So I have a simple fork and exec program. It works pretty good but I want to be able to detach the process that is started, I try a fork with no wait:
if((pid = fork()) < 0)
perror("Error with Fork()");
else if(pid > 0) {
return "";
}
else {
if(execl("/bin/bash", "/bin/bash", "-c", cmddo, (char*) 0) < 0) perror("execl()");
exit(0);
}
It starts the proc fine but when my main app is closed - so is my forked proc.
How do I keep the forked process running after the main proc (that started it) closes?
Thanks :D
Various things to do if you want to start a detached/daemon process:
fork again and exit the first child (so the second child process no longer has the original process as its parent pid)
call setsid(2) to get a new session and process group
reopen stdin/stdout/stderr to dereference the controlling tty, if there was one. Or, for example, you might have inherited a pipe stdout that will be broken and give you SIGPIPE if you try to write it.
chdir to / to get away from the ancestor's current directory
Probably all you really want is to ignore SIGHUP in your fork()ed process as this is normally the one which brings the program down. That is, what you need to do is
signal(SIGHUP, SIG_IGN);
Using nohup arranges for a reader to be present which would avoid possibly writing to close pipe. To avoid this you could either arrange for standard outputs not to be available or to also ignore SIGPIPE. There are a number of signals which terminate your program when not ignore (see man signal; some signals can't be ignored) but the one which will be sent to the child is is SIGHUP.
Related
Im trying to emulate shell through C program. In my program whenever I run any normal (foreground) commands it works fine. Also I have handled background process with commands ending with '&'. Now to handle this I have avoided the parent waiting for a child process.
The problem is whenever for the first time in my shell I run any background command(i.e ending in '&') then it works fine. But then after that each command(normal) doesnot terminate. I guess it waits for the previously opened process. How to rectify. Please you can ask questions so that i can make myself more clear to you. This is the snippet which is doing the above mentioned task.
child_id=fork();
if(child_id==0){
//logic fo creating command
int ret=execvp(subcomm[0],subcomm);
}
//Child will never come here if execvp executed successfully
if(proc_sate!='&'){
for(i=0;i<count_pipe+1;i++){
waitpid(0,&flag,0);
}
//something to add to make it not wait for other process in my scenario for second time
}
Here proc_state just determines whether it is background or foreground.It is just a character. count_pipe is just a variable holding number of pipes (e.g ls -l|wc|wc this contains 2 pipes). Dont worry this all is working fine.
waitpid(0, &flag, 0) waits for any child process whose process group ID is equal to that of your shell. So if you have not called setsid() after the fork() of the disconnected child process, the code above will wait for that too.
pid_t pid = fork();
if (pid == 0) { /* child process */
setsid(); /* Child creates new process group */
... /* redirections, etc */
execvp(...);
}
I am creating a pipe using popen() and the process is invoking a third party tool which in some rare cases I need to terminate.
::popen(thirdPartyCommand.c_str(), "w");
If I just throw an exception and unwind the stack, my unwind attempts to call pclose() on the third party process whose results I no longer need. However, pclose() never returns as it blocks with the following stack trace on Centos 4:
#0 0xffffe410 in __kernel_vsyscall ()
#1 0x00807dc3 in __waitpid_nocancel () from /lib/libc.so.6
#2 0x007d0abe in _IO_proc_close##GLIBC_2.1 () from /lib/libc.so.6
#3 0x007daf38 in _IO_new_file_close_it () from /lib/libc.so.6
#4 0x007cec6e in fclose##GLIBC_2.1 () from /lib/libc.so.6
#5 0x007d6cfd in pclose##GLIBC_2.1 () from /lib/libc.so.6
Is there any way to force the call to pclose() to be successful before calling it so I can programmatically avoid this situation of my process getting hung up waiting for pclose() to succeed when it never will because I've stopped supplying input to the popen()ed process and wish to throw away its work?
Should I write an end of file somehow to the popen()ed file descriptor before trying to close it?
Note that the third party software is forking itself. At the point where pclose() has hung, there are four processes, one of which is defunct:
USER PID %CPU %MEM VSZ RSS TTY STAT START TIME COMMAND
abc 6870 0.0 0.0 8696 972 ? S 04:39 0:00 sh -c /usr/local/bin/third_party /home/arg1 /home/arg2 2>&1
abc 6871 0.0 0.0 10172 4296 ? S 04:39 0:00 /usr/local/bin/third_party /home/arg1 /home/arg2
abc 6874 99.8 0.0 10180 1604 ? R 04:39 141:44 /usr/local/bin/third_party /home/arg1 /home/arg2
abc 6875 0.0 0.0 0 0 ? Z 04:39 0:00 [third_party] <defunct>
I see two solutions here:
The neat one: you fork(), pipe() and execve() (or anything in the exec family of course...) "manually", then it is going to be up to you to decide if you want to let your children become zombies or not. (i.e. to wait() for them or not)
The ugly one: if you're sure you only have one of this child process running at any given time, you could use sysctl() to check if there is any process running with this name before you call pclose()... yuk.
I strongly advise the neat way here, or you could just ask whomever responsible to fix that infinite loop in your third party tool haha.
Good luck!
EDIT:
For you first question: I don't know. Doing some researches on how to find processes by name using sysctl() shoud tell you what you need to know, I myself have never pushed it this far.
For your second and third question: popen() is basically a wrapper to fork() + pipe() + dup2() + execl().
fork() duplicates the process, execl() replaces the duplicated process' image with a new one, pipe() handles inter process communication and dup2() is used to redirect the output... And then pclose() will wait() for the duplicated process to die, which is why we're here.
If you want to know more, you should check this answer where I've recently explained how to perform a simple fork with standard IPC. In this case, it's just a bit more complicated as you have to use dup2() to redirect the standard output to your pipe.
You should also take a look at popen()/pclose() source codes, as they are of course open source.
Finally, here's a brief example, I cannot make it clearer than that:
int pipefd[2];
pipe(pipefd);
if (fork() == 0) // I'm the child
{
close(pipefd[0]); // I'm not going to read from this pipe
dup2(pipefd[1], 1); // redirect standard output to the pipe
close(pipefd[1]); // it has been duplicated, close it as we don't need it anymore
execve()/execl()/execsomething()... // execute the program you want
}
else // I'm the parent
{
close(pipefd[1]); // I'm not going to write to this pipe
while (read(pipefd[0], &buf, 1) > 0) // read while EOF
write(1, &buf, 1);
close(pipefd[1]); // cleaning
}
And as always, remember to read the man pages and to check all your return values.
Again, good luck!
Another solution is to kill all your children. If you know that the only child processes you have are processes that get started when you do popen(), then it's easy enough. Otherwise you may need some more work or use the fork() + execve() combo, in which case you will know the first child's PID.
Whenever you run a child process, it's PPID (parent process ID) is your own PID. It is easy enough to read the list of currently running processes and gather those that have their PPID = getpid(). Repeat the loop looking for processes that have their PPID equal to one of your children's PID. In the end you build a whole tree of child processes.
Since you child processes may end up creating other child processes, to make it safe, you will want to block those processes by sending a SIGSTOP. That way they will stop creating new children. As far as I know, you can't prevent the SIGSTOP from doing its deed.
The process is therefore:
function kill_all_children()
{
std::vector<pid_t> me_and_children;
me_and_children.push_back(getpid());
bool found_child = false;
do
{
found_child = false;
std::vector<process> processes(get_processes());
for(auto p : processes)
{
// i.e. if I'm the child of any one of those processes
if(std::find(me_and_children.begin(),
me_and_children.end(),
p.ppid()))
{
kill(p.pid(), SIGSTOP);
me_and_children.push_back(p.pid());
found_child = true;
}
}
}
while(found_child);
for(auto c : me_and_children)
{
// ignore ourselves
if(c == getpid())
{
continue;
}
kill(c, SIGTERM);
kill(c, SIGCONT); // make sure it continues now
}
}
This is probably not the best way to close your pipe, though, since you probably need to let the command time to handle your data. So what you want is execute that code only after a timeout. So your regular code could look something like this:
void send_data(...)
{
signal(SIGALRM, handle_alarm);
f = popen("command", "w");
// do some work...
alarm(60); // give it a minute
pclose(f);
alarm(0); // remove alarm
}
void handle_alarm()
{
kill_all_children();
}
-- about the alarm(60);, the location is up to you, it could also be placed before the popen() if you're afraid that the popen() or the work after it could also fail (i.e. I've had problems where the pipe fills up and I don't even reach the pclose() because then the child process loops forever.)
Note that the alarm() may not be the best idea in the world. You may prefer using a thread with a sleep made of a poll() or select() on an fd which you can wake up as required. That way the thread would call the kill_all_children() function after the sleep, but you can send it a message to wake it up early and let it know that the pclose() happened as expected.
Note: I left the implementation of the get_processes() out of this answer. You can read that from /proc or with the libprocps library. I have such an implementation in my snapwebsites project. It's called process_list. You could just reap off that class.
I'm using popen() to invoke a child process which doesn't need any stdin or stdout, it just runs for a short time to do its work, then it stops all by itself. Arguably, invoking this type of child process should rather be done with system() ? Anyway, pclose() is used afterwards to verify that the child process exited cleanly.
Under certain conditions, this child process keeps on running indefinitely. pclose() blocks forever, so then my parent process is also stuck. CPU usage runs to 100%, other executables get starved, and my whole embedded system crumbles. I came here looking for solutions.
Solution 1 by #cmc : decomposing popen() into fork(), pipe(), dup2() and execl().
It might just be a matter of personal taste, but I'm reluctant to rewrite perfectly fine system calls myself. I would just end up introducing new bugs.
Solution 2 by #cmc : verifying that the child process actually exists with sysctl(), to make sure that pclose() will return successfully. I find that this somehow sidesteps the problem from the OP #WilliamKF - there is definitely a child process, it just has become unresponsive. Forgoing the pclose() call won't solve that. [As an aside, in the 7 years since #cmc wrote this answer, sysctl() seems to have become deprecated.]
Solution 3 by #Alexis Wilke : killing the child process. I like this approach best. It basically automates what I did when I stepped in manually to resuscitate my dying embedded system. The problem with my stubborn adherence to popen(), is that I get no PID from the child process. I have been trying in vain with
waitid(P_PGID, getpgrp(), &child_info, WNOHANG);
but all I get on my Debian Linux 4.19 system is EINVAL.
So here's what I cobbled together. I'm searching for the child process by name; I can afford to take a few shortcuts, as I'm sure there will only be one process with this name. Ironically, commandline utility ps is invoked by yet another popen(). This won't win any elegance prizes, but at least my embedded system stays afloat now.
FILE* child = popen("child", "r");
if (child)
{
int nr_loops;
int child_pid;
for (nr_loops=10; nr_loops; nr_loops--)
{
FILE* ps = popen("ps | grep child | grep -v grep | grep -v \"sh -c \" | sed \'s/^ *//\' | sed \'s/ .*$//\'", "r");
child_pid = 0;
int found = fscanf(ps, "%d", &child_pid);
pclose(ps);
if (found != 1)
// The child process is no longer running, no risk of blocking pclose()
break;
syslog(LOG_WARNING, "child running PID %d", child_pid);
usleep(1000000); // 1 second
}
if (!nr_loops)
{
// Time to kill this runaway child
syslog(LOG_ERR, "killing PID %d", child_pid);
kill(child_pid, SIGTERM);
}
pclose(child); // Even after it had to be killed
} /* if (child) */
I learned in the hard way, that I have to pair every popen() with a pclose(), otherwise I pile up the zombie processes. I find it remarkable that this is needed after a direct kill; I figure that's because according to the manpage, popen() actually launches sh -c with the child process in it, and it's this surrounding sh that becomes a zombie.
Very strange bug, perhaps someone will see something I'm missing.
I have a C++ program which forks off a bash shell, and then passes commands to it.
Periodically, the commands will contain nonsense and the bash process will hang. I detect this using semtimedwait, and then run a little function like this:
if (kill(*bash_pid, SIGKILL)) {
cerr << "Error sending SIGKILL to the bash process!" << endl;
exit(1);
} else {
// collect exit status
long counter = 0;
do {
pid = waitpid(*bash_pid, &status, WNOHANG);
if (pid == 0) { // status not available yet
sleep(1);
}
if(counter++ > 5){
cerr << "ERROR: Bash child process ignored SIGKILL >5 sec!" << endl;
}
} while (pid != *bash_pid && pid != -1);
if(pid == -1){
cerr << "Failed to clean up zombie bash process!" << endl;
exit(1);
}
// re-initialized bash process
*bash_pid = init_bash();
}
Assuming I understand the workings of waitpid correctly, this should first send SIGKILL to the shell, and then essentially sit in a spinlock, trying to reap the resulting process. Eventually, it succeeds and then a new bash process is started with init_bash().
At least, that's what should happen. Instead, the child process's exit status is never collected, and it continues to exist as a zombie process. In spite of this, the parent does exit the loop and manages to restart the bash process, and continues with normal execution. Eventually too many zombies are generated and the system runs out of pids.
Additionally:
Fork is called in exactly one place in the program, inside init_bash.
Checks prevent init_bash from being called except once at the program's start and after a call to the function above.
Thoughts?
Articles that I read indicate that the reason for a zombie process is that a child process does an exit however the parent never collects the child's exit.
This article provides several ways to kill a zombie process from the command line. One technique is to use other signals besides SIGKILL for instance SIGTERM.
This article has an answer which suggests SIGKILL should not be used.
One of the techniques is to kill the parent thereby also killing its child processes including any zombies. The author indicates that there appear to be child processes that just remain as zombies until the OS is restarted.
You do not mention the mechanism used to communicate the commands to the child process. However one option may be to turn the child process loose by disconnecting it from its parent similar to the way a child of a terminal process can be disconnected from the terminal session. That way the child will become its own process and if there is a problem may exit without becoming a zombie.
Looking to fork a process, in c++, that wont hang its parent process - its parent is a daemon and must remain running. If i wait() on the forked process the forked execl wont defunt - but - it will also hang the app - not waiting fixes the app hang - but the command becomes defunt.
if((pid = fork()) < 0)
perror("Error with Fork()");
else if(pid > 0) {
//wait here will hang the execl in the parent
//dont wait will defunt the execl command
//---- wait(&pid);
return "";
} else {
struct rlimit rl;
int i;
if (rl.rlim_max == RLIM_INFINITY)
rl.rlim_max = 1024;
for (i = 0; (unsigned) i < rl.rlim_max; i++)
close(i);
if(execl("/bin/bash", "/bin/bash", "-c", "whoami", (char*) 0) < 0) perror("execl()");
exit(0);
}
How can I fork the execl without a wait(&pid) where execl's command wont defunct?
UPDATE
Fixed by adding the following before the fork
signal(SIGCHLD, SIG_IGN);
Still working with my limited skills at a more compatible solution based on the accepted answer. Thanks!
By default, wait and friends wait until a process has exited, then reap it. You can call waitpid with the WNOHANG to return immediately if no child has exited.
The defunct/"zombie" process will sit around until you wait on it. So if you run it in the background, you must arrange to reap it eventually by any of several ways:
try waitpid with WNOHANG routinely: int pid = waitpid(-1, &status, WNOHANG)
install a signal handler for SIGCHLD to be notified when it exits
Additionally, under POSIX.1-2001, you can use sigaction set the SA_NOCLDWAIT on SIGCHLD. Or set its action to SIG_IGN. Older systems (including Linux 2.4.x, but not 2.6.x or 3.x) don't support this.
Check your system manpages, or alternative the wait in the Single Unix Specification. The Single Unix Spec also gives some helpful code examples. SA_NOCLDWAIT is documented in sigaction.
I think a signal handler would be the best way as indicated. I would like to point out another way this could be handled: Fork twice and have the child exit while the grandchild would call execl. The defunct process would then be cleaned up by the init process.
As said in comment, double fork saves process from defunct state.
What is the reason for performing a double fork when creating a daemon?
is there a way for a forked child to examine another forked child so that, if the other forked child takes more time than usual to perform its chores, the first child may perform predefined steps?
if so, sample code will be greatly appreciated.
Yes. Simply fork the process to be watched, from the process to watch it.
if (fork() == 0) {
// we are the watcher
pid_t watchee_pid = fork();
if (watchee_pid != 0) {
// wait and/or handle timeout
int status;
waitpid(watchee_pid, &status, WNOHANG);
} else {
// we're being watched. do stuff
}
} else {
// original process
}
To emphasise: There are 3 processes. The original, the watcher process (that handles timeout etc.) and the actual watched process.
To do this, you'll need to use some form of IPC, and named shared memory segments makes perfect sense here. Your first child could read a value in a named segment which the other child will set once it has completed it's work. Your first child could set a time out and once that time out expires, check for the value - if the value is not set, then do what you need to do.
The code can vary greatly depending on C or C++, you need to select which. If C++, you can use boost::interprocess for this - which has lots of examples of shared memory usage. If C, then you'll have to put this together using native calls for your OS - again this should be fairly straightforward - start at shmget()
This is some orientative code that could help you to solve the problem in a Linux environment.
pid_t pid = fork();
if (pid == -1) {
printf("fork: %s", strerror(errno));
exit(1);
} else if (pid > 0) {
/* parent process */
int i = 0;
int secs = 60; /* 60 secs for the process to finish */
while(1) {
/* check if process with pid exists */
if (exist(pid) && i > secs) {
/* do something accordingly */
}
sleep(1);
i++;
}
} else {
/* child process */
/* child logic here */
exit(0);
}
... those 60 seconds are not very strict. you could better use a timer if you want more strict timing measurement. But if your system doesn't need critical real time processing should be just fine like this.
exist(pid) refers to a function that you should have code that looks into proc/pid where pid is the process id of the child process.
Optionally, you can implement the function exist(pid) using other libraries designed to extract information from the /proc directory like procps
The only processes you can wait on are your own direct child processes - not siblings, not your parent, not grandchildren, etc. Depending on your program's needs, Matt's solution may work for you. If not, here are some other alternatives:
Forget about waiting and use another form of IPC. For robustness, it needs to be something where unexpected termination of the process you're waiting on results in your receiving an event. The best one I can think of is opening a pipe which both processes share, and giving the writing end of the pipe to the process you want to wait for (make sure no other processes keep the writing end open!). When the process holding the writing end terminates, it will be closed, and the reading end will then indicate EOF (read will block on it until the writing end is closed, then return a zero-length read).
Forget about IPC and use threads. One advantage of threads is that the atomicity of a "process" is preserved. It's impossible for individual threads to be killed or otherwise terminate outside of the control of your program, so you don't have to worry about race conditions with process ids and shared resource allocation in the system-global namespace (IPC objects, filenames, sockets, etc.). All synchronization primitives exist purely within your process's address space.