static local variable address in C

Question

I know that a static local variable is in existence for the life of a program. But does a static local variable maintain the same memory address?

Or does the compiler just ensure that it exists and can be accessed within the local scope?

melpomene · Accepted Answer · 2019-02-17 23:23:28Z

4

In C objects don't move around during their lifetime. As long as an object exists, it will have the same address.

Variables with static storage (this includes variables with block scope declared as static) have a lifetime that covers the whole execution of the program, so they have a constant address.

answered Feb 17, 2019 at 23:23

melpomene

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Petr Skocik · Accepted Answer · 2019-02-17 23:37:18Z

There's very little difference between local statics and regular globals.

int x = 42; //static lifetime, external name
static int y = 43; //static lifetime, no external name, 
                   //referencable in all scopes here on out
                   //(unless overshadowed)
int main()
{
   static int z = 44; //like y, but only referencable from within this scope
                      //and its nested scopes
   {
       printf("%p\n", (void*)&z));
   }
}

All of these have fixed addresses once the program has been linked and loaded.

Local statics are like globals, except they're only referencable (by their name) from within their scope and its nested subscopes. (You can refer to them from unrelated scopes via pointers.)

proxict · Accepted Answer · 2019-02-18 01:14:14Z

2

Yes, the address offset of every static variable is known at the compile time. When the binary is loaded into the memory, local variables are stored in the .data segment of the program's address space.

In other words, the address of a static variable won't change during the code execution.

edited Feb 18, 2019 at 1:14

answered Feb 17, 2019 at 23:23

proxict

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8 Comments

melpomene Over a year ago

.data is platform specific (and static variables often go in .bss).

proxict Over a year ago

@melpomene Ah, true! Isn't it also dependant on whether or not the variable is initialized?

melpomene Over a year ago

Yes, and whether it is const (which might make the compiler place it in .rodata). But all of this is implementation specific.

Eric Postpischil Over a year ago

The actual address in the machine is generally not known at compile time. The compiler may determine an offset within some section of the program, but the sections are often assigned addresses at link time or at execution time. (Especially with address space layout randomization, which is intended to frustrate attackers that would take advantage of known addresses, the address is changed on each execution of the program.)

Eric Postpischil Over a year ago

Local variables may be stored in various places, not just a .data segment. A local automatic variable is most often stored on the stack or in a register if optimization permits (or may even vanish when optimization folds it into other expressions). A static variable might be stored in a constant section if it is constant, a data section if it is not constant but has an initial value, or an uninitialized segment like BSS if it does not have an initial value.

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