BSS, Stack, Heap, Data, Code/Text - Where each of these start in memory?

Question

Segments of memory - BSS, Stack, Heap, Data, Code/Text (Are there any more?).

Say I have a 128MB RAM, Can someone tell me:

• How much memory is allocated

Answer 1

That question depends on the number of variables used. Since you did not specify what compiler or language or even operating system, that is a difficult one to pin down on! It all rests with the operating system who is responsible for the memory management of the applications. In short, there is no definite answer to this question, think about this, the compiler/linker at runtime, requests the operating system to allocate a block of memory, that allocation is dependent on how many variables there are, how big are they, the scope and usage of the variables. For instance, this simple C program, in a file called simpletest.c:

#include 
int main(int argc, char **argv){
   int num = 42;
   printf("The number is %d!\n", num);
   return 0;
}

Supposing the environment was Unix/Linux based and was compiled like this:

gcc -o simpletest simpletest.c

If you were to issue a objdump or nm on the binary image simpletest, you will see the sections of the executable, in this instance, 'bss', 'text'. Make note of the sizes of these sections, now add a int var[100]; to the above code, recompile and reissue the objdump or nm, you will find that the data section has appeared - why? because we added a variable of an array type of int, with 100 elements.

This simple exercise will prove that the sections grows, and hence the binary gets bigger, and it will also prove that you cannot pre-determine how much memory will be allocated as the runtime implementation varies from compiler to compiler and from operating system to operating system.

In short, the OS calls the shot on the memory management!