allocation and initialization that is determined at linking stage itself will all happen before your code starts executing.

But I see the point you are making. That runtime allocation can also happen before main executes and I agree. Perhaps I should have rephrased that and said _ _startup or _ _crt0 instead of main

Is learncpp.com is good website to learn c++

Yes it is a decent website and much better than all the other ones like tutorialspoint, geeksforgeeks and stuff. I would recommend a book like C++ Primer by Barbara Moo however.

tell me about strstr(); string function in c

Use Google. We are not here to spoonfeed you

Use Google. We are not here to spoonfeed you