Definition

The sequential execution of the instructions of a program in a namespace.

Namespace:

• code
• stack
• variables and constants
• files

Different processes run on different namespace.

Notion of Process

• Corresponds to the execution of a binary program
• Identified in a unique way by its pid number
• 3 segments: code (which can be kept in shared memory), data, stack
• Runs under the identity of a user
• Has a current directory

Data

Data segment

• Data - initialized data
• BSS (Block Started by Symbol) - Non initialized data
• Heap - malloc(), calloc()

Namespace of a process

text	executable code
data	initialized data
bss	non initialized data
heap	dynamic allocation
stack	execution (function calls, local variables, parameters)
U-area	argv[], envp[], etc

States of a process

（we should have a graph here)

Process Control Block (PCB)

Infomation associated with each process

• Process state
• PID - pid_t getpid(void)
• Program counter
• CPU registers
• CPU scheduling info
• Memory management info
• Accounting info
• I/O info

Process Creation with a fork

pid_t fork(void)

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

int main(){
    int pid;
    pid = fork();
    printf("The pid of the program is %d\n", getpid());
    return 0;
}

Return value of fork():

• Parent: child pid
• Child: 0
• Error: -1

getppid() - get parent pid.