![]() ![]() When using recursion, with each function call a new stack frame is created, and all local variables are allocated on the stack. If you statically allocate an object, the whole object is allocated on the stack, including all of its member variables. Local variables are allocated on the current stack frame (belonging to the current function). ![]() Or consider a still more complex example, // command line arguments may be stored in a separate area this is an auto variable that is allocated in stack ![]() dynamic memory allocation, done in heap but it points to code area where code of strlen() is stored fp is an auto variable that is allocated in stack all are stored in initialized to zero segment(BSS) Lets look at a sample program to show which variables get stored where, int initToZero1 Therefore, whatever happens to be there in the memory becomes the initial (garbage) value for the objects created in that space. The stack and heap are uninitialized areas. Stack frames are created in stack for functions and heap for dynamic memory allocation. int main()įor execution, the program uses two major parts, the stack and heap. For example (I'm going to give only C based examples) the following code may result in runtime error or even crash the system. The attempt to write to code area leads to undefined behavior. Another important information to note here is that the system may consider this area as read only memory area and any attempt to write in this area leads to undefined behavior.Ĭonstant strings may be placed either in code or data area and that depends on the implementation. This can be accessed only be function pointers and not by other data pointers. The program code is the code area where the executable code is available for execution. However, the variables in the data area does not have such runtime overload because the initialization is done only once and that too at loading time. Auto variables have overhead in initializing the variables each time they are required and code is required to do that initialization. This area can be addressed and accessed using pointers from the code. This information may be useful in applications such as embedded systems. Mostly the layout of the data segment is in the control of the underlying OS, still some loaders give partial control to the users. ![]() This bulk initialization can greatly reduce the time required to load the executable file. Instead of that, the job of initializing the variable is left to the OS. However, the variables that are not initialized need not be explicitly initialized with 0's one-by-one. The data area is separated into two areas based on explicit initialization because the variables that are to be initialized can be initialized one-by-one. That is how the uninitialized global data and static get default value as zero. It is the part of memory where the OS initializes the memory block to zeros. The other part of data segment is called BSS (because of the old IBM systems had that segment initialized to zero). The data segment contains the global and static data that are explicitly initialized by the users containing the intialized values. Uninitialized data segment (initialized to zero data segment - BSS ).Initialized data segment (initialized to explicit initializers by programmers).The following is the basic structure of any program when loaded in the memory. ![]()
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