02 Processes 1

UNIX Process Creation¶

If \(P_1\) spawns \(P_2\)

• \(P_1\) is parent
• \(P_2\) is child

fork()¶

function using which new processes become child processes of the caller

• No parameters
• returns 0 to child process
• returns process ID of the child to the parent

Both parent and child will immediately execute after the fork()

UNIX makes an exact copy of the parent’s Stack, Heap, Data, and Code in another sequence of memory locations.

Any change of variables by parent process won’t affect the child process’ values, and vice-versa

If there are \(n\) fork() one after each other,

void main()
{
  fork(); // 1
  fork(); // 2
  ...;
  fork(); // n
}

• Total number of processes \(= 2^n\)
• Total number of child processes \(= 2^n - 1\)

Code 1¶

The output after fork()

#include <unistd.h>
#include <stdio.h>

void main()
{
  int pid;
  pid = fork();

  print("Hello\n");
}

Hello
Hello

Concept 2¶

The order of execution may be

• Parent then child
• or, Child then parent

This depends on the system

Value of pid

• Child (=0)
• Parent (>0)
• Unsuccessful (<0)

#include <unistd.h>
#include <stdio.h>

void main()
{
  int pid;
  pid = fork();

  if(pid == 0)
    printf("Child\n");
  else if(pid > 0)
    printf("Parent\n");
  else
    printf("Unsuccessful Fork");
}

Concept 3¶

Let’s say both the parent and child get the same variable, then

• Changes in parent process will only affect the value in the parent process
• Changes in child process will only affect the value in the child process

This is because each process has its own address space; any modifications will be independent of each other

#include <unistd.h>
#include <stdio.h>

void main()
{
  int a = 0;

  int pid;
  pid = fork();

  if(pid == 0)
  {
    a = a+10;
    printf("Child %d\n", a);
  } else if(pid > 0)
  {
    a = a+5;
    printf("Parent\n");
  } else
    printf("Unsuccessful Fork");
}

Parent 5
Child 10
(or)
Child 10
Parent 5

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