Programming :: Understanding Pid() And Fork() Programming?
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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Apr 19, 2011
For this code:
Code:
class B1
{
[code]....
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Dec 3, 2010
var param = function() { <whatever> } ;What exactly does the above MEAN, and why have they included this in the language this way?
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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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Jun 20, 2011
2 Questions:
1) Is there a dedicated forum on the net for people writing Linux drivers?
2) I've been reading over how Linux drivers are put together and even made my own dummy driver. However, before I begin writing the "real" driver I set out to write, I would like someone with some driver writing experience to verify that my knowledge is correct.
The driver I intend to write is for a PCI card. The very first thing I need is to know is the vendor ID and device ID of the card. After having that information, I can then use the pci_register_driver command to open a connection to the card. At this point, I need to use the pci_*_config_* commands (an example of a pci_*_config_* command is pci_read_config_byte) to figure out where the device is mapped in memory and what I/O ports I need. Now, the pci_* breed of commands take a parameter integer (which is an address) as there second argument. The address(es) which I will use and what they accomplish is device dependent. At this point, after having otained the I/O ports I need, I can then begin writing to them via the inb, outb, etc commands. Writing to X port will have Y result but this is vendor specific.
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Nov 20, 2010
I have been reading some books about device driver development etc.I made a char driver of bond (dummy) device. My book says that PCI devices contain three addressable regions,configuration space,IO ports,and device memory,the book talks about a file include/linux/pci_ids.h and PCI addressing etc.I read about following functions
1) pci_read_config_
2) pci_write_config_ some thing known as offset is defined to be passed on as an argument to above functions
3) IRQ number assigned to a card function pci_read_config_byte_,configuration register offsets
4) pci_request_region I want to write a pci driver for my own understanding and I am reading some books about it.
[Code]...
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Mar 30, 2010
As part of my job I often have to write queries in Oracle (10g) and I am experienced enough to see where I should be using an index where possible etc. Most of the time I just let the CBO come up with the best plan however sometimes it obviously doesn't!Now, I know in general that full scans of large tables aren't good however I don't fully understand an execution plan - ie, I could not look at a complex piece of code, look at it's plan and say "oh, the reason that is performing badly there is because the SBO has chosen to use nested loops rather than a hash join, so if I hint it then it will run quicker!" - this ultimately is where I'm trying to get to
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Jan 6, 2011
I'm reading "Understanding the Linux Kernel" and came upon this assembly instruction:
movl $(__KERNEL_CS << 16), %eax
I am curious as to what "<<" means/does. I tried to gooogle, but google doesn't search for "<<".
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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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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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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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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 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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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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Apr 9, 2010
I have a TCL framewrok for my test cases automation. This is been working for last 1 year. But yesterday it exited with exception when a function was called. The exception was given as fork: not enough memory while executing "spawn bash"(procedure "runtrigger" line 36) invoked from within "runtrigger $fnAfter $sid_l $mapver" ("trigger" arm line 6) invoked from within
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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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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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Aug 27, 2010
I am interested in learning 3D programming. The thing is, I would hate to put too much effort to learn something that doesn't have future and is dying. My favorite language at the time is Java. My goal is professional programming.
So I have several questions:
1. Should I learn JOGL or start learning C++ and do C++ openGL programming?
2. Is there a big difference between JOGL and C++ openGL programming?
3. Is it worth to learn openGL? Does it have a future?
4. Is it a big difference between openGL and directX coding?
5. If choosing Java, then JOGL or LWJGL?
Why and what is the main difference between them?
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Mar 7, 2010
Groovy is an object-oriented programming language for the Java platform. I do not have experience in Java, only perl and shell scripts. Recently I have been asked to maintain a software written in groovy (also to make enhancements). So can I learn groovy without knowing java language. or isit I have to learn java before venturing into groovy.
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May 30, 2010
I did searched you tube but my results were not great.I have 2 books on KernelProgramming.I feel I need if some where I can get a video tutorial which can help me to understand how to develop a Linux Device driver that will be great.I had a look at Greg Kroah Hartmans video lecture of developing patches on ......I have been reading books and a lot of stuff.So I wish if I could get a video lecture that would be better
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Mar 8, 2011
I have a server listening on incoming client connections. Once the client establishes SSL connection with the server, the server waits on read() from the client. Only Client can disconnect the connection. I want to have a timer in the server program to wait for x secs after read() and then disconnect the Client connection.
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Feb 9, 2011
I want that I click with the mouse on the video, it paused.I notice that there is "BaconVideoWidget" which I guess is the video rendering widget but it don't have signal named "clicked":
Code:
vd = totem_object.get_video_widget()
vd.connect("clicked", vd.hide)
[code]....
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May 16, 2011
i have a server program which accept multiple client connection and am using polling. like every 2 secs it will look to client whether any data is received after it binded. i have used setitimer but there is runtime error i got.. the server accept all client connection but doesn't execute any msg which client sent.
#include<stdio.h>
#include<stdlib.h>
#include<sys/socket.h>
#include<sys/types.h>
#include<netdb.h>
#include<unistd.h>
#include<pthread.h>
#include<string.h>
[Code]....
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Oct 27, 2010
I am learning network programming via a book of Richard Stevens.The sample source codes are given here
http://www.unpbook.com/unpv13e.tar.gz
I downloaded and unzipped the file in /usr/src folder.As per the instructions given in README of downloaded archive I did.
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Sep 28, 2010
A simple TCP based chat server could allow users to use any TCP client (telnet, for example) to communicate with each other. For this question you should consider a single process, single thread server that can support exactly 2 clients at once, the server simply forwards whatever is sent from one client to the other (in both directions). Your server must not insist on any specific ordering of messages as soon as something is sent from one client it is immediately forwarded to the other client. As soon as either client terminates the connection the server can exit
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