I was reading about processes and I came across this:
Usually, the child process then executes execve or a similar system call to change its memory image
what I can derive from this is this pseudocode:
if(child_created_sucessfully)
{
do_ABC_and_ignore_the_part_of_the_parent's_control_flow //is this what it meant to "change its memory image"?
}
(Question asked in the pseudocode's comment)
I completely don't understand this other part:
example, when a user types a command, say, sort, to the shell, the
shell forks off a child process and the child executes sort. The reason for this twostep
process is to allow the child to manipulate its file descriptors after the fork but
before the execve in order to accomplish redirection of standard input, standard
output, and standard error.
Regarding the first part
Usually, the child process then executes execve or a similar system
call to change its memory image
This simply means that when you create a child process it initializes it's own stack and heap memory although this is not 100% true. Since the new process is forked at time T at the time T + 1 when the process starts to run it is pretty much identical when it comes to the data in memory so there is a smart optimization called 'copy on write' more here.
Regarding the second part
example, when a user types a command, say, sort, to the shell, the
shell forks off a child process and the child executes sort. The
reason for this twostep process is to allow the child to manipulate
its file descriptors after the fork but before the execve in order to
accomplish redirection of standard input, standard output, and
standard error.
Simply put this means that when you execute a shell command (like ls, ps, grep, nstat...) the OS forks the current process which executes the command and the command itself is executed by this new process. An easy way to understand this is by using ps | grep ps this will first fork and create a new process, then this part comes to play
this twostep process is to allow the child to manipulate its file
descriptors after the fork but before the execve
and the standard output of the process is changed. After the new ps process executes the ps it will then fork and create one more process for the grep ps which will execute the grep and you should be able to see the ps process which created this grep process.
I am looking for C code to use on a Linux based system to start another process asynchronously. The second process should continue, even if the first ends. I've looked through the "fork" and "system" and "exec" options, but don't see anything that will spawn a peer process that's not communicating with or a child of the original process.
Can this be done?
Certainly you can. In the parent fork() a child, and in that child first call daemon() (which is an easy way to avoid setsid etc.), then call something from the exec family.
In Linux (and Unix), every process is created by an existing process. You may be able to create a process using fork and then, kill the parent process. This way, the child will be an orphan but still, it gets adopted by init. If you want to create a process that is not inherited by another, I am afraid that may not be possible.
You do a fork (man 2 fork) followed by an execl (man 2 execl)
For creates a new process of the same image as the calling process (so a perfect twin), where execl replaces one of the twins with a new image.
If you search google for "fork execl" you will find many text book examples -- including how to use correctly fork() and exec()
The most common fork-execl you will still have the new process associated to the terminal -- to create a perfect background process you need to create what is called a daemon process -- the template for that can be fornd in this answer here Creating a daemon in Linux
I am using pthread_create() to create a thread and inside that thread i am using fork+execlp to load a new script.
But the problem is some time fork() call is fine but it is not executing the execlp call. So i have several process with the parent name is running and that is the reason some of the scripts are getting missed.
For example: If from my main program should execute 4 scripts.
I create 4 threads, and inside that i use fork+execlp to execute the scripts.
But when i see what are the scripts it is running, it only shows 3 scripts and one process iwth the parent name.
Can you please let me know what is the best way to deal with this situation?
It sounds like the execlp() call might be failing. If it succeeds, it never returns, so any return from execlp() indicates failure. Immediately after the call, you should call perror("execlp") to show the error, then _exit(1); to have the new child process exit.
When combining multiple threads with fork() make sure that you only execute only one fork in parallel. The pthreads method to do this is to lock a mutex with pthread_mutex_lock().
In the child, you will only have one thread -- the one that called fork(). Don't do anything but call async-signal-safe calls after the fork and before the exec.
See this SO question for more information.
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.