Memory Segments, Storage Classes, Static Functions and Linkage
1. Memory Segments
Storing the program at different blocks of memory
The commonly discussed segments are:
Text (Code) Segment
Data Segment
BSS Segment
Heap Segment
Stack Segment
###1.1 Text Segment:
The text segment typically stores the machine instructions generated by the compiler.
Example:
c
#include <stdio.h>
int main()
{
printf("Hello World");
return 0;
}
The compiled instructions for `main()` are typically stored in the text segment.
### 1.2 Data Segment:
The initialized data segment typically stores global and static variables initialized with nonzero values.
Example:
c
int n = 5; // Global variable
void func()
{
static int y = 4; // Static local variable
}
Both `n` and `y` typically reside in the initialized data segment.
### 1.3 BSS Segment
The BSS segment typically stores global and static variables that are uninitialized or initialized to zero.
Example:
c
int x; // Global variable
int y = 0; // Global variable
void func()
{
static int z; // Static local variable
}
These variables have an initial value of zero.
### 1.4 Heap Segment
The heap stores the dynamic memory allocation during program execution.
Example:
c
#include <stdlib.h>
int main()
{
int *p = malloc(sizeof(int));
if (p != NULL)
{
*p = 10;
free(p);
}
return 0;
}
### 1.5 Stack Segment
The stack typically stores function-call information, parameters, and local automatic variables.
Example:
c
void func()
{
int a = 10;
}
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2. Storage Classes
A storage class specifies properties such as a variable's scope, lifetime, and linkage, depending on how it is declared.
The four commonly taught storage classes in C are:
`auto`
`register`
`static`
`extern`
### 2.1 auto
`auto` is the default storage class for ordinary local variables.
Example:
c
int main()
{
auto int i = 10;
return 0;
}
This is equivalent to writing:
c
int i = 10;
- Scope: The block where the variable is declared.
- Lifetime: During execution of that block.
- An uninitialized automatic variable has an indeterminate value.
### 2.2 register
The `register` keyword suggests that a variable may benefit from being stored in a CPU register for faster access.
Example:
int main()
{
register int i = 10;
return 0;
}
Important points:
- The compiler may ignore this suggestion.
- Modern compilers already optimize register allocation.
- It does not guarantee faster execution.
- In standard C, the address-of operator (`&`) cannot be applied to a variable declared with the `register` storage-class specifier.
Therefore, ordinary variables are generally sufficient in modern C programming.
### 2.3 static
A local static variable retains its value between function calls. It is initialized only once.
Example:
#include <stdio.h>
void func()
{
static int i = 0;
i++;
printf("%d ", i);
}
int main()
{
func();
func();
func();
return 0;
}
Output:
1 2 3
Explanation:
- First call: `i` becomes 1.
- Second call: `i` becomes 2.
- Third call: `i` becomes 3.
### 2.4 extern
`extern` declares a variable that is defined elsewhere. It is useful when one source file needs to access a global variable defined in another source file.
Example:
**File 1: `file1.c`*\*
c
int x = 5; // Definition of the global variable
**File 2: `file2.c`*\*
#include <stdio.h>
extern int x; // Declaration of the variable defined elsewhere
int main()
{
printf("Global value is %d\n", x);
return 0;
}
Output:
Global value is 5
## 3. Static Function
A static function is a function declared with `static` at file scope. It can be called by name only within the same source file.
Example:
#include <stdio.h>
static void func()
{
printf("Hello World\n");
}
int main()
{
func();
return 0;
}
Output:
Hello World
Here, `func()` has internal linkage. Another source file cannot call this particular function by referring to its name.
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4. Linkage
Linkage determines whether declarations of the same name refer to the same variable or function.
C has three types of linkage.
### 4.1 External Linkage
A name with external linkage can refer to the same entity across multiple source files. Ordinary file-scope function definitions have external linkage by default.
### 4.2 Internal Linkage
A name with internal linkage can be referred to only within the same source file.
Example:
static void func()
{
// Function definition
}
The `static` keyword gives this file-scope function internal linkage.
### 4.3 No Linkage
A name with no linkage does not refer to the same entity as another declaration in a different scope merely because the names match.
Example:
void func()
{
int a = 10;
}
The local variable `a` has no linkage.
Summary:
| Text segment -> Typically stores machine instructions
| Data segment -> Typically stores initialized global/static variables
| BSS segment -> Typically stores uninitialized or zero-initialized global/static variables
| Heap -> Dynamic memory allocation
| Stack -> Typically stores function-call information and local automatic variables
| `auto` -> Default storage class for local variables
| `register` -> Suggests register storage; no guaranteed speed improvement
| `static` local variable -> Retains its value between calls
| `extern` -> Declares a variable defined elsewhere
| Static function -> Has internal linkage
| External linkage -> Allows a name to refer to the same entity across source files
| Internal linkage -> Limits a name to the current source file
| No linkage -> Common for ordinary local variables
