Where are constant variables stored in C?

I wonder where constant variables are stored. Is it in the same memory area as global variables? Or is it on the stack?

Solution 1:

How they are stored is an implementation detail (depends on the compiler).

For example, in the GCC compiler, on most machines, read-only variables, constants, and jump tables are placed in the text section.

Solution 2:

Depending on the data segmentation that a particular processor follows, we have five segments:

1. Code Segment - Stores only code, ROM
2. BSS (or Block Started by Symbol) Data segment - Stores initialised global and static variables
3. Stack segment - stores all the local variables and other informations regarding function return address etc
4. Heap segment - all dynamic allocations happens here
5. Data BSS (or Block Started by Symbol) segment - stores uninitialised global and static variables

Note that the difference between the data and BSS segments is that the former stores initialized global and static variables and the later stores UNinitialised ones.

Now, Why am I talking about the data segmentation when I must be just telling where are the constant variables stored... there's a reason to it...

Every segment has a write protected region where all the constants are stored.

For example:

• If I have a const int which is local variable, then it is stored in the write protected region of stack segment.
• If I have a global that is initialised const var, then it is stored in the data segment.
• If I have an uninitialised const var, then it is stored in the BSS segment...

To summarize, "const" is just a data QUALIFIER, which means that first the compiler has to decide which segment the variable has to be stored and then if the variable is a const, then it qualifies to be stored in the write protected region of that particular segment.

Solution 3:

Consider the code:

const int i = 0;
static const int k = 99;

int function(void)
{
    const int j = 37;
    totherfunc(&j);
    totherfunc(&i);
  //totherfunc(&k);
    return(j+3);
}

Generally, i can be stored in the text segment (it's a read-only variable with a fixed value). If it is not in the text segment, it will be stored beside the global variables. Given that it is initialized to zero, it might be in the 'bss' section (where zeroed variables are usually allocated) or in the 'data' section (where initialized variables are usually allocated).

If the compiler is convinced the k is unused (which it could be since it is local to a single file), it might not appear in the object code at all. If the call to totherfunc() that references k was not commented out, then k would have to be allocated an address somewhere - it would likely be in the same segment as i.

The constant (if it is a constant, is it still a variable?) j will most probably appear on the stack of a conventional C implementation. (If you were asking in the comp.std.c news group, someone would mention that the standard doesn't say that automatic variables appear on the stack; fortunately, SO isn't comp.std.c!)

Note that I forced the variables to appear because I passed them by reference - presumably to a function expecting a pointer to a constant integer. If the addresses were never taken, then j and k could be optimized out of the code altogether. To remove i, the compiler would have to know all the source code for the entire program - it is accessible in other translation units (source files), and so cannot as readily be removed. Doubly not if the program indulges in dynamic loading of shared libraries - one of those libraries might rely on that global variable.

(Stylistically - the variables i and j should have longer, more meaningful names; this is only an example!)