Questions tagged [fork]
The fork tag has no usage guidance.
0
votes
0answers
15 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
0answers
19 views
What does fork() do exactly? [duplicate]
According to the manual, fork() creates a clone of the program in the given position of the code, but compiling the code below in C gives us "AB\nAB\n". How is this possible if fork() is after the ...
0
votes
0answers
29 views
How to force all processes with a known name D, started from a given process K have equal CPU ticks scheduled?
Say we have a process K that in itself starts docker images D. We want to make sure that CPU ticks are provided to each and every one from that pool of processes D in a round-robin, or fair meaning ...
-1
votes
2answers
83 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
96 views
Measure memory usage of forked processes
In order to measure memory usage of a process, Iam using the VmPeak and VmSize fields of /proc/[pid]/status. What happens to memory reporting if this process is forked after allocating memory?
My ...
0
votes
0answers
38 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/...
52
votes
3answers
10k 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
196 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
198 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
votes
0answers
142 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
76 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
110 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
1answer
105 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 ?
44
votes
4answers
7k 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 ...
1
vote
0answers
101 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
225 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 ...
0
votes
2answers
62 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
89 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
397 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
318 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
122 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
473 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
194 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
306 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 ...
1
vote
2answers
680 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 ...
1
vote
1answer
224 views
Linux open() syscall and folder permissions
I have a process P which is spawned by a process owned by root.
After P is created setguid() and setuid() are called and it runs as user U.
The process P attempts to create a file f on a folder F (in ...
1
vote
1answer
1k views
Systemd timeout because it doesn't detect daemon forking
I'm currently developping a systemd daemon. The problem I'm facing is that the daemon is killed 1m30s after beeing launched because the forking is not detected.
I'm using the int daemon(int nochdir, ...
8
votes
4answers
2k views
When to call fork() and exec() by themselves?
I'm new to Unix system calls and am learning about fork() and exec() commands. It seems like fork() and exec() are usually called together. (fork() creates a new child process, and exec() replaces the ...
1
vote
1answer
245 views
how does pte's flags are restored when a page is swapped in from swap-area?
I would like to understand properly the swapping in process and yet, couldn't find a thorough explanation how pte's flags of a page are restored once a page is swapped in back to memory- since it's ...
0
votes
1answer
249 views
Yad forking and return results
I'm trying to make a yad process collaborate with another (a mounter but that's not very important).
I have a code structure like this (simplified) in the critical part:
`
yad --list --button=gtk-...
0
votes
1answer
395 views
How does a parent process know the process IDs of the child processes it started?
The title was a question in an exam I had recently.
I could not find the answer afterwards in the slides (also not on the web).
In the course slides it is only described that the parent process holds ...
-1
votes
1answer
170 views
What exactly is the function piping into the other function in this fork bomb :(){ :|: & };:?
There are a couple of questions related to the fork bomb for bash :(){ :|: & };: , but when I checked the answers I still could not figure out what the exactly the part of the bomb is doing when ...
1
vote
1answer
503 views
:(){:|:&};: How does this BASH fork bomb work in detail? [duplicate]
I have a very fragmentary understanding of shells, subshells, functions, background processes and therefore I am insecure in understanding how this fork bomb works in detail,
However I tell you how ...
3
votes
2answers
593 views
Prevent a shell fork from living longer than its initiator?
If I have a Bash script like:
function repeat {
while :; do
echo repeating; sleep 1
done
}
repeat &
echo running once
running once is printed once but repeat's fork lives forever,...
3
votes
1answer
857 views
shell script process fork
I am doing some experiments to know how environment variables are inherited from parent process to child process by executing shell scripts in zsh and then use pstree <username> to see the ...
3
votes
2answers
3k views
Why does fork sometimes return parent and sometimes child?
When running the fork call to create a new process, if it succeed it returns either 0 (the child) or the parent.
I didn't get the idea behind this. Why doesn't fork just always return child or always ...
0
votes
0answers
312 views
How do you send stdout to the journal when using a process type of forking
I have a custom systemd unit file that uses the Type of forking. The initial process's stdout will be logged to the journal but the forked process's stdout is not logged to the journal. How can the ...
1
vote
1answer
717 views
docker fork/exec /proc/self/exe: cannot allocate memory
After a few days, docker fails with errors
docker[8927]: level=error msg="Error running exec in container: execcantrun: Cannot run exec command 34a26 in container 4972e576: [8] System error: fork/...
3
votes
2answers
457 views
Systemd restart service if one of the processes in the control group fails
I have a systemd service unit:
[Unit]
After=network.target
[Service]
User=<user>
Type=forking
ExecStart=/opt/app/start.sh
Restart=on-failure
[Install]
WantedBy=multi-user.target
The /opt/app/...
3
votes
1answer
832 views
How does fork system call really works
I've a very specific question about fork system call. I've this piece of code:
int main (void)
{
for (int i = 0; i < 10; i++) {
pid_t pid = fork ();
if ( !pid ) {
...
1
vote
0answers
591 views
Tracing a fork bomb
I am currently having an issue with a server running CentOS 6. For the past two days around midday we run into what I believe is one of our applications fork bombing the server. We start getting the ...
1
vote
1answer
210 views
When exactly does context_switch() switch control to a new process?
If I understand context switching correctly, the process involves two major steps:
The MMU is switched to one that maps the new processes virtual memory space to physical memory space.
The processor ...
-1
votes
1answer
93 views
Redirection in self created linux shell [closed]
I am writing my own shell. I want to implement redirection( > and >>). For that I have used dup2() system call. But if a command I enter has redirection, another command follows the previous ...
15
votes
1answer
2k views
How exactly does the typical shell “fork bomb” call itself twice?
After going through the famous Fork Bomb questions on Askubuntu and many other Stack Exchange sites, I don't quite understand what everyone is saying like it's obvious.
Many answers (Best example) ...
3
votes
1answer
325 views
Unexpected parent process id in output
My code is forking a process and printing each process' PID and PPID. I was expecting the child's PPID to be same as the parent's PID, but it is not coming up as such.
I'm using Ubuntu 14.04.
#...
3
votes
1answer
338 views
Confused by this fork bomb script [duplicate]
There's a shell script floating around the Internet that looks like this:
:(){ :|: & };:
Basically this shell script just creates a function that calls itself, leading to infinite recursion. ...
1
vote
1answer
3k views
wrapper script: fork: retry: No child processes
"myapplication" needs some setup or clean up done, so I use the following wrapper script:
#!/bin/bash
echo "Do important set up stuff"
myapplication
echo "Clean up"
and put it in my path, named "...
16
votes
4answers
2k views
How to pass a password to a child process?
Passing a password on command line (to a child process started from my program) is known to be insecure (because it can be seen even by other users with ps command). Is it OK to pass it as an ...
3
votes
0answers
324 views
Theoric: How to kill a forever forking process?
Let's say you have a process that forks every time it has the chance.
It launches itself in niceness -20, so it forks faster than you can kill it.
Also, it remembers its childrens and it parents, so ...
12
votes
1answer
497 views
How does ksh93 avoid forks in command substitution
Given
cmd='fun(){ echo "$@"; }; fun $(fun $(fun hi))'
shells tend to need to make 2 forks to make it happen
strace-f(){ strace -f "$@" 2>&1; };
for sh in dash bash zsh ksh; do
printf ...
