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.
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)
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:
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) {
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..
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.
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);}
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
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?
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
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:
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.?
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.
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.
I'm struggling with the issue of passing a vector of a class to itself, here's what state its in now... (tried many variations, but without direction).
I'm trying to work out the best way to achieve the following.
1)php page that grabs data from a local database. (not a problem)
2)It then needs to send this data to a c program/service running on a remote server. (I probably need it to be able to handle 4+ million reocrds in an array)
3) The c service then needs to process the data and send it back to the initial php script that called it. i was hoping this could be in a an array like structure of some kind. 4)update the db with the results.
I was thinking of using gsoap to write a simple c soap service that php can communicate with. Would this be the right way of doing this or would something like sockets in php be a better way of sending this volume of data as an array or struct to linux c socket if thats even possible.
i'm programming a small tcp client, but i need that the server knows the source ip. My client receives a external connection and forwards, but the ip seen by the server, of course, is the client ip. I would like, change this ip to original source ip.
I don't know how to do this. I tried with the connect and select function.
I have a file with around 1000 IP addresses in it and I need to be able to ssh into each one of them, run a single command, and then exit. I already know the ssh command I want to run and it looks like this:
(I know shpass is not good to use and keys are the correct way but I don't have any other options in this scenario.) if these ip addresses were in a .csv file, by themselves with no other information, how would I create a script to do the above command to each ip until the end of the file?
and lang.php has a link to page2.html, how do I pass on the value of the lang parameter from lang.php to page2.html? The value needed is the one passed in to lang.php.
An input filename ($1) is fed into mediainfo, which by the use of grep and cut spits out a single number which is the aspect ratio. This is then divided by bc into 320, which gives the desired height dimension for the file that I want ffmpeg to create for me. Finally, ffmpeg runs using the calculated dimensions... Basically, it's the passing of the $ASPECT variable to bc that seems to fail. It looks like bc won't read the output from the mediainfo line... It always crashes out with:
Code:
(standard_in) 1: illegal character: ^M I've tried doing something even simpler like this to debug by just trying it to display the calculation on the screen:
I want to build on the code from /etc/apt/apt.conf.d/05etckeeper to work with Snapper, the new-in btrfs (et al) snapshot package.
Code: Select allDPkg::Pre-Invoke    { "if [ -x /usr/bin/etckeeper ]; then etckeeper pre-install; fi"; }; DPkg::Post-Invoke   { "if [ -x /usr/bin/etckeeper ]; then etckeeper post-install; fi"; };
The etckeeper code will work well as a template, but I need to pass a parameter between the pre- and post- instances. The parameter is obtained from the pre- invocation and passed to the post- invocation.I know that something similar to my quest is done with the 'pid' but how to do it in the 'standard' way. Happily there can't be multiple dpkg instances running concurrently (prevented by dpkg?) so I don't have to worry about that issue.
Q1. What is the 'standard' way of passing parameters about?
Code: Select allsnapper -c etc create -t pre -p   (which 'prints' the parameter (int) to pass to the following invocation) snapper -c etc create -t post --pre-number <parameter> in place of the two etckeeper calls.
Q2. How do I pick up the 'printed' output of the 'pre' call? I think it's just a 'get' from the stream but perhaps I've missed something.?
I am trying to simulate a shell. So what I do is checking of having the parameters from standard input, suc as "/bin/ls -l /home/france/Documents", and then passing them to function execute, which at some point calls execvp(argv[0],argv)The problem is that I don't succeed in using these arguments, while if I call execvp(paramList[0],paramList) it works!!!! Where paramList is exactly what I would put on standard input, but defined statically.
I started to run a C++ my program in Linux and I don't know how to test my program that works fine with a text file. This is a project for my uni and to explain more clear, My program is a sample database for modify or add and search and it read data from a binary file and also write into a binary file. I have got an input from my lecture and she said after compiling in Linux I must write:
1-to compile: Code: g++ (files with cpp) 2-to see the output: Code: ./a.out 3-to test with input: Code: ./a.out < input.txt >
My problem is in step three. What I must write in the command part after" step2" to see the output?
