What is the purpose of fork()?

问题:

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?

回答1:

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:

1. Your shell uses fork to run the programs you invoke from the command line.
2. 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.
3. 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.
4. 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.
5. 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.

回答2:

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.

回答3:

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

回答4:

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.

回答5:

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.

回答6:

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.

回答7:

Fork creates new processes. Without fork you would have a unix system that could only run init.

回答8:

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.

回答9:

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.

回答10:

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

回答11:

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.

回答12:

First one needs to understand what is fork () system call. Let me explain

1. 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.

2. 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():

1. 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.

回答13:

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

回答14:

The fork() function is used to create a new process by duplicating the existing process from which it is called. The existing process from which this function is called becomes the parent process and the newly created process becomes the child process. As already stated that child is a duplicate copy of the parent but there are some exceptions to it.

• The child has a unique PID like any other process running in the operating system.

• The child has a parent process ID which is same as the PID of the
  process that created it.

• Resource utilization and CPU time counters are reset to zero in child process.

• Set of pending signals in child is empty.

• Child does not inherit any timers from its parent

Example :

    #include <unistd.h>
    #include <sys/types.h>
    #include <errno.h>
    #include <stdio.h>
    #include <sys/wait.h>
    #include <stdlib.h>

    int var_glb; /* A global variable*/

int main(void)
{
    pid_t childPID;
    int var_lcl = 0;

    childPID = fork();

    if(childPID >= 0) // fork was successful
    {
        if(childPID == 0) // child process
        {
            var_lcl++;
            var_glb++;
            printf("\n Child Process :: var_lcl = [%d], var_glb[%d]\n", var_lcl, var_glb);
        }
        else //Parent process
        {
            var_lcl = 10;
            var_glb = 20;
            printf("\n Parent process :: var_lcl = [%d], var_glb[%d]\n", var_lcl, var_glb);
        }
    }
    else // fork failed
    {
        printf("\n Fork failed, quitting!!!!!!\n");
        return 1;
    }

    return 0;
}

Now, when the above code is compiled and run :

$ ./fork

Parent process :: var_lcl = [10], var_glb[20]

Child Process :: var_lcl = [1], var_glb[1]

回答15:

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.