Questions tagged [fork]
The fork tag has no usage guidance.
159
questions
1
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1answer
28 views
Can I force some process to execute fork system call, from outside of that process?
Let's say there is a program, that waits for user input. If user input is "A", program goes to state s_a, and if user input is "B", program goes to state s_b. I don't know which state I need, ...
1
vote
1answer
24 views
How can I use fork to calculate partial results of a calculation?
The task that has been given to me is that I have to find all numbers that divide a number (let's call it x) and find out how many times does it divide it.
I have to use fork to do the task.
What I ...
0
votes
0answers
12 views
Can I efficiently fork a ramfs partition?
I want a ramfs (or equivalent, but strictly RAM only) partition which will serve as a read-only master instance from which any number of identical children can be forked.
When a process forks, this ...
1
vote
1answer
21 views
Recurrent output of printf() to stdout each time fork() is invoked allthough printf() is invoked prior to fork(). And why does '\n' fixes this? [duplicate]
While I was playing around with fork() I noticed a rather strange behavior but I couldn't figure out myself why this happens.
In the example below, each time fork() is invoked the output from the ...
0
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0answers
30 views
Rewrite program using POSIX threads rather than fork
Beginning to learn more on pthreads and how they could be implemented within my program. Below is a program which uses fork however would like to change the code so instead uses POSIX. Honestly do not ...
0
votes
1answer
48 views
Can I wait for all forked processes from a command to finish?
Suppose running FOO stars BAR in the background. How can I wait for BAR to finish in the shell? I want a solution where I don’t change the internals of FOO (though I can grudgingly change BAR’s.).
...
0
votes
0answers
50 views
Receiving SIGKILL when children terminate
I want to receive SIGKILL instead of SIGCHLD when any of my forked children terminate. On Linux this is possible by passing a different child termination signal to clone()1.
Is this possible on any ...
0
votes
1answer
64 views
Interacting programs stdin and stdout (create a robot program to play a game)
The setup:
I have an executable file, that I "cannot edit", since I don't have the source code.
When I execute the program it reveals a game, where it hands me some numbers via stdout, for me to ...
2
votes
0answers
37 views
Make Linux reboot when it runs out of PID
I want to configure a headless server to automatically reboot under catastrophic circumstances. Right now I have configured to make it panic when running out of RAM, and reboot automatically on kernel ...
0
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0answers
38 views
Why is “&” necessary to make a Unix fork bomb work? [duplicate]
Consider the canonical example of a Unix fork bomb:
DANGER: running the following command, might crash your system.
:(){ :|: & };:
I wonder why & (background) is needed in order to use up ...
0
votes
1answer
619 views
Cgroup memory limits and process killing
I have the following situation: (The following functions are ones taken from python)
I have a process A which is running and has a cgroup memory limit set on it.
I fork a child process from A using ...
1
vote
1answer
47 views
Do subprocesses keep pipes open?
I am trying to understand the behaviour of programs that launch subprocesses, when run in a pipeline.
This bash program, fork.sh, prints and returns immediately:
(sleep 1) &
echo 'here'
But ...
0
votes
2answers
103 views
kill( ) won't stop subprocess if done from process started by /etc/rc.local?
On a raspberry pi (a recent raspian) I wrote an application that, on demand via network request, spawns an application, and on demand via different network request, kills it. The spawning mechanism is ...
0
votes
1answer
40 views
fork(), exec() sequence with object file formats?
It's my understanding that POSIX systems rely on certain linker formats to organize memory during a fork() exec() sequence of calls. a.out, COFF and ELF are widely known formats of binary object ...
0
votes
0answers
109 views
what's the execution sequence of multiple child processes
use the follwing code to fork four chile processes
#define _GNU_SOURCE
#include <stdio.h>
#include <unistd.h>
#include <sys/wait.h>
#include <stdlib.h>
pid_t pid;
int ...
0
votes
1answer
52 views
Difference between pid_max/ulimit. How to create fake processes?
I am trying to learn the nuances between pid_max and setting user limits through ulimit. My understanding is that, no matter how high you set the user limits (for example setting user max processes to ...
0
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0answers
26 views
How does this code execute more than one case of the if - else statements? [duplicate]
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
int main(int argc, char *argv[]) {
printf("hello world (pid:%d)\n", (int) getpid());
int rc = fork();
if (rc < 0) {
// ...
1
vote
0answers
35 views
How can I create a function that memoizes other commands?
I have created the following function to memoize any command:
# zsh
(( $+functions[ec] )) || {
ec() print -r -- "$@"
ecerr() { ec "$@" 1>&2 }
ecdbg() ecerr "$@"
gq() { ec "${(q+...
16
votes
4answers
3k views
Why should fork() have been designed to return a file descriptor?
On his web page about the self-pipe trick, Dan Bernstein explains a race condition with select() and signals, offers a workaround and concludes that
Of course, the Right Thing would be to have fork(...
0
votes
1answer
517 views
How to create a process in Linux?
I am a research student working on an experiment involving Linux, and for it I need to create processes. I know how to create them with the fork() system call, but I was wondering if there was an ...
1
vote
2answers
78 views
Listen to a folder and modify incoming files on change
I want to convert pcap files to json files using shell script (and tshark). As soon as one or more new pcap files are copied into a folder, the command tshark should be executed and the resulting json ...
-1
votes
1answer
879 views
Two processes with identical PID after a fork call
According to the fork documentation (man 2 fork):
The child process is an exact duplicate of the parent process except for the following points:
The child has its own unique process ID, and this PID ...
0
votes
0answers
359 views
systemd: process with fork() only shows logs after child exits
I have a program that fork()s and exits. Its child remains running for about 50 seconds, while it prints out messages each 5 seconds. When I run from a terminal it runs as expected, the parent exits, ...
3
votes
1answer
989 views
Why forking is used in a unit file of a service?
My nginx unitfile is following,
[root@arif ~]# cat /usr/lib/systemd/system/nginx.service
[Unit]
Description=The nginx HTTP and reverse proxy server
After=network.target remote-fs.target nss-lookup....
0
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0answers
49 views
how to get the program and parent program of do_fork()
I'm trying to make a kernel module that would gather statistics about which process forks what subprocess. Kprobe seems like it might work, but I'm struggling to understand the structs that I get.
I ...
1
vote
1answer
198 views
Bash and Sudo forking continuously, hidden fork bomb?
I am experiencing a frustrating problem where my bash and sudo programs seem to be replicating thousands of processes on Mac. I have searched for all kinds of ways to stop them. I don't know what to ...
0
votes
2answers
963 views
Question about global environment variables and fork() & exec() [duplicate]
What I'm trying to understand is where does a child process inherit exported variables from it's parent process
What I'm trying to understand is where a child process inherits exported environment ...
0
votes
2answers
433 views
Would a forked child always get same File ID as parent's file ID?
This might be a naive question, but given in parent open call returned me File ID 4.
Is it guaranteed that child process will always get a File ID of 4 as well ?
0
votes
1answer
284 views
Why can't a fork bomb be killed in zsh in the same way as it's killed in bash?
I wanted to make my system a little bit more resilient to fork bombs. This can be partially achieved by using systemd's DefaultTasksMax parameter, or by using cgroup's pids controler for all valid ...
0
votes
2answers
2k views
Is copy-on-write not implemented based on page fault?
Operating System Concepts say
fork() we can use a technique known as copy-on-write,
which works by allowing the parent and child processes initially to share the
same pages. ...
When it is ...
0
votes
0answers
122 views
Listen for child process creation/fork
Say I have a process like so:
#!/usr/bin/env bash
node foo.js &
pid=$!
is there a way to listen to child processes that get created from pid?
I want to create a list of pids that get created/...
55
votes
3answers
13k views
Why can't I crash my system with a fork bomb?
Recently I've been digging up information about processes in GNU/Linux and I met the infamous fork bomb :
:(){ : | :& }; :
Theoretically, it is supposed to duplicate itself infinitely until the ...
1
vote
1answer
817 views
fork() and COW behavior after exec()
We understand the COW behavior after a fork (as for example described here) as follows: fork creates a copy of the parent's page table for the child and marks the physical pages read only, so if any ...
1
vote
1answer
764 views
How to change working directory of a child process by posix_spawn? [closed]
In Linux (CentOS 7.5, kernel 3.10, gcc 7.3), is it possible to change the working directory of a child process created by posix_spawn before it runs a given process image (an executable)? If yes, how? ...
0
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0answers
283 views
can two running processes share the complete process image in physical memory, not just part of it?
can two running processes share the complete process image in physical memory, not just part of it?
Here I am talking about the Linux operating systems(eg. Ubuntu).
My thinking:
I think it is ...
0
votes
0answers
132 views
AIX kernel internal stats of nmon: fork vs exec
Why the values of fork and exec stats shown on the kernel internal stats of nmon utility in AIX systems (k option) are so different?
Usually the shown fork value is greater than the exec value, ...
0
votes
1answer
198 views
What happens if you fork before switch_root?
So typically an initramfs /init script will end with something like
exec switch_root /newroot /sbin/init
effectively replacing itself with systemd or whatever floats your boat. But what would happen ...
0
votes
2answers
352 views
Does a forked process copy all mapped memory when writing?
I understand that a forked process maps the parent's memory and copies it when writing to it. Does it copy just what it needs written or does it copy the entire mapped memory ?
54
votes
4answers
15k views
Why does a program with fork() sometimes print its output multiple times?
In Program 1 Hello world gets printed just once, but when I remove \n and run it (Program 2), the output gets printed 8 times. Can someone please explain me the significance of \n here and how it ...
2
votes
0answers
236 views
Is it possible to fork the current pane in tmux and put it in a new pane?
Often I have set up some temporary environment variables/modules and I would like them to persist as I create a split in Tmux. Is it possible to fork the current running program (i.e. bash) and put ...
0
votes
2answers
1k views
Physical Address of a variable in processes
When we create a child process by using the system call fork(), address space of parent process is copied to child process. The code below demonstrates this clearly.
#include <stdio.h>
#include ...
1
vote
2answers
270 views
Making a bind-mount take effect only in the context of the current process and its descendants
I have 2 files: /MyDir/a and /MyDir/MySubDir/b and am running a bash script, to which I want to add code to make file /a point to file /b, but only in the current process and its descendants.
In ...
1
vote
2answers
98 views
Is it possible to findout if our command is sourced from a script or manually typed on command line?
We have an implementation to set environment for a particular tool with our custom command.
Eg: custom_command tool_name
This command logs the invokation time, user and tool_name in a log file.
And ...
1
vote
1answer
2k views
Creating a specific process tree and terminating it
I am currently taking a Computer Systems class and am having trouble with a homework problem. I have to create this specific process tree:
I also need it to stay in this state for a while (using ...
1
vote
0answers
524 views
Why did ulimit not stop my fork bomb that was running as the superuser?
I'm studying the fork bombs and I have some troubles to find a way to protect from this type of attack.
As far as I know there are two possible solutions:
ulimit
cgroup
I'm studying the first one on ...
0
votes
0answers
306 views
Creating an exact copy of a whole process?
fork() forks a process, but it will not copy all of its threads; nor will it create a clone of all its fds: the fds are shared between parent and child.
I want a way of do a complete fork. So after I ...
1
vote
2answers
2k views
Why is the shell not waiting for a child process of a command after the command exits?
I have been learning about UNIX processes and could not understand one point. Say we have code like this,
void fork_child()
{
if (fork() == 0) {
/* Child */
printf("Running Child, ...
1
vote
0answers
259 views
How to configure new RSS value to a process in Linux 3.10?
I want to limit the RSS of my python process to 5GB. The python process is running C++ code in every iteration (there are 10000 such iterations) and executing some commands which interact with the C++ ...
-1
votes
2answers
639 views
fork() and execve() efficiency [closed]
In terms of memory allocation efficiency is it right to say that after a fork() in the code of the child if I execute a program with execve() this would be more efficient than the same program ...
2
votes
2answers
2k views
Why are file descriptors shared between forked processes?
When we fork() a process, the child process inherits the file descriptors. The question is, why?
As I am seeing it, sharing the file descriptor is a headache when every process is trying to keep ...
