Programming :: Fork/exec Versus Posix_spawn

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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Programming :: Passing Usb_dev_handle After Fork Exec?

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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Programming :: Re-mapping Stdio For A Fork'd/exec'd Executable Doesn't Seem To Behave

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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Programming :: How To Use Globals When Using Fork

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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Programming :: Can't Run Second Child In C (Fork)

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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Programming :: Creating Multiple Processes Using Fork

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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Programming :: Why Fork Is Executing The Program Twice From Start

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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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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Programming :: Exec And Ssh : Command In Scripting?

Aug 4, 2010

I am a newbie, I am writing a script file to execute some programs.

# ! bin/ bash

clear

echo "*************************************"
echo " ACOUSTIC MODELLING - BELLHOP "
echo "*************************************"
#exec ssh automatix
code....

problem 1 : I can ssh to the required space but after the terminal prompts me for password it stops ececuting the script.

problem 2: after performing the 'exec' command for the first time. its not executing anything after that line. is there any work around for this.

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Programming :: Fork()ing Multi Child Process In C - Stuck

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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Programming :: Bash Redirection With Exec: Specify Fd With Variable?

Feb 21, 2010

Within a bash script, I'm attempting to redirect file descriptors with exec, e.g. exec 3>&1 1>&2; however, I'd like to do something like exec $FD>&1 1>&2, which doesn't work because bash tries to execute the value of $FD. Various placements of eval fail, as well. Is there a way around this, or am I stuck hard-coding the descriptor?

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Programming :: Gdb Debugging Exec From Child Process?

Jul 14, 2011

Im using gdb for debugging my application.. I was able to debug child process(when fork comes) .. and in child process we have an exec call to .... So the problem is, when the control come to exec , the exec process is executing at a time... I could not debug the exec. process... error is stack curruption due to same frame So, is there any way to debug the exec process

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Programming :: Python's Thread & Fork Support Is Compared To Perl?

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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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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Programming :: Fork() - Variable Values Between Child And Parent Process?

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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Programming :: `find . -exec' Handling Backticks In Bash

May 15, 2011

i had a problem with the find command in bash (which i deem is close enough to a promming language, if not please move this thread :P). i tried to reduce the command to the problem. i want the backticks, or $() for that matter; to be evaluated by -exec of find, not by bash. is that a caveat of find?

Code:

$ find testd -exec echo `basename {}` ; #confused me
test
test/a
test/b

[code]...

edit: i found out whats causing this. `basename {}` gets evaluated by bash before find is invoked, returns {} and `find . -exec echo {} ;" is run. now my question is, how to escape this eveluation from happening before.

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Programming :: Redirecting Stdin / Stdout And Stderr For Program Run As Fork()/execl()?

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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Programming :: Spaces Versus Tabs

Feb 24, 2010

Which do you prefer and why? Also, I remember someone telling me once that while using space indentation use either 4 spaces or 2 spaces. Don't use 3 spaces. Anyone know why they would say not to use 3 spaces?

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Programming :: Fork: Not Enough Memory While Executing "spawn Bash"

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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Programming :: Creates A Child Process With Fork And, When The Child Ends, Receives The SIGCHLD Signal And Wait For Its Termination?

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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Programming :: Keep The Bash-script Running If "exec Command" Fails

Aug 13, 2010

Is there a way to use exec, but if exec fails to go on with the script?

Example:

Code:
#!/usr/bin/env bash
exec startx
echo "Starting of X failed"

If startx fails, the echo will be seen on the screen. I tried all kind of stuff, but guess it ain't of much use to post it here. I searched the web, but searching for "exec and bash" in one sentence does give results which are not what i am looking for.

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May 6, 2011

I need to build an https client in 'C' language on linux platfrom and my app is a multithreaded one...in each thread it need to send a seperate https request to the same server..and receive response.... I used openssl library for this, i am able to send request to server. but when i am trying to read the socket (using SSL_Read) it's closing connection(returning 0 with error code 29).

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Programming :: Under Which Circumstances A Child Process Creates Another Child Process Using Fork

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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Programming :: Physical Versus Virtual Mac Address And Mac Address From NIC's Firmware?

Apr 6, 2011

I need a small shell based program that prints the mac address of physical ethernet adapter from it's firmware. I need this utility for license generation and appliance activation. I have tried several example but none of them is flawless, The easiest method I have found is to parse the output of "ifconfig" command but it has also some drawbacks.

1. Firstly program should differentiate between physical and virtual adapters. Physical means installed on board(wired or wireless) or installed additionally. Virtual adapters are those created by VPN or created by virtualization apps such as VirtualBox/VMWare etc. I am not interested in virtual ones.

2. In case of more them one physical adapters(wired and wireless), it should print the mac address and description(name & vendor) of both/all adapters.

3. If media is disconnected then also it should be able to read the mac address and description(name, vendor) of card.

4. This one is bit complex. I know that 'ethtool' can show you the universal mac address but it's limited to use only 2 types of drivers and won't work in all cases.

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Programming :: SORT Command Versus Unix SORT

May 4, 2010

We switched from unix to linux and we have an old report that extracted data from a database, output to an ascii file and then sorted the results in the file based on different arguments. The report now blows up when it runs,and I can only guess it is because the options for sort on linux differ slightly from unix.For example, here is one of the commands issued from within the report app that ran on the old unix box:

if sort-sequence = "descending" then
'sort -t~" -f +3.0f -4.0 +5.0r -6.0 -f '
else
'sort -t~" +3.0f -4.0 +1.0f -2.0 -f'

I will eventually rewrite the report to store the data in a local table, but I can simply adjust the options to suit the requirments of linux. Basically, I need to know if this can be a quick fix for the short term.

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Programming :: Atomic Variable Versus Atomic Operation

May 19, 2010

have two shared variables a,b which are related to each other.When multiple applications share these shared variables, access to them needs to be an atomic operation, otherwise the relation may break. So to ensure mutual exclusion, I'll put their modification under a critical section protected by lock.

Code:

critical_code
{
P(mutex)
a := something
b := something
V(mutex)
}

Lets say my hardware/OS/compiler supports atomic variables. Then I modified my above code as follows.

Code:

code
{
atomic a := something
atomic b := something
}

Can this code ensure mutual exclusion, when accessed by multiple applications?

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Programming :: Eclipse PDT 2.0 Versus Zend Studio For Eclipse

Jan 28, 2009

I have been writing PHP using Adobe Dreamweaver; I have been looking around for an application that focus more in PHP.I have found:

[URL]

and:

[URL]

I know Zend stands behind PHP and that Eclipse is a popular open source IDE. What is the difference tho?Is the PDT Project only a functionality that could be added to Eclipse IDE? Is Zend for Eclipse a stand alone IDE for PHP? if so, Why the Eclipse in the name? I have downloaded Zend Studio for Eclipse, so far it seems very complete (no that have learned how to use all its features).

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Programming :: Static Linking Versus Dynamic Linking

Apr 18, 2010

I have written an OpenGL based program which uses i.a. libjpeg that I want to link statically so I can send it to someone who doesn't have all the libraries needed installed to allow for a dynamically linked executable to run. As such I tried with giving the [b]-static[b] parameter to gcc.

Code:

x86_1$ make
gcc -O3 -c jpegloader.c
gcc --std=c99 -O3 -c --std=c99 -I/usr/include -L/usr/X11R6/lib main.c

[code].....

Why would gcc claim that I'm missing GLUT? This works perfectly fine when I do dynamic linking -- i.e. omitting the -static argument.

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General :: How To Use Fork In My Programme

Aug 31, 2009

1) Write a C program using the fork() system call that that generates the Fibonacci sequence in the child process. The number of the sequence will be provided in the command line. For example, if 5 is provided, the first five numbers in the Fibonacci sequence will be output by the child process. Because the parent and child processes have their own copies of the data, it will be necessary for the child to output the sequence. Have the parent invoke the wait() call to wait for the child process to complete before exiting the program. Perform necessary error checking to ensure that a non-negative number is passed on the command line.

2) Repeat the preceding exercise, this time using the CreateProcess () in the Win32 API. In this instance, you will need to specify a separate program to be invoked from CreateProcess(). It is this separate program that will run as a child process outputting the Fibonacci sequence. Perform necessary error checking to ensure that a non-negative number is passed on the command line.i have done with Fibonacci sequence .but i dont know how to include tht fork() function and win32 api .any one can help to finish?

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General :: Fork Process Will Not Exit - Ksh

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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