I am having a little trouble with understanding execution flow of fork(). My main question is that when fork() is called does the calling process pause execution flow, or continue execution? Here is an example of what I'm working on
for (i = 0; i < hosts; i++)
{
if (fork() == 0)
{
forward_data(client_sock, remote_sock[i]);
}
}
Here I use the fork() function to create separate processes that handle connections between remote hosts. The function forward_data() sends data from client_sock to remote_sock, and I am designing the program to send to multiple hosts at the same time.
fork() will duplicate the process and both processes (original and clone) will continue to execute from there, the only difference is that in the parent process, fork() will return the PID of the new process that was created (or -1 if error), while in the child process fork() will have returned 0.
It doesn't quite count as multithreading as once this split happens, the processes are no longer in the same virtual memory space.
First, as noted elsewhere, fork() makes a copy of the current process, and then both the new and the old process continue after the fork() returns -- the new process will see fork() return 0, the old process will see fork() return the pid of the new (child) process.
In what you've written, the original process will spawn hosts children, and each child will run forward_data(). If forward_data() returns, then each child will then spawn hosts - 1 grandchildren, who will in turn each spawn hosts - 2 further greatgrandchildren and so on.
Second, the short answer to the question "does the calling process pause execution flow, or continue execution?" is yes. The longer answer is that the calling process may or may not execute at the same time as the newly created process, you cannot tell and it may be different every time -- if you care, then you need to use some IPC mechanism to synchronise as required.
Third, since the question is tagged "multithreading", if the old process is running more than one pthread, then the new process inherits all the mutexes, conditions etc of the old process, in the state they were in when fork() was called. However, the new process has only one pthread, and that is a copy of the pthread in the old process which executed the fork().
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.
As I am currently doing this project in only C, I've up untill this point only used my webserver as a single threaded application. However, I dont want that anymore! So I have the following code that handles my Work.
void BeginListen()
{
CreateSocket();
BindSocket();
ListenOnSocket();
while ( 1 )
{
ProcessConnections();
}
}
Now I've added fork(); before the start of ProcessConnection(); which helpes me allowing multiple connections! However, when I add code for daemoning the application found in this answer. I've encounted a little problem, using fork() will create a copy of my whole running app, which is the purpose of fork(). So, I'd like to solve this problem.
My ProcessConnection() looks like this
void ProcessConnections()
{
fork();
addr_size = sizeof(connector);
connecting_socket = accept(current_socket, (struct sockaddr *)&connector, &addr_size);
if ( connecting_socket < 0 )
{
perror("Accepting sockets");
exit(-1);
}
HandleCurrentConnection(connecting_socket);
DisposeCurrentConnection();
}
How would I do to simply just add a couple of lines above or after connecting=socket = accept... in order to make it accept more than one connection at the time? Can i use fork(); but when it comes down to DisposeCurrentConnection(); I want to kill that process and just have the parent-thread running.
I'm not a 100% sure what it is that you're trying to do, buy off the top of my head, I'd prefer to do the fork after the accept, and simply exit() when you're done. Keep in mind though, that you need to react to the SIGCHLD signal when the child process exits, otherwise you'll have a ton of zombie-processes hanging around, waiting to deliver their exit-status to the parent process. C-pseudo-code:
for (;;) {
connecting_socket = accept(server_socket);
if (connecting_socket < 0)
{
if (errno == EINTR)
continue;
else
{
// handle error
break;
}
}
if (! (child_pid = fork ()))
{
// child process, do work with connecting socket
exit (0);
}
else if (child_pid > 0)
{
// parent process, keep track of child_pid if necessary.
}
else
{
// fork failed, unable to service request, send 503 or equivalent.
}
}
The child_pid is needed to (as already mentioned) to kill the child-process, but also if you wish to use waitpid to collect the exit status.
Concerning the zombie-processes, if you're not interested in what happened to the process, you could install a signal hander for SIGCHLD and just loop on waitpid with -1 until it there are no more child-processes, like this
while (-1 != waitpid (-1, NULL, WNOHANG))
/* no loop body */ ;
The waitpid function will return the pid of the child that exited, so if you wish you can correlate this to some other information about the connection (if you did keep track of the pid). Keep in mind that accept will probably exit with errno set to EINTR, without a valid connection if a SIGCHLD is caught, so remember to check for this on accepts return.
EDIT:
Don't forget to check for error conditions, i.e. fork returns -1.
Talking about fork() and threads on unix is not strictly correct. Fork creates a whole new process, which has no shared address space with the parent.
I think you are trying to achieve a process-per-request model, much like a traditional unix web server such as NCSA httpd or Apache 1.x, or possibly build a multi-threaded server with shared global memory:
Process-per-request servers:
When you call fork(), the system creates a clone of the parent process, including file descriptiors. This means that you can accept the socket request and then fork. The child process has the socket request, which it can reply to and then terminate.
This is relatively efficient on unix, as the memory of the process is not physically copied - the pages are shared between the process. The system uses a mechanism called copy-on-write to make copies on a page-by-page basis when the child process writes to memory. Thus, the overhead of a process-per-request server on unix is not that great, and many systems use this architecture.
Better to use select() function which enables u to listen and connect from different
requests in one program.... It avoids blocking but forking creates a new address space
for the copy of the program which leads to memory inefficiency....
select(Max_descr, read_set, write_set, exception_set, time_out);
i.e u can
fd_set* time_out;
fd_set* read_set;
listen(1);
listen(2);
while(1)
{
if(select(20, read_set, NULL,NULL, timeout) >0)
{
accept(1);
accept(2); .....
pthread_create(func);....
}
else
}
Check the return value of fork(). If it is zero, you are the child process, and you can exit() after doing your work. If it is a positive number then it's the process ID of the newly created process. This can let you kill() the child processes if they are hanging around too long for some reason.
As per my comment, this server is not really multi-threaded, it is multi-process.
If you want a simple way to make it accept multiple connections (and you don't care too much about performance) then you can make it work with inetd. This leaves the work of spawning the processes and being a daemon to inetd, and you just need to write a program that handles and processes a single connection. edit: or if this is a programming exercise for you, you could grab the source of inetd and see how it does it
You can also do what you want to do without either threads or new processes, using select.
Here's an article that explains how to use select (pretty low overhead compared to fork or threads - here's an example of a lightweight web server written this way)
Also if you're not wedded to doing this in C, and C++ is OK, you might consider porting your code to use ACE. That is also a good place to look for design patterns of how to do this as I believe it supports pretty much any connection handling model and is very portable.