code reads something like :
pid=fork()
if(pid){
print parent_id
print parent_id
}
else{
print child_id
print child_id
}
when it was executed it was
child
parent
child
parent
I don't understand how this could happen because child and parent should be printed consecutively.
You probably have a mutli-core CPU, so both parent and child can both be running on separate CPU cores. Printing is slow enough that the parent process has time to gets its 1st print started before your child process's 2nd print starts.
Or if not, on a single-core machine then scheduling could context-switch after one print.
The whole point of fork is to create a 2nd process that's also running; there should be no expectation that their system calls don't interleave with each other.
Existing near-duplicates about fork race conditions / timing, although those primarily about which runs first, not any expectation of continuing to run for multiple system calls before the other process can do anything.
fork(), runs always the parent first and then the child
Who executes first after fork(): parent or the child?
In fork() which will run first, parent or child?
Which process runs first when a fork() is called
And more generally Understanding parent and child process execution order about making multiple system calls including read in both parent and child, causing one or both to block.
And two where people had the opposite problem: they expected interleaving of output but didn't get it. (Perhaps because of buffered I/O resulting in only one write system call, if run from an IDE with output connected to a pipe instead of tty perhaps.)
fork() does not run parallel
Child process starts after parent process
I need to create a multi-process program that:
Creates 5 processes using fork();
Sends stuff to do the the child processes using pipes
When a child process completes its stuff, it should receive new stuff to do from parent process until all stuff are completed.
Right now my idea is to wait() on completed child tasks (and it exits) and then to create a new child process so I always have a maximum of 5 processes.
Is there a way to "re-use" the already existing process? Maybe "signaling" something? Cannot find it on Google.
Using C.
I solved my problem in this way:
Children write the result of their calculation on a pipe A (this pipe is not blocking). Then they wait their next input on a pipe B (this pipe is blocking).
Parent loops on all children trying to read something from pipe A (this pipe is not blocking so parent goes on if child didn't send anything). If pipe A contains a result, the parent sends another task to that child on pipe B.
There is a pipe A and a pipe B for each child.
I'm looking for a way to read from STDIN(console) in child process, after closing parent process.
My program should be like:
parent process forks and creates child process.
After creating child process, parent MUST be closed. I can not use functions such as wait() etc.
The thing is, when I exit from parent process, I can not read from console anymore. Is there any way to 'pass the control' to child process, instead of passing it back to shell ?
Instructions:
Process 1: reads data (sigle lines) from standard input stream and passes it with ipc message queue to process 2.
Process 2: receives data send by process 1 and prints it in standard output stream.
Two processes should be executed automatically from 1 initiative process. After executing child processes, initiative process should immediatelly close.
How can I split processes using C?
For example, in bash, I can use the command:
screen
You need to use fork() function.
The only way in C to create a new child process is the system call fork. All new process on UNIX/Linux are created with fork.
The child process is a copy of the parent process, the return value of fork call indicate if the current process is the parent (>0 : PID of the child process) or the child (0) or if the child process cannot be created (-1).
i have a c problem can any one help me . i wrote a code of process creation in c language , it uses pid & fork() call. the fork call is use for making child process now can any body tell me how to make parent process? i know that creating a child in process make the process parent automatically but i want to make a parent process right from the start so can any one tell me how to do this,
Secondly i just create the process i don't know how to use it i cant assign any work(computation) to it.so can any one tell me how to use the process for work?
Third one i want to give a name to my process how can i do it & how i can control their execution?
please if anyone can enlighten me than please help me to understand all this.
i shall be thank full for this forever
thanks in advance
The fork call creates a new process that is identical to the existing process except for a few minor differences such as its pid, parent pid. The original process carries on from exactly the same place and this is the parent process. Which means your question is basically meaningless. You don't create the parent process, the original process becomes the parent process once it creates a child process.
It's a bit like asking "I created a child by getting pregnant and giving birth, but how do I create the parents?" You are a parent automatically.
Back to computers...
When you fork, the system call returns the pid of the child to the parent and 0 to the child, so you should have code something like:
int pid = fork();
if (pid == 0)
{
// in child, do child processing - normally exec an executable
}
else if (pid > 0)
{
// in parent, do some processing - often wait for child to complete
}
else
{
// fork failed - handle the error
}
When you fork a process, one process becomes two processes. Each continues running at exactly the same place. The only difference is that the fork returns the PID of the child process to the parent process and returns the value 0 to the child process.
Without any help, the child process does not know its parent. If the two processes need to communicate with one another then they will need to use some sort of IPC mechanism.
A common form of IPC is pipe. If one opens pipes before forking, then both the child and the parent keep the open file descriptors. This will allow both processes to communicate with one another. The parent is now free to communicate its PID to the child process if so desired.
Secondly i just create the process i don't know how to use it i cant assign any
work(computation) to it
You need to use fork and exec in pair make it run the program you want to execute.
Here is the wiki link for more information.
Fork-exec is a commonly used technique in Unix whereby an executing process spawns a new program. fork() is the name of the system call that the parent process uses to "divide" itself ("fork") into two identical processes. After calling fork(), the created child process is actually an exact copy of the parent - which would probably be of limited use - so it replaces itself with another process using the system call exec().
To create a parent process, take your code for creating a child and reverse the roles of parent and child. Presto change-o, the new process is the parent and the old process is the child.
For communications between the processes, use a pipe(2), or several. Also, there is shared memory.
To control execution, use kill(2) and wait(2). I'm not sure about assigning names, we might need to know what platform you're on.
About renaming (I assume you mean the name displayed by ps), to "rename" a process, just copy your new name into argv[0]