I have the following piece of code
#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>
int main() {
int pid, status, number = 300; // Create a process (fork)
pid=fork();
if(pid==0) {
// Child process
number = 400;
printf("Child process: Number is %d\n",number); // Terminate OK
exit(0);
} else {
// Father process
number = 500;
printf("Father process: Number is %d\n",number); // Wait for child to finish
wait(&status);
}
return 0; // Terminate OK
}
I'm doubting if wether or not the scheduler might affect the output of this code. I believe the scheduler might execute an entire code block first and then execute the other one, hence changing the value of number and changing the print of any of the two threads to the other thread's number.
Is this true? Or is the result deterministic and will always output 400 and 500 in their respective execution threads?
This is not multi-threading. fork() creates a duplicate of the process.
When you run the program, the operating system loads it into memory, creates a process and starts it. The process uses (at least) one memory block that contains the code and one that contains its data (n.b. it is not that simple but the full explanation is beyond the scope of this question).
The code segment contains the code of the program and it is read-only. The data segment contains the data (the variables) and its content can be modified.
The fork() system call creates a new process that is a clone of the original process. The new process uses the same code segment as the original process but it has its own data segment. The data segment is a clone of the original data segment.
The new process does not start running the code from the beginning but its status is the same as the one of the parent process; just after its creation, the new process is running inside the fork() function.
The new process runs independent of its parent process. In the parent process, the function fork() returns a value that is greater than 0 (the process ID of the child process). Inside the child process, fork() returns 0. This way the code can continue with different things in the parent and in the child process.
Each of the two processes then live its own life, without influencing the life of the other process. In your example, the parent changes the value of number to 400 in its data segment then prints it. This change does not affect in any way the value of number (or of any other variable) of the child process; the child process has its own data segment. The child process changes its copy of number to 500 and prints it.
Because the two processes run in parallel, the order of the two displayed values is not determined. It can be 400 then 500 on one execution and the other way around on another execution.
Welp, we just asked the teacher lol.
The memory spaces are totally different, which means the other thread shouldn't affect the print output.
I want to know how fork, wait and exit communicate with each other. What is passed in &n and exit(0) returns what to the parent process?
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
int main(void){
int n=5;
fork();
wait(&n);
printf("%d\n", n);
exit(0);
}
The output I am getting is
sh-4.3$ main
5
0
sh-4.3$
fork() creates a new process which is almost the exact copy of the one that makes the call. The newly created process is called the child. Beware that exact copy is a strong assumption! That really means that the newly created process is at the time of its creation in the exact state of its parent (this is a slightly rough assumption, but for the sake of simplicity think of it as is). Then one process makes a call to fork and then two processes return from the call! fork is a kind of cloning machine (one sheep enters, and then two exit from the machine!). Normally you should test the returned value of fork (see man for details).
Then from now two processes are running independently!
One does a call to wait, while the other does the same on its own. Effect of wait is different in both. For the original process, wait will wait for the child to terminate and then capture information about that event. The child calls wait but as it has no child wait does nothing and returns immediately. It then print the value of its variable n which as not been modified: 5. And then it exits with a value 0. So it terminates. This will permit the parent process to awake from it's wait call and capture the exit value of its child (0). It then prints it 0 and exits.
Beware that some variants of this scenario may happen, while this will not change the observable behavior, concurrent runs may run differently.
You can find out about those functions on man pages, fork, wait, exit.
For fork I would look up usage examples however, as it is kind of complicated to explain.
Good morning,
I'm trying to learn how to us Peterson's solution for critical section protection. Each process is trying to increment total to 100,000 and I have to make sure each child calls process#(). I also need to use the "wait" function so the parent knows when the child finishes. Once a child finishes I need to print the process ID and the amount of times process 1 interrupts process 2, and vise versa. I really have no idea what I'm doing even though I've been reading around a lot. What is this "Waiting" function I'm supposed to use? How do I use it? Why is my code incrementing to 200,000 instead of 100,000?
Code removed, unnecessary for question.
Apparently somewhere in the main function, I need to loop for the parent to wait for the child and then print the process ID of the children, but I have no idea how to do this.
The wait() command you are referring to (and waitpid()) is a command you use in the parent process to "wait" for a child to terminate (blocking, meaning the parent won't continue execution until the child changes state). If your child terminates and you do not wait() in the parent, the child process will become a "zombie". wait()ing effectively "reaps" the child process.
This is the signature for waitpid():
pid_t waitpid(pid_t pid, int *status, int options);
status is a variable you can use to return some info from the child to the parent (e.g., the number of times this process was interrupted (assuming you keep track of this in the child?)); it is the exit code of the child. Let's assume you have a child with PID of 1234, you would call waitpid(1234, &status 0) (if you wanted to wait non-blocking, you'd have to use WNOHANG for options (it immediately returns if no child has exited). Check out http://linux.die.net/man/2/waitpid because there's cool values you can use for waitpid() such as -1 to wait for any child to exit (this is the same as regular wait()). Please post comments if you have any more questions, but hopefully this is enough to point you in the right direction :).
Since you want to "loop" in the parent to wait for your children, you can either skip the loop altogether and use the normal, "blocking" wait, or you can use an infinite loop with a non-blocking wait (or a series of them, one for each child if you don't use regular wait()).
Just in case you didn't know, you can determine whether you are in the parent or child based on the return value of fork(), but I think you already knew this. So in the body of an if (checking for parent) is where you would do the wait().
Also, is process2() supposed to have while (k < 200000)? Could this by why you say it's incrementing to 200,000?
In many programs and man pages of Linux, I have seen code using fork(). Why do we need to use fork() and what is its purpose?
fork() is how you create new processes in Unix. When you call fork, you're creating a copy of your own process that has its own address space. This allows multiple tasks to run independently of one another as though they each had the full memory of the machine to themselves.
Here are some example usages of fork:
Your shell uses fork to run the programs you invoke from the command line.
Web servers like apache use fork to create multiple server processes, each of which handles requests in its own address space. If one dies or leaks memory, others are unaffected, so it functions as a mechanism for fault tolerance.
Google Chrome uses fork to handle each page within a separate process. This will prevent client-side code on one page from bringing your whole browser down.
fork is used to spawn processes in some parallel programs (like those written using MPI). Note this is different from using threads, which don't have their own address space and exist within a process.
Scripting languages use fork indirectly to start child processes. For example, every time you use a command like subprocess.Popen in Python, you fork a child process and read its output. This enables programs to work together.
Typical usage of fork in a shell might look something like this:
int child_process_id = fork();
if (child_process_id) {
// Fork returns a valid pid in the parent process. Parent executes this.
// wait for the child process to complete
waitpid(child_process_id, ...); // omitted extra args for brevity
// child process finished!
} else {
// Fork returns 0 in the child process. Child executes this.
// new argv array for the child process
const char *argv[] = {"arg1", "arg2", "arg3", NULL};
// now start executing some other program
exec("/path/to/a/program", argv);
}
The shell spawns a child process using exec and waits for it to complete, then continues with its own execution. Note that you don't have to use fork this way. You can always spawn off lots of child processes, as a parallel program might do, and each might run a program concurrently. Basically, any time you're creating new processes in a Unix system, you're using fork(). For the Windows equivalent, take a look at CreateProcess.
If you want more examples and a longer explanation, Wikipedia has a decent summary. And here are some slides here on how processes, threads, and concurrency work in modern operating systems.
fork() is how Unix create new processes. At the point you called fork(), your process is cloned, and two different processes continue the execution from there. One of them, the child, will have fork() return 0. The other, the parent, will have fork() return the PID (process ID) of the child.
For example, if you type the following in a shell, the shell program will call fork(), and then execute the command you passed (telnetd, in this case) in the child, while the parent will display the prompt again, as well as a message indicating the PID of the background process.
$ telnetd &
As for the reason you create new processes, that's how your operating system can do many things at the same time. It's why you can run a program and, while it is running, switch to another window and do something else.
fork() is used to create child process. When a fork() function is called, a new process will be spawned and the fork() function call will return a different value for the child and the parent.
If the return value is 0, you know you're the child process and if the return value is a number (which happens to be the child process id), you know you're the parent. (and if it's a negative number, the fork was failed and no child process was created)
http://www.yolinux.com/TUTORIALS/ForkExecProcesses.html
fork() is basically used to create a child process for the process in which you are calling this function. Whenever you call a fork(), it returns a zero for the child id.
pid=fork()
if pid==0
//this is the child process
else if pid!=0
//this is the parent process
by this you can provide different actions for the parent and the child and make use of multithreading feature.
fork() will create a new child process identical to the parent. So everything you run in the code after that will be run by both processes — very useful if you have for instance a server, and you want to handle multiple requests.
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. Please note that Unix 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.
Let us understand it with an example to make the above points clear. This example does not distinguish parent and the child processes.
#include <stdio.h>
#include <string.h>
#include <sys/types.h>
#define MAX_COUNT 200
#define BUF_SIZE 100
void main(void)
{
pid_t pid;
int i;
char buf[BUF_SIZE];
fork();
pid = getpid();
for (i = 1; i <= MAX_COUNT; i++) {
sprintf(buf, "This line is from pid %d, value = %d\n", pid, i);
write(1, buf, strlen(buf));
}
}
Suppose the above program executes up to the point of the call to fork().
If the call to fork() is executed successfully, Unix will make two identical copies of address spaces, one for the parent and the other for the child.
Both processes will start their execution at the next statement following the fork() call. In this case, both processes will start their execution at the assignment
pid = .....;
Both processes start their execution right after the system call fork(). Since both processes have identical but separate address spaces, those variables initialized before the fork() call have the same values in both address spaces. Since every process has its own address space, any modifications will be independent of the others. In other words, if the parent changes the value of its variable, the modification will only affect the variable in the parent process's address space. Other address spaces created by fork() calls will not be affected even though they have identical variable names.
What is the reason of using write rather than printf? It is because printf() is "buffered," meaning printf() will group the output of a process together. While buffering the output for the parent process, the child may also use printf to print out some information, which will also be buffered. As a result, since the output will not be send to screen immediately, you may not get the right order of the expected result. Worse, the output from the two processes may be mixed in strange ways. To overcome this problem, you may consider to use the "unbuffered" write.
If you run this program, you might see the following on the screen:
................
This line is from pid 3456, value 13
This line is from pid 3456, value 14
................
This line is from pid 3456, value 20
This line is from pid 4617, value 100
This line is from pid 4617, value 101
................
This line is from pid 3456, value 21
This line is from pid 3456, value 22
................
Process ID 3456 may be the one assigned to the parent or the child. Due to the fact that these processes are run concurrently, their output lines are intermixed in a rather unpredictable way. Moreover, the order of these lines are determined by the CPU scheduler. Hence, if you run this program again, you may get a totally different result.
You probably don't need to use fork in day-to-day programming if you are writing applications.
Even if you do want your program to start another program to do some task, there are other simpler interfaces which use fork behind the scenes, such as "system" in C and perl.
For example, if you wanted your application to launch another program such as bc to do some calculation for you, you might use 'system' to run it. System does a 'fork' to create a new process, then an 'exec' to turn that process into bc. Once bc completes, system returns control to your program.
You can also run other programs asynchronously, but I can't remember how.
If you are writing servers, shells, viruses or operating systems, you are more likely to want to use fork.
Multiprocessing is central to computing. For example, your IE or Firefox can create a process to download a file for you while you are still browsing the internet. Or, while you are printing out a document in a word processor, you can still look at different pages and still do some editing with it.
Fork creates new processes. Without fork you would have a unix system that could only run init.
Fork() is used to create new processes as every body has written.
Here is my code that creates processes in the form of binary tree.......It will ask to scan the number of levels upto which you want to create processes in binary tree
#include<unistd.h>
#include<fcntl.h>
#include<stdlib.h>
int main()
{
int t1,t2,p,i,n,ab;
p=getpid();
printf("enter the number of levels\n");fflush(stdout);
scanf("%d",&n);
printf("root %d\n",p);fflush(stdout);
for(i=1;i<n;i++)
{
t1=fork();
if(t1!=0)
t2=fork();
if(t1!=0 && t2!=0)
break;
printf("child pid %d parent pid %d\n",getpid(),getppid());fflush(stdout);
}
waitpid(t1,&ab,0);
waitpid(t2,&ab,0);
return 0;
}
OUTPUT
enter the number of levels
3
root 20665
child pid 20670 parent pid 20665
child pid 20669 parent pid 20665
child pid 20672 parent pid 20670
child pid 20671 parent pid 20670
child pid 20674 parent pid 20669
child pid 20673 parent pid 20669
First one needs to understand what is fork () system call. Let me explain
fork() system call creates the exact duplicate of parent process, It makes the duplicate of parent stack, heap, initialized data, uninitialized data and share the code in read-only mode with parent process.
Fork system call copies the memory on the copy-on-write basis, means child makes in virtual memory page when there is requirement of copying.
Now Purpose of fork():
Fork() can be used at the place where there is division of work like a server has to handle multiple clients, So parent has to accept the connection on regular basis, So server does fork for each client to perform read-write.
fork() is used to spawn a child process. Typically it's used in similar sorts of situations as threading, but there are differences. Unlike threads, fork() creates whole seperate processes, which means that the child and the parent while they are direct copies of each other at the point that fork() is called, they are completely seperate, neither can access the other's memory space (without going to the normal troubles you go to access another program's memory).
fork() is still used by some server applications, mostly ones that run as root on a *NIX machine that drop permissions before processing user requests. There are some other usecases still, but mostly people have moved to multithreading now.
The rationale behind fork() versus just having an exec() function to initiate a new process is explained in an answer to a similar question on the unix stack exchange.
Essentially, since fork copies the current process, all of the various possible options for a process are established by default, so the programmer does not have supply them.
In the Windows operating system, by contrast, programmers have to use the CreateProcess function which is MUCH more complicated and requires populating a multifarious structure to define the parameters of the new process.
So, to sum up, the reason for forking (versus exec'ing) is simplicity in creating new processes.
Fork() system call use to create a child process. It is exact duplicate of parent process. Fork copies stack section, heap section, data section, environment variable, command line arguments from parent.
refer: http://man7.org/linux/man-pages/man2/fork.2.html
Fork() was created as a way to create another process with shared a copy of memory state to the parent. It works the way it does because it was the most minimal change possible to get good threading capabilities in time-slicing mainframe systems that previously lacked this capability. Additionally, programs needed remarkably little modification to become multi-process, fork() could simply be added in the appropriate locations, which is rather elegant. Basically, fork() was the path of least resistance.
Originally it actually had to copy the entire parent process' memory space. With the advent of virtual memory, it has been hacked and changed to be more efficient, with copy-on-write mechanisms avoiding the need to actual copy any memory.
However, modern systems now allow the creation of actual threads, which simply share the parent process' actual heap. With modern multi-threading programming paradigms and more advanced languages, it's questionable whether fork() provides any real benefit, since fork() actually prevents processes from communicating through memory directly, and forces them to use slower message passing mechanisms.