Unix - Pipe, forks, execlp, dup2, c program
Things to be done: Main Process create a pipe (see: man pipe()) start 2 child processes (see: man fork) wait for both of them to exit (see: man wait) exit Chid 1 redirect stdout to write end of pipe (see: man dup) run ps -aux (see: man exec) exit Child...
signal handler reading incorrect value
While there certainly is a race condition here, it's not what causing trouble. The problem is that your signal is triggered while the read() call in the child process is blocked. You can add a sufficiently long pause in the parent process to let child's read() complete: if (pid ==...
Trouble Understanding Fork Logic
Two things here: First, fork() return 0 in child process while it returns a non zero pid to the parent process. Second, short circuit of &&. So in the beginning of the first process (p0), it runs to i < 5 && !fork(). Now i = 0 and another process...
Pipes, child processes, and the scope of a child process
Standard C (before C99) needs variables to be declared at the beginning of a bloc. A function (including main) is a bloc in that sense. The switch could be a block, but a case is not, so you cannot declare new variables in a case. The real error is at...
understanding fork in c
By adding a newline character at the end of your string you are implicitly flushing the output buffer before you fork. Your other option is to explicitly flush it with fflush(stdout). Otherwise when you fork(), both processes are just spitting out whatever was still left in the buffer beforehand (in...
Linux, forked process hangs immediately
I took the sample from Adrien Descamps' link (see also comments above) and C++-ified and modified it a little: #include <thread> #include <iostream> #include <atomic> #include <unistd.h> #include <syslog.h> #include <sys/wait.h> std::atomic<bool> go(true); void syslogBlaster() { int j = 0; while(go) { for(int i = 0; i < 100; ++i)...
C++ Fork child, ask child for process list, kill a process in Linux
Compilation Errors occurred on line number:9,53,64 can be solved by using these: line 9: FILE* pipe = popen(cmd.data(), "r"); line 53: write(C2P[1], getlistOfProcesses("ps").data(), 1024); line 64: printf("%s",listOfProcesses.data()); Reason: These popen,write,printf requires char* as their arguments but you are passing them std::string. You have to use std::string.data() function instead as it...
correct output for this fork concept in C
Many of the questions you have asked can be answered by just looking into the man pages. Anyway I will try to explain them. 1) int atoi(const char *str) str -- This is the string representation of an integral number. This function atoi returns the converted integral number as an...
GITHUB: How to convert forked repository that was turned standalone back t a fork
You can fork again, add the forked repository as remote and then git push --all --force from your local clone of your "unforked" repo.
Shared Memory UNIX Segmentation Fault (fork()) in C++
c++,unix,segmentation-fault,fork,shared-memory
Check the return value of shmget (which you are storing in idShMem). It is possible for the call to fail. If it is negative then you are never successfully allocating the memory in the first place. If idShMem is negative, that is your problem. This is analogous to checking a...
Interpreting STRACE output - pipes and forks
execve("./forks", ["./forks"], [/* 55 vars */]) = 0 The shell calls execve with your executable and ./forks as argv[0]. The /* 55 vars */ are the environment variables inherited from the shell. arch_prctl(ARCH_SET_FS, 0x7f2b0e498700) = 0 Probably sets up thread local storage for the newly launched process. pipe([3, 4]) The...
Pyramid, fork, Java and sockets
java,python,sockets,fork,pyramid
Is there a simpler way of avoiding this "child process inheriting sockets" problem? Yes, just pass close_fds=True to the Popen constructor or wrapper function. This closes all fds except 0, 1, and 2.* If you need to keep some different set alive instead of 0, 1, and 2, use...
how the fork() function works?
Remember fork() forks your process, resulting in two more-or-less identical processes, the difference in each is the return value of fork() is 0 for the child, and the child's pid for the parent. Your while loop only iterates for the parent processes (it ends for the child processes since in...
pthread_atfork fails to compile when declaring -nostartfiles
c,pthreads,shared-libraries,fork,glibc
Your problem is that the outer wrapper function pthread_atfork seems to have been moved to libpthread_nonshared.a so that it can identify the library calling it, presumably so that the installed handlers can be removed if/when the library is unloaded, and it achieves this by referencing the __dso_handle symbol which is...
Why does closing a pipe take so long to terminate a child process?
Child process inherits open file descriptors. Every subsequent gzip child process inherits not only pipe file descriptors intended for communication with that particular instance but also file descriptors for pipes connected to previous child process instances. It means that stdin pipe is still open when the main process performs close...
Fork() to performe different processes
How many process have been created (I assume that we have 4 since it's 2 forks!)? Depending on the result of your forks it should be 0 to 2. Probably 2 if nothing goes wrong. There's a parent process that forks 2 children processes. Does "//in parent" mean both...
pipe can not read/write all the chars
You invoke std::strlen() on an uninitialized char array, which is your mistake. std::strlen() looks for the first occurence of a null byte in the array and returns its position. But the array is uninitialized, thus making the first occurence of the null byte undefined. Besides, you should check the return...
How to accurately kill child process after certain time limit in C?
Ignore what I said in comments: I believe you are looking for setitimer with ITIMER_VIRTUAL. You would call this in the child before execve. It can trigger a fatal signal after a certain amount of CPU time elapsed, with a resolution in microseconds, and (unlike timer_create and ualarm) is documented...
fork and exec combination not working if I take some input in child process
The child is trying to read from the terminal after its parent has died. That doesn't work because the child is no longer in the foreground process group. Ok, what's a foreground process group? The basic idea is that a process group the set of processes that are in the...
Segmentation fault 11 printing string from struct
Your main problem is that you don't quite have the right understanding of C strings. You cannot do sizeof(char_pointer). That will just give you the pointer size (4 in a 32 bit system) and not the size of the string it points to. Use strlen to get the length of...
How to fork, make significant changes, and give proper credit? [closed]
Copyright is cumulative (i.e. as long as you keep using some of the original code in the new code, the original's author still applies to that part of the code). So don't replace his copyright with yours, but just add yours next to his instead.
C Socket/Client fork(), share struct memory
c,sockets,struct,fork,shared-memory
First just a comment on your example: `printf("Dipartimento: %s", appello[0]-> dipartimento);` this space does not belong in any form ^ Note:, for the comments below, I did not have the definition of your struct member struct Esami esame[10];, so had to simplify representations of your struct in all illustrations. Next...
Why is the output for the id variable 1?
Because you are setting it equal to the result of fork() == 0 which is a logical test. Fork will succeed (return zero) inside of the forked thread. The outer thread will have the PID....
Fork() printing multiple case in switch-case
c++,linux,linux-kernel,switch-statement,fork
Question1:I don't understand why both case 0: and default: gets executed ! The case 0 is executed by the child process, where fork returns 0. The default case is executed in the parent process, where the return value of fork is the pid of the new child process. Fork,...
what happens when I exec() from a forked child in c
"foo calls fork() and now I have two, call them foo_parent and foo_child." Less confusing: you still have the process foo (parent) and now you also have a child process (foo_child). "Is foo overwritten in memory with bar?" When foo_child calls exec(), then its memory space is overwritten with that...
why is the total number of processes 2
There should be 4 processes created. You can easily verify this by placing an extra printf call after the if statement. The printf("Hello") is only run twice, because the condition is only true for two of those processes. Specifically, the root process spawns two child processes, and the second child...
Understanding how fork() system call works
This line isn't doing what you think: if(fork() != -1) { // #1 That will succeed for both the parent and the child (as long as fork is possible, which is almost always the case). You mean to test against 0 here. The parent will get 0, the child will...
fork and pipe confusion from quiz
You really should RTFM but :- fork() creates an identical copy of the current procedure running from the same line of code. The only difference between the two copies is the return code from fork(). This will be 0 if you are in the newly created copy or the process...
Record and write processes onto a file using C (linux)
Open a file for write+append. Each forked process will inherit the file descriptor. In each forked child process, write a single null byte to the file descriptor. When everything crashes, the size of the file, in bytes, will tell you how many processes were started....
How to send a variable between two C programs
c,pipe,exec,fork,argument-passing
You have serveral ways to transfer data to another program. (pipin', files, and way more. I can edit my answer later and give a more standard guide about sending data between two C programs if it's necessary) Let's look execl() for your case. We have to agree first that the...
Concurrent program in C(sequential issue)
c,linux,concurrency,fork,system-calls
This is pretty much a sequential execution indeed. The parent-process enters a loop, forks a child-process, and then it won't continue to the next loop until that child-process is done. What you could do is create a pid_t array of size argc, to store each fork()'s return value. Also create...
fork is giving weird output when trying to add variable from child to father after killing child in c
Not really orthodox but you could do this: if(pid!=0){ for(i=0;i<max/2;i++){ if(vet[i]==k){ l++; } } //printf("trovati figlio %i\n",l); wait(&j); j = WEXITSTATUS(j); }else{ for(i=max/2;i<max;i++){ if(vet[i]==k){ j++; } } return j; } ...
Fork creates a new process that is exactly the same as its parent
As others mentioned in the comments, a technique called Copy-On-Write mitigates the heavy cost of copying the entire memory space of the parent process. Copy-on-write means that memory pages are shared read-only between parent and child until either of them decides to write -- at which point the page is...
SIGINT ignored by multiple processes
c,linux,signals,fork,signal-handling
In this loop: for(;count < 3;count++){ pid = fork(); } you will create a total of 8 processes, as you are executing fork() 3 times, i.e. 1. call to fork() gives you 1*2=2 processes, 2nd call to fork() gives you 2*2=4 processes and 3rd call to fork() gives you 4*2=8...
GNU C Multi-process handling with pipes
c,pipe,multiprocessing,fork,posix
You're write logic is questionable, and the layout of your pipes is overtly complicated. Below is your code tailored down to as simple a case as I can muster. Comments are included to help yo along. I find it easiest when dealing with chained pipes (which is for all intents...
Manually set 'forked from' to GitHub project
No, there is no way to turn an existing repo into a fork. You can permanently switch to A' and abandon A, it's a one-time procedure. And there is no need to merge A into A'; you can simply push the whole A into A' instead.
How can I write to a child process stdin obtained with fork() in python?
For completeness here is another answer with a working example by Thomas Orozco: http://stackoverflow.com/a/12309286/491827
signal() in C doesn't work
It seems to be a bug here: if (pidA != 0 & pidB != 0) { The correct should be "&&" and not "&": if (pidA != 0 && pidB != 0) { Edit: I got it working, for the kill function, As Chris Dodd said, you have to pass...
C, 'fork: resource temporarily unavailable' on TCP server
It's a fork bomb because you never terminates the child, so it continues to run by looping in the while where accept() gives an error but doesn't terminates the process. So it continues to fork() and doing so forever. Modify the code like this: if (pid == 0) { close...
Drawing with ncurses, sockets and fork
The problem is that your parent process is trying to do two different things that need waiting: "barber sleeps until someone wakes him" accept() Without some kind of joint waiting, this architecture does not work. Both pieces of code sleeps waiting for a single kind of event, disregarding the other...
What happens if an MPI process crashes?
process,fork,mpi,poco-libraries,fault-tolerance
If you want collectives, you're going to have to wait for MPI-3.something (as High Performance Mark and Hristo Illev suggest) If you can live with point-to-point, and you are a patient person willing to raise a bunch of bug reports against your MPI implementation, you can try the following: disable...
The behavior of the fork() system call on Linux in this code [duplicate]
When you call fork() the operating system creates a copy of the current processes entire memory (it doesn't actually copy the memory since it can use the MMU to do this efficiently). Since stdout is buffered by default it will only print the message once a newline characters is written...
Creating non blocking pipes to child that does not block and can be reused
There are several issues to be considered. First, pipes have a finite maximum length (4096 was common in the distant past); any write to a pipe which has more data in it than that will block, as will any read from an empty pipe. These are fundamental to the way...
Avoid variable duplication using fork() in C
c,multithreading,sockets,fork,shared-memory
fork does not duplicate variables but the entire address space (by definition of fork) of the invoking process. You might want to use some shared memory, but then you should care about synchronization. Read shm_overview(7) and sem_overview(7) (you could share some memory using mmap(2) but you need to synchronize anyway)....
GDB and LLDB “swallow” status set by child process in OS X
gdb,fork,osx-yosemite,lldb,sigsegv
That's just a bug. Given it affects both gdb & lldb it is probably in CoreOS not the debuggers (though the same folks did the Mach specific layer of both debuggers so that's not a guarantee...) Anyway, please file a bug report with http://bugreporter.apple.com....
Exec after forking a process doesn't return the result before the program finishes
The problem is that you call pipe in the grandparent process. After the grandchild process (ls -a) exits, the parent process (sort -r) blocks indefinitely waiting to read more input from the pipe since some process - the grandparent - holds an open descriptor to the write end of the...
Fork and child/parent processes
It'll print ONE, followed by: For the parent process: TWO FOUR For the child process: TWO THREE The two processes are distinct with no synchronization between them. They run their due course, at their own timings. So say if the parent was faster than the child, you could get TWO...
why does fork() return all possible outputs in this combination?
I guess you are overcomplicating it. What happens after a fork(), is that you have two instances (father and son) executing, each of them performing two output operations. The two instances are independent and don't know anything about each other, so one will not wait for the other to complete...
How to create a child process with a different name
You can use the prctl system call for this. It has a horrible interface but if you get over that, using it for this task is rather straight forward. Here is a minimal example. #include <stdio.h> /* perror() */ #include <stdlib.h> /* NULL */ #include <sys/prctl.h> /* prctl(), PR_SET_NAME */...
How to take advantage of each process when using two fork()'s in C
Each process needs to know which letter it is supposed to print. That means you have to analyze the values in child1 and child2. For example, one process has two zeros; it might print D. Arguably, each process needs to know whether it is a parent or not. If it...
C pipes write/read priority
According to man 7 pipe, "If a process attempts to read from an empty pipe, then read(2) will block until data is available.". So if the read occurs before the write, it will wait until the write is done. Reciprocally, if the read occurs after the write, it's obvious it...
How to get return value from a forked / spawned process in Ruby?
ruby,process,output,fork,spawn
There are a whole bunch of ways to run commands from Ruby. The simplest for your case is to use backticks, which capture output: `sleep 10; date` # "Tue Jun 23 10:15:39 EDT 2015\n" If you want something more similar to Process.spawn, use the open3 stdlib: require 'open3' stdin, stdout,...
Active process when it should be in sleep
Comments turned into an answer with expanded information. Initial asides Your return; statement should be return 0; and your compiler should be complaining about a return with no value in a function that returns a value. Why write contorted C? What's wrong with if ((p = fork()) == 0) as...
Concurrent programming - Is it necessary to manually lock files that multiple processes will be accessing?
Short answer: you have to do it manually. There are certain guarantees on the atomicity of each write, but you'll still need to synchronize the processes to avoid interleaving writes. There are a lot of techniques for synchronizing processes. Since all of your writers are descendants of a common process,...
Printing an array in order with fork()
Here a little modification of your code to print in order. /* Create children procs */ pid = fork(); wait(); As suggested above, all you need is to use wait for the next child process to be created....
How to tell if child Node.js Process was from fork() or not?
javascript,node.js,process,fork
It's a bit of a hack, but you check if process.send exists in your application. When it was started using fork() it will exist. if (process.send === undefined) { console.log('started directly'); } else { console.log('started from fork()'); } Personally, I would probably set an environment variable in the parent and...
unable to use fork() in recursion? [closed]
You could use fork() to have a child process execute each binary search and have the child return the insertion point to the parent through a pipe (or the child's exit status if the array will always be 255 elements or smaller). That's the only thing I can see that...
Printing in SIGALRM handler
If you for whatever reason want to display something printed without end of line it will most likely help to do fflush(stdout); as stdout is buffered and normally flushes at each end of line.
I'm confused how this execvp() is handled in this sample function which uses fork() to clone a process
You can have a look at the manpage for execvp, it says: The exec() family of functions replaces the current process image with a new process image. So, what does that mean? It means, if execvp succeeds, your program wont be in memory anymore, thus it wont ever reach the...
Messing with signals, pipes and forks in C
A simple (but wasteful) option is that the initial program forks and then wait for input. The child process can update the counter. When the parent program receive "pause" it sends the signal SIGTSTP to the child. When the parent program receive "continue" it sends the signal SIGCONT to the...
fork() in C; which should be parent process which should be child process
Read Advanced Linux Programming which has an entire chapter dedicated to processes (because fork is difficult to explain); then read documentation of fork(2); fork is not about multi-threading, but about creating processes. Threads are generally created with pthread_create(3) (which is implemented above clone(2), a Linux specific syscall). Read some pthreads...
os.listdir throws Error 5 (IO Error) when in background
Apparently it turned out to be related to the print command! Everything my server sent to me using the fSay went to the fLog as well. Wrapping the print inside fLog made all problems go away. Things get really weird sometimes....
Fork and execlp not executing my program?
c,fork,parent-child,pid,child-process
I think that your program is actually working perfectly. And also, "test" is a terrible name for a command. When you are calling execlp("test",...), the kernel looks for a program named test along your PATH environment variable (that's the the p means in execlp). It will find one in /bin:...
multithreading or forking in perl
multithreading,perl,fork,forking
It's difficult to give an answer to your question, because it depends. Yes, Perl supports both forking and threading. In general, I would suggest looking at threading for data-oriented tasks, and forking for almost anything else. And so what you want to so is eminently achievable. First you need to:...
reading a value from a pipe isn't working
The pipes are actually working fine... it's scanf() that's failing. When your second child process starts, it calls: if (myJump == 0) { pause(); scanf("%d", &num1); printf("CHild 2: %d\n", num1); /* <== problem here */ myJump = 1; } ...and the printf() there leaves "CHild 2: " as the next...
Program stuck on Pipe (exec ls grep sort)
You never close pipe_fd1 on the parent, so grep and sort doen't know when to stop reading input: because the pipe read and write ends are never closed on the parent, the reader blocks waiting for more input that will never arrive. You need to close it. Also, you don't...
Multiple pipes and processes
c,operating-system,pipe,fork,child-process
You're probably thinking that some specific pipe[2] is shared by the parent and it's respective child process. That's true ... However it is also shared by all the other children processes you create along the way - and because it's opened, those other children processes also inherit it as opened....
How do I fork an external process in java
java,process,operating-system,fork
I suggest you prefer a ProcessBuilder over Runtime.exec. Also, if I understand your qestion, then you can pass the full path to the exe file to your ProcessBuilder. Something like, ProcessBuilder pb = new ProcessBuilder("C:\\Windows\\system32\\calc.exe"); pb.inheritIO(); // <-- passes IO from forked process. try { Process p = pb.start(); //...
Simple fork and piping
In process b you probably reading from wrong fd read(pipefd[1], buf,BSIZE). Replace it with read(pipefd[READ_END], buf,BSIZE).
Forking server, single file - r/w access
Use lockf() to lock the block of the file that you're writing to in each process. Since the section to lock starts at the current file offset, I think it would be best to open the file separately in each process, rather than sharing the same file handle. This way,...
Why is main called twice?
Good question! I was a little bit confused at first. When you use printf, the output is buffered. That means that things printed by printf won't be actually flushed to the console until a newline is reached, or even until the program terminates if stdout is redirected. In this case....
How do I detect if a system supports forking of processes in python?
Yes, that returns False. That's a good way to do it.
How to add library to forked cdnjs repo?
There is a file in cdnjs repo root, that is named CONTRIBUTING. All necessary information about contributing to cdnjs can be found in this file. It contains lots of guidelines you have to take care of!! To create a new folder, just enter your forked and downloaded repository and create...
Killing a forked Windows process in Perl
windows,multithreading,perl,fork,kill
The Forks::Super module provides many useful facilities for handling forked processes, as well as offering a more portable interface. This program runs notepad to edit the program's own source file and kills the child process after five seconds. If you need to pass parameters to the command then you should...
C, Some statements in fork() not executing
Forked child processes get a copy of their parent's memory (and share most open file descriptors with their parent and siblings). They do not all share the same memory, so a variable change in a child will have no effect in the parent or a sibling. To get the effect...
Stuck on wait() after fork()
It is an error for a process to call wait() when it has no uncollected children (wait() will return -1 and set errno to ECHILD in that case), so the fact that wait() hangs indicates that there is a child process that has not been collected. Once that child process...
fork() understanding, about execution and children
This program creates five descendant processes, and makes six calls to printf, of which four will be the IF(TRUE) message and two will be IF(FALSE). Here is an ASCII-art diagram of the control flow; every time it branches, both sides are executed, with the parent going straight down and the...
Memory issue sklearn pairwise_distances calculation
python,out-of-memory,fork,scikit-learn,cosine-similarity
What version of scikit-learn are you using? And does it run with n_jobs=1? The result should fit in memory, it is 8 * 42588 ** 2 / 1024 ** 3 = 13 Gb. But the data is about 2gb, and will be replicated to each core. So if you have...
C fork program behaviour explanation
Your information is basically correct. When the output goes to the terminal, the output is line buffered. When the output goes to a pipe, the output is fully buffered, so one of the processes finishes its output first. You could add fflush(stdout) after each iteration in writeloop(), or use setvbuf()...
Infinite Loop with execvp in C
c,shell,fork,infinite-loop,execvp
After getting the first input you are not clearing the input buffer. so this is the reason for the infinite loop. After entering the first input new line will be there in input buffer. After processing first input then buffer will give the \n. scanf("%[^\n]",buffer); // This scanf will not...
Process Emulation
windows,fork,posix,simulation,createprocess
In Windows, you simulate fork with CreateThread, unless it's followed by exec, in which case you ues CreateProcess. The fork system call creates a clone of your process. The exec system call loads a new program into your process, replacing the old program....
Fork() cannot be called from a Device Driver in linux
c,linux,kernel,fork,device-driver
You cannot use syscalls(2) inside the Linux kernel (because syscalls are the interface from user-land applications to kernel), and you really should not start a process inside a driver, since with a few painful exceptions (like /sbin/init of pid 1, or maybe in some particular cases /sbin/hotplug etc...) every process...
Counter not increasing inside a for using fork()
To get the effect you want, you have to control pos in the parent process. Any changes made by a child are local to the child, and are lost when the child exits. int i = 0; int w; int pos = 0; pid_t pid; char c[1]; for (i =...
redirect unnamed pipe output to log file in c
To redirect output from a pipe to a file, somebody needs to read from the read end of the pipe and write to the file's file descriptor. It can't be done merely by duping the file descriptors. For instance, say you have a pipe int filedes[2]; pipe (filedes); and a...
fork() not working as expected
Yes, you are fork bombimg... your function calls fork() twice, and then both children will eventually call the makeSiblings() recursively. When the first child, the one that calls run_ping() finishes, it does not actually finish, but it will call run_monitor(). Then it will call makeSiblings() and repeat until it explodes!...
fork() bomb explanation in terms of processes? [closed]
linux,operating-system,fork,system,virus
How does it work in terms of processes? It creates so many processes that the system is not able to create any more. Do children keep being produced and then replicating themselves? Yes, fork in the name means replication. is the only way to get out of it is...
Execute a function in a new process node
You can fork new processes using child_process.fork. It accepts a separate module, but if you must have it all contained in one file you could give your script a mode parameter: var child_process = require('child_process'); var mode = process.argv[2] ? process.argv[2] : 'default'; var modes = { default: function() {...
How to fork and create a specific number of children that perform the same task?
You need to move the call to parentProcess() outside the loop: for (i = 0; i < num_forks; i++) { child_pid = fork(); if (child_pid < 0) { perror("fork\n"); } else if (child_pid == 0) { childProcess(i, goal); } } parentProcess(); Otherwise, the parent waits for each child in turn...
Wait for all processes reach some point in program, then resume
This seems tailor-made for a semaphore. Specifically, this is easy to implement with "System V semaphores". See semget(2) and semop(2). The idea is that you obtain a semaphore in the parent, initialize its value to N, then have each child as it's "ready" decrement the value by 1. All children...