Operating Systems Lab

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1 Operating Systems Lab Islamic University Gaza Engineering Faculty Department of Computer Engineering Fall 2012 ECOM 4010: Operating Systems Lab Eng: Ahmed M. Ayash Lab # 3 Fork() in C and C++ programming September 22, 2012

2 Duplicating a process image: To use processes to perform more than one function at a time we can either use threads or create an entirely separate process from within program by calling fork. System call fork() is used to create processes. It takes no arguments and returns a process ID. The purpose of fork() is to create a new process, which becomes the child process of the caller. After a new child process is created, both processes will execute the next instruction following the fork() system call. Therefore, we have to distinguish the parent from the child. This can be done by testing the returned value of fork(): If fork() returns a negative value, the creation of a child process was unsuccessful. fork() returns a zero to the newly created child process. fork() returns a positive value, the process ID of the child process, to the parent. The returned process ID is of type pid_t defined in sys/types.h Normally, the process ID is an integer. Moreover, a process can use function getpid() to retrieve the process ID assigned to this process. Therefore, after the system call to fork(), a simple test can tell which process is the child. Note that Linux will make an exact copy of the parent's address space and give it to the child. Therefore, the parent and child processes have separate address spaces Execution - Parent and children execute concurrently. - Parent waits until children terminate. 1

3 Fork examples: Fork 1 program #include <stdio.h> #include <sys/types.h> #include <unistd.h> int main(void) int pid=0; printf("hello \n"); pid=fork(); printf("bye\n"); return 0; P CH pid>0 pid=0 Output: Hello is printed once by parent process bye - is printed twice, once by the parent and once by the child Note <sys/types.h> : this header file defines a collection of data types including (pid_t). <unistd.h> : this header file defines system call functions including (fork()). Practical: - Create the file : $ vi fork1.cpp Or $ gedit fork1.cpp - Write the code. - Compile and execute the code in Shell as shown in figure1. Figure (1) 2

4 Fork 2 program #include <stdio.h> #include <sys/types.h> #include <unistd.h> int main(void) int pid1, pid2, pid3; pid1 = 0; pid2 = 0; pid3 = 0; pid1 = fork(); if (pid1 == 0) pid2 = fork(); pid3 = fork(); else pid3 = fork(); if (pid3 == 0) pid2 = fork(); if ((pid1 == 0) && (pid2 == 0)) printf("level1\n"); if (pid1!= 0) printf("level2\n"); if (pid2!= 0) printf("level3\n"); if (pid3!= 0) printf("level4\n"); Output: Level1: 2 Level2: 3 Level3: 3 Level4: 3 Practical: - Create the file : $ vi fork2.cpp Or $ gedit fork2.cpp - Write the code. - Compile and execute the code in Shell as shown in figure 2. 3

5 Figure (2) Fork 3 program using namespace std; #include <iostream> #include <sys/types.h> #include <unistd.h> #include <stdio.h> int main() pid_t pid; char *message; int n; cout<<"fork is starting"<< endl; pid = fork(); switch(pid) case -1: perror("fork failed"); return 1; case 0: message = "This is the child"; n = 5; break; default: message = "This is the parent"; n = 2; break; 4

6 for(; n > 0; n--) cout<<(message)<< endl; sleep(1); return 0; Output: This program runs as two processes. A child is created (born?) and prints a message five times. The original process (the parent) prints a message only two times and (fork is starting) paragraph. The parent process finishes before the child has printed all of its messages, so the next shell prompt appears mixed in with the output. Here parent don't wait for the child. Practical: - Create the file : $ vi fork3.cpp Or $ gedit fork3.cpp - Write the code - Compile and execute the code in Shell as shown in figure 3. Figure (3) Fork 4 program (waiting for a process): When we start a child process with fork, it takes on a life of its own and runs independently. Sometimes, we would like to find out when a child process has finished. For example, in the previous program, the parent finishes ahead of the child and we get some messy output as the child continues to run. We can arrange for the parent process to wait until the child finishes before continuing by calling wait. 5

7 #include <sys/types.h> #include <sys/wait.h> pid_t wait(int *stat_val); The wait system call causes a parent process to pause until one of its child processes is stopped. The call returns the PID of the child process. This will normally be a child process that has terminated. The status information allows the parent process to determine the exit status of the child process, that is, the value returned from main or passed to exit. If stat_val is not a null pointer, the status information will be written to the location to which it points. Fork4.c #include <sys/types.h> #include <sys/wait.h> #include <stdio.h> void main() int status; pid_t pid; pid = fork(); if(pid == -1) printf("\nerror child not created "); else if (pid == 0) /* child process */ printf("\n I'm the child!"); exit(0); else /* parent process */ wait(&status); printf("\n I'm the parent!"); printf("\n Child returned: %d\n", status); Practical: - Create the file : $ vi fork4.c Or $ gedit fork4.c - Write the above code - Compile and execute the code in Shell as shown in figure 4. 6

8 Figure (4) Exercises: 1) For fork3.cpp >> if you exchange: - n to 5 for parent - n to 2 for child To avoid the messy output as the child continues to run after parent finishes. What will be the output? 2) Using fork3.cpp, make all children execute before all parent. Show the code and the output. 7

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