Programming :: Fork() Copies The Address Space Of The Calling Process?
May 13, 2010
I know that fork() copies the address space of the calling process. Say, however, i have a linked list allocated. Will the list be copied over to the child process's space? If so, i would have to free them in the child process as well as the parent process, correct? Or will the variables be copied but not be pointing to any valid address? Or would it just kind of not do anything?example:
Code:
struct ll_ex {
struct ll_ex * next;
[code]....
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Feb 21, 2011
if i attach a shared memory to my process whch part of the address space it will add(like stack, heap, data, code...).
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May 12, 2011
i got basic knowledge about creating a single child from a parent using fork(). But when it comes into creating multiple children, i am simply stuck. I am trying to create two processes from a parent and it would wait for both two processes to finish. my attempt is as below
[Code]....
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Feb 24, 2010
Code:
int main()
{
int pi;
int i=10;
pid=fork();
[code]....
Q 1. The value of the variable pid returned by the fork() function will be greater than 0 in the parent process and equal to zero in the child process? but during forking, there values are exactly copied so what's went wrong here?
Q 2. "changes to the variable in one process is not reflected in the other process" why it is so? >> Even if we have variable i declared as a pointer or a global it wont make a difference.
Code:
int main()
{
int i, pid;
i=10;
printf("before fork i is %d
[code]....
Q. Through this program it is clear that both process is using the data from the same location, so where's the original value is residing when the child process is in execution.?
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Dec 7, 2009
Consider the following code:
Code:
int main()
{
int i=0;
pid_t pid;
for(i=0;i<2;i++)
[code]....
I get the following output:
Parent: chid_pid=4356 i=0 parent's pid=4355
This is child 4356 i=0
This is child 4357 i=1
[code]....
I can observe instead of two children(as I expect) processes there are three. This is because child process 4356 creates its own child. Why all the messages of the type "This is child X i=Y" are concentrated one under another? How exactly fork works? Is affected by the fact that I have a dual-core processor?
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Feb 17, 2011
Suppose a process creates multiple shared mem segments as part of it job.Now is it always possible that on restart, that process can reattach to those previously created shared mem segments to the same virtual address spaces (returned on first time creation) ? My intention is use pointers over shared memory.So on restart I want those pointers absolute value to remain valid.
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May 23, 2011
I have a doubt about signals in C programming. I have done this little program to explain it. It creates a child process with fork and, when the child ends, receives the SIGCHLD signal and wait for its termination.Ok, quite easy, BUT when I execute this code the SIGCHLD signal is received twice, first as an error (returns -1) and the second one to finish the child process.I don't understand the meaning of the first received signal. Why is it generated? Is the code wrong? (if you add the SIGINT and press Ctrl+C during the execution it also receives two signals instead of one)
Code: #include <stdio.h>
#include <unistd.h>
#include <string.h>
[code]....
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Sep 22, 2009
I am writing device driver in which i have to call callback function from kernel space, which are saving my data. But the callback functions are in userspace. While accessing them i am getting segmentation fault.
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Aug 19, 2010
I've a script that forks to another script.
The problem is that the child will not return to main process.
Main script "fork.sh"
Code:
The child process "smb_fork.sh"
Code:
when running it from the terminal "sh -x ./foork.sh"
Code:
My goal is via cron connect to ~100 remote winXP computers and download a single file. There fore I like to fork the process.
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Aug 2, 2011
trying to fork multiple child but the program does not go as i want...
My code :
Code:
#include <stdio.h>
#include <sys/types.h>
#include <unistd.h>
[code].....
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May 9, 2011
I recently inherited a server that I'm now looking after, and when browsing the running processes I came across a number of processes labelled:"/usr/local/apache-2.2.6/bin/.libs/lt-httpd -k start"
At any one time there are between 10 and 15 of them running.Googling the issue has been surprisingly unhelpful... can anybody even tell me what this process is, much less what sort of issue might result in multiple running copies? I'm running Debian Etch (release 4.0).
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Mar 18, 2011
my code is very simple:
int pid, status;
printf("parent process id=%d, to call fork
",getpid());
if(pid=fork()<0)
[Code].....
but everytime I can only get two fork return pid=0. In parent process, the pid returned by fork() should be >0 and equal to the child process id.
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Dec 10, 2008
I am writing a program which i dont want the kernel scheduler to preempt before certain time duration. I am using the system call sched_get_priority_max to set the maximum priority. However it is not producing results.
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Nov 21, 2010
I am trying to understand working of pci_resource_start function So I browsed code via cscope and searched for string pci_resource_start and got following in pci.h
Code:
#define pci_resource_start(dev, bar) ((dev)->resource[(bar)].start)
I am not able to understand how does this above macro works. How does it above macro gets appropriate base address register in configuration space?
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Aug 31, 2010
I would like to run versions of Mozilla Firefox 3.6.5+ as a single process, just as it was in versions 3.6.3 and prior. The reason is that, on Linux, I am running within proxychains - which doesn't bind to forked processes. Because the plugins in versions 3.6.5+ run in a forked process I can't use proxychains to redirect Flash streams. Is there a setting in modern versions of Firefox that allow me to run plugins in the main process?
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Jun 26, 2010
I have a strange problem on my MythTV PC: From time to time when I watch TV or a video in VLC the picture freezes but the sound will go on. After some time the picture comes back, first with some ugly artifacts in it and then completely. I've found out by letting htop run in the mean time that during these time CPU usage spikes up to a 100%, I also identified the process causing it (see attached screenshot)
Code:
/usr/X11R6/bin/X :0 -br -audit 0 -auth /var/lib/gdm:0.Xauth - nolisten tcp vt7"
but I have no idea what it does or why it behaves that way. Can someone fill my gaps here? By the way I'm not running Mythbuntu, but Ubuntu (gnome) with a mythback- and -frontend (don't know whether that's relevant though). I have a nVidia graphics card and a dual screen setup.
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Sep 14, 2009
Im an academic (university networks and security lecturer) studying/teaching network and operating system security, and inspired by the work of Hovav Shacham set about testing ASLR on linux. Principley I did this by performing a brute force buffer overflow attack on Fedora 10 and Ubuntu 9. I did this by writting a little concurrent server daemon which accidently on purpose didnt do bounds checking.
I then wrote a client to send it a malicious string brute forcing guessed addresses which caused a return-to-libc to the function usleep with a parameter of 16m causing a delay of 16 seconds as laid out in [URL] Once I hit the delay I new I had found the function and could calculate delta_mmap allowing me to create a standard chained ret-to-libc attack. All of that works fine. However .... To complete my understanding I am trying establish where I can find the standard base address for ubuntu 9 (and other distros) for the following, taken from Shacham:-
Quote:
[code]....
/proc/uid/maps gives me some information but not the base address ldd also gives me the randomised starting address for sections in the user address space but neither gives me the base address. Intrestingly ... when a run ldd with aslr on for over (about) 100 times and checked the start point of libc I determined that the last 3 (least significant) hex digits were always 0's and the fist 4 (most significant) where between 0xB7D7 and 0xB7F9. To me this indicated that bits 22-31 were fixed and bits 12-21 were randomized with bits 11-0 fixed. Although even that doesnt define the boundaries observed correctly.
Note: I am replicating the attack to provide signatures to detect it using IDS, and for teaching purposes. I am NOT a hacker and if needed to could reply from my .ac.uk email address as verification.
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Jun 21, 2010
In a 32-bit system, max memory addressable is 4GB. Now Linux kernel does memory mapping division of 1GB for kernel address space and 3GB for user address space. That means 4GB of virtual address space is divided between kernel (1GB) and user (3GB).
Q1. All virtual mapping utilizes the available physical RAM without any division? I mean to say that if RAM is 512MB then a page in kernel space can lie any where RAM (leave aside old PCI dma accesses)? (How this fits to fact that kernel memory is non-pageble)
Q2. If a process is created in user space, it has visibility 4GB address space or 3GB address space?
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Sep 16, 2010
As part of my ongoing project I need to put together a messaging server of sorts. So far I have coded a server which accepts multiple connections, which then runs a function that will perform a task, see below.On this sever I have a global structure which is populated using the data found in a read() buffer. However with each fork() that runs the below function each process is handed a blank structure. How could I go about allowing each fork() child process access the same structure?
My code should hopefully show what I am trying to do?
Code: void listen_for_client(int sock)
{
[code]....
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Apr 10, 2011
Now,I created one parent and two children. I must create some value as random in child1 and child1 has to send these values to child2.Child2 must read them.. now,I cant create some random values and i can send them.But child2 doesnt work. I have two functions. One them is writing, another one is reading. Child2 uses reading function,but it doesnt work (child2 cant call it, because writing function in endless loop) What i must do? I used wait,usleep... No way.. I attached my file and also there is code..
PHP Code: #include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <errno.h>
#include <string.h>
#include <signal.h>
#include <stdarg.h>
#include <sys/types.h>
[Code].....
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Sep 30, 2010
Just coming over from Windows here... where I've been using "CreateProcess" which returns a value depending on whether the process was created successfully or not.
Now I'm trying to create a process on linux and I've been learning about fork/exec, and I've been struggling with the fact that there seems to be no easy way to know (within the original parent process) if exec succeeded or not. (without forcing the parent process to hang around polling it or something).
Anyway, I've now just discovered the posix_spawn function, and it seems exactly what I need. However I'm finding the documentation a little hard to understand. In particular, I can't find actual confirmation that it will definitely return an error if creating the process fails (like CreateProcess does on windows). So can anyone confirm that for me?
The text on the documentation states: "If posix_spawn() or posix_spawnp() fail for any of the reasons that would cause fork() or one of the exec family of functions to fail, an error value shall be returned as described by fork() and exec"
This makes me think it uses exec/fork under the hood, and so I wonder if it can be trusted..?
And also, most of the examples on the internet for creating processes use fork/exec, so I wonder if there is some catch with posix_spawn I haven't read about... (since it seems a much simpler way of doing things)
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Aug 19, 2010
I have been struggling with this for about a week now. I'm working with QuickUSB to send video streams to and from a device. I get the fastest results by fork()ing and exec()ing new processes everytime I run a pipeline, which works great when reading from files, but now i have to transition it so that i am reading/writing data over a QuickUSB bus. I have it working for each individual process, but i can't run multiple processes because it won't let me open the device. After looking through QuickUSB's API a little closer, i found that the QHANDLE type that they use to describe the usb device is actually just a usb_dev_handle *. From what i have found, this is an incomplete structure, so i am having trouble with opening it in the main process and passing it to each new process to write to it. I already have semaphores set up to prevent multiple processes using it at the same time. I just CANT find out how to utilize the same usb device between two process that are forked and exec. i also must do the exec() because otherwise my gtk and gstreamer fails for trying to share resources.
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Oct 23, 2010
As an assignment i was doing a program to create two process using fork and pass messages between them using message queue.Did it worked well until my friend tried to copy it using scp.suddenly all hell broke loose as processes without ran syncronisation ie. in tech terms the process just wont wait wen a message queue is empty.it keeps on executing randomly.but after a reboot .. everything worked fine. until again i tried to do scp on my system on purpose. and again the program just went mad.
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May 25, 2011
I am learning about OS and I wrote this simple forking program... Here's the code..
Code:
#include <sys/types.h>
#include <unistd.h>
#include <stdio.h>
[Code].....
How can it print the first statement, that is BEFORE the fork() statement twice?
I am running Natty 11.04 btw..
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Mar 28, 2011
I have been doing programming since the last 5 years. however, i have used to cut and paste the template without knowing in depth since i am chasing the time to finish a certain project. The title above seems easy if we read the manual, its just that im kinda confuse just for a second. Hope you guys can help me to give a better understanding for me so that i can upgrade my knowledge.Well, during my normal practice cut and paste, i try to understand this code below :-
Code: pid_t pid;
pid=fork();
if (pid < 0) {perror("Erro spawning process : "); exit(EXIT_FAILURE);}
[code]....
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Jul 13, 2010
I have a directory with some data files in it. I did an md5sum find, and built an index of all the files contained:
Code:
find ./* -type f -print0 | xargs --null md5sum > MD5SUM
Now, based on my new index, I want to find the copies of these files as they appear in a new directory, where they have been renamed and reorganized.
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Mar 24, 2011
I'm wondering how python's thread & fork support is compared to perl? Which one is best option among Perl and Python
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Nov 24, 2010
I'm trying to write a program that will fork a series of FTP sessions. For each session, there should be separate input and output files associated with stdin and stdout/stderr.
I keep reading how I should be able to do that with dup2() in the child process before the execl(), but it's not working for me. Could someone please explain what I've done wrong? The program also has a 30-second sniper alarm for testing and killing of FTPs that go dormant for too long.
The code: (ftpmon.c)
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
[code]....
The output:
$ ftpmon
Connected to gila-crstest.gilacorp.com (172.16.20.8).
220 (vsFTPd 2.0.1)
ftp> waitpid(): Interrupted system call
Why am I getting the ftp> prompt? If the dup2() works, shouldn't it be taking input from my script and not my terminal? In stead, it does nothing, and winds up getting killed after 30 seconds. The log file is created, but it's empty after the run.
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Jan 27, 2011
I have a monitoring program ( GIT link to sourceforge ) which I'm trying to use to track when a child exits/dies/whatever. I'm calling fork(), then close for 0,1,2, and then opening /dev/null, monitored.stdout, and monitored.stderr as a replacement. I'm not sure if I've done something incorrect (perhaps I should use dup2 for explicit assignment?) but it appears that printf() messages are just being blackholed. I've tried setting the line buffering as a last ditch effort. On a different system, using code similar to the spawn_monitor() function, this appears to work fine, which makes me think I'm relying on some implementation specific detail.
Relevant function, for those who don't click links:
Code:
26 int spawn_monitored(char *argv[])
27 {
28 int exit = 0;
29 pid_t child;
30 const struct rlimit inf = {
[code].....
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Jun 4, 2010
I have a question about calling an asm function from C....It doesn't work unless I create an asm variable to hold the value of the function in....Why?Here's the code that doesn't work...
asmfile.s - version one Code: .section .data
mydata: .ascii "this is the message!
.equ mylen, . - mydata
[code]...
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