Scope, Storage Duration, and Linkage in C

Scope, Storage Duration, and Linkage in C
an Introduction

Prof. David Bernstein
James Madison University

Computer Science Department

bernstdh@jmu.edu

Scope, Storage Duration, and Linkage

Scope:
- The portion of a translation unit (i.e., the code produced by the preprocessor from the source file and the header files) in which an identifier is visible (i.e., represents an entity)
Storage Duration:
- The (minimum potential) lifetime of the storage containing an entity
Linkage:
- Determines whether declarations in different scopes can refer to the same entity

Declarations in C

Purpose:
- Associates an entity with a name, either introducing a new name into the translation unit or re-using a name from earlier (in the case of a redeclaration)
Loose Syntax:
- [qualifier] specifiers declarator [= initializer];
Kinds of Specifiers:
- Type specifiers (e.g., char, int, void)
- Function specifiers (e.g., inline)
- Storage class specifiers (e.g., auto, extern, register, static)
- Other Specifiers (e.g., struct, union, enum)

Examples of Declarations

  // Type Specifier
  int i;

  // Type Specifier with a Modifier
  double grades[10];
  void error_exit();

  // Type Specifier with a Type Qualifier
  volatile int size;

  // Type Specifier with a Type Qualifier and Modifier
  const int *fp;

Scope in C

File Scope:
- Identifiers that are declared in the outermost part of a translation unit
Function Prototype Scope:
- Identifiers that are declared in the formal parmeter list of a function
Block Scope:
- Identifiers declared within a function
- Identifiers declared within a block

Scope in C (cont.)

#include <stdio.h>

int
main(void)
{
  n += 5;
  printf("%d\n", n);

  return 0;
}

Compile?
- No, because n is not declared in the scope of main()
Output?
- N/A

Scope in C (cont.)

#include <stdio.h>

int n = 0;

int
main(void)
{
  n += 5;
  printf("%d\n", n);

  return 0;
}

Compile?
- Yes, because n has file scope
Output?
- 5

Scope in C (cont.)

#include <stdio.h>

int n = 0;

int
main(void)
{
  int n;

  n += 5;
  printf("%d\n", n);

  return 0;
}

Compile?
- Yes, there is an n with file scope and an n with local scope
Output?
- Garbage, because the local n is not initialized.

Scope in C (cont.)

#include <stdio.h>

int n = 0;

int
main(void)
{
  int n = 1;

  n += 5;
  printf("%d\n", n);

  return 0;
}

Compile?
- Yes, there is an n with file scope and an n with local scope
Output?
- 6

Storage Duration in C

Automatic:
- Begins at entry into the block and ends upon exit from the block
Static:
- Begins at execution and ends at termination
Dynamic:
- Begins at allocation (e.g., malloc()) and ends at deallocation (e.g., free())

Using Storage Specifiers to Specify Duration

Storage Duration in C (cont.)

#include <stdio.h>

int 
total(int x)
{
  static int total = 0;

  total += x;
  return total;
}

int 
main(void)
{
  total(5);
  total(10);
  printf("%d\n", total(15));
  return 0;
}

Compile?
- Yes, the static specifier can be used in the declaration of a varaible with block scope
Output?
- 30

Linkage in C

No Linkage:
- The entity can't be referenced by name from anywhere else
Internal Linkage:
- The entity can be referenced by names declared in the same scope and/or names declared in other scopes of the same translation unit (which is useful if you want to have multiple entities with the same name in different translation units)
External Linkage:
- The entity can also be referenced by name in other translation units (which is useful if you want the entity to be "global")

Specifying Linkage for Functions

Specifying Linkage for Functions (cont.)

library.h

#ifndef LIBRARY_H
#define LIBRARY_H

int 
net_price(int price, int discount);

static int 
validate(int n);

#endif

library.c

#include "library.h"

int 
net_price(int price, int discount)
{
  return validate(price - discount);
}

static int 
validate(int n)
{
  if (n < 0) return 0;
  else       return n;
}

Specifying Linkage for Functions (cont.)

driver.c

#include "library.h"

int 
main(void)
{
  int i, j;

  i = validate(-5);
  j = net_price(6, 2);
}

Compile?
- Yes
Link?
- No, the reference to validate() in main() is undefined because validate() has internal linkage

Specifying Linkage for Variables

Specifying Linkage for Variables (cont.)

#include <stdio.h>

int n = 0;

int
main(void)
{
  extern int n;

  n += 5;
  printf("%d\n", n);

  return 0;
}

Compile?
- Yes
Output?
- 5

Definitions in C

Definitions:
- A definition of an entity allocates storage for it
An Important Constraint:
- An entity must be defined exactly once (in a translation unit if the entity has internal linkage; in the entire program if the entity has external linkage)
An Important Point:
- All definitions are implicitly declarations

Examples of Definitions

The Following are at the Top Level of a Translation Unit

  // A definition (and implicit declaration) of a function
  int 
  max(int a, int b)
  {
    if (a > b) return a;
    else       return b;
  }

  // A definition (and implicit declaration) of a variable with file scope
  int total = 0;

Declarations vs. Definitions (cont.)

Functions:
- A function heading without a body is a declaration
- A function heading with a body is both a declaration and a definition
Variables with File Scope (i.e., External Variables):
- The situation is more complicated

Declarations/Definitions of External Variables

Initializer == Yes:
- Makes the statement a definition (and a declaration)
Initializer == No and extern == Yes:
- Makes the statement a declaration only (with external linkage)
Initializer == No and (extern == No or static == Yes):
- A tentative definition (which acts as a definition with the initializer = 0 or = {0} if there is no definition in the same translation unit)

Declarations/Definitions of External Variables (cont.)

The Following are at the Top Level of a Translation Unit

  // A declaration with external linkage
  extern int i;  

  // A definition with external linkage
  int total = 0; 

  // A definition with internal linkage
  static const char *university = "JMU";

  // A tentative definition with external linkage
  int  j;

  // A tentative definition with internal linkage
  static int  k;

A Common "Problem"

The General Situation:
- You are creating a library of functions all of which assign a value to a variable with file scope
A Specific Example:
- The library is organized into the files account.h and account.c
- There is a file named driver.c that uses the library

A Common "Problem": Attempt 1

account.h

  #ifndef ACCOUNT_H
  #define ACCOUNT_H

  #define ACCT_OK      0
  #define ACCT_INVALID 1
  #define ACCT_CLOSED  2

  int acct_status;
  #endif

account.c

  #include "account.h"

  // Use acct_status

driver.c

  #include "account.h"

  // Use acct_status

A Common "Problem": Attempt 1 (cont.)

Compile?
- Yes, because account.h is in both translation units
Link?
- No, you will get a message like multiple definition of 'acct_status' because acct_status has been defined in both translation units

A Common "Problem": Attempt 2

account.h

  #ifndef ACCOUNT_H
  #define ACCOUNT_H

  #define ACCT_OK      0
  #define ACCT_INVALID 1
  #define ACCT_CLOSED  2

  // Don't define acct_status since that caused the problem
  #endif

account.c

  #include "account.h"

  // Use acct_status

driver.c

  #include "account.h"

  // Use acct_status

A Common "Problem": Attempt 2 (cont.)

Compile?
- No, you will get a message like 'acct_status' was not declared in this scope
Link?
- Not Applicable

A Common "Problem": Attempt 3

account.h

  #ifndef ACCOUNT_H
  #define ACCOUNT_H

  #define ACCT_OK      0
  #define ACCT_INVALID 1
  #define ACCT_CLOSED  2

  // Don't define acct_status
  #endif

account.c

  #include "account.h"

  int acct_status;

  // Use acct_status

driver.c

  #include "account.h"

  // Use acct_status

A Common "Problem": Attempt 3 (cont.)

Compile?
- account.c will compile but driver.c won't
Link?
- Not Applicable

A Common "Problem": Attempt 4

account.h

  #ifndef ACCOUNT_H
  #define ACCOUNT_H

  #define ACCT_OK      0
  #define ACCT_INVALID 1
  #define ACCT_CLOSED  2

  extern int acct_status;
  #endif

account.c

  #include "account.h"

  // Use acct_status

driver.c

  #include "account.h"

  // Use acct_status

A Common "Problem": Attempt 4 (cont.)

Compile?
- Yes, because acct_status is only declared in account.h, not defined
Link?
- No, you'll get a message like undefined reference to 'acct_status' because it has been declared but not defined

A Common "Problem": Attempt 5

account.h

  #ifndef ACCOUNT_H
  #define ACCOUNT_H

  #define ACCT_OK      0
  #define ACCT_INVALID 1
  #define ACCT_CLOSED  2

  extern int acct_status;
  #endif

account.c

  #include "account.h"

  int acct_status = ACCT_OK;

  // Use acct_status

driver.c

  #include "account.h"

  // Use acct_status

A Common "Problem": Attempt 5 (cont.)

Compile?
- Yes
Link?
- Yes