Please consider:
scripttmp=$(mktemp -d)
cleanup() {
rm -rf "${scripttmp}"
}
trap cleanup EXIT
I understand cleanup is a call(ed)back function, as it is being called just before exiting from the main function, from which it is part (I grasp the main function as a function in the general sense even though there is no function syntax around its code).
If I never actually called cleanup before --- I don't really "call it back"; I just call it before exiting, but not "back".
Is the term trap more accurate than the generic "callback" term in programming?
"Callback" comes from the fact that you give a routine a piece of code for later execution (for whenever some condition is fulfilled), and the routine "calls back" by executing that code.
Compare this with giving someone your phone number for when they need it. When they need it, they call you back. At that point, they may never have called you before. The process of "calling" is the callback.
In a shell script, trap is used to install a callback that will be evaluated when a signal is "caught" or "trapped". A standard shell also allows trapping a special event, namely when the shell exits (EXIT), and the bash shell additionally supports trapping errors (ERR), exit from a function (RETURN), and every simple command (DEBUG).
The trap utility does not allow for calling a callback function for generic asynchronous events.
In your example, cleanup could be called a callback function. It is installed with trap and will execute just before the current shell exits. In other words, the trap utility installs an EXIT trap that will call the callback function cleanup when the EXIT event is caught.
The code installed by trap action event will be executed in a manner equivalent to eval action when the given event occurs. The action could therefore be any shell code, not necessarily just a function call.
Another word for your cleanup function would be a "handler", a routine that handles something (in this case, handling the termination of the script), and possibly more specifically "an EXIT handler". If it was used to handle a caught signal, it would be "the signal handler for that particular signal". It is also common to call this function a "trap handler" (a handler installed by trap) or just "trap", although this is not "more accurate".
The term "call back" does not imply anyhow that it has to be called again (it's NOT called callagain or such).
It just means that the caller tells the callee how to notify him at some defined condition (so how to call him back then) - or which action to take at that condition (in behalf, the execution of the action goes back to the caller as the caller initiated that action through handing it over to the calle as the callback).
So the wording callback is very clear and consistent.
So for #Kusalananda example there are different possibilities.
Normally a single (or a single batch of) callback(s) is handed over to the callee (the routine, someone). It is well defined which type(s) of information(s) the callee will supply to the callback (so give it as arguments to the routine or tell it on phone to the recipient of the call). Possibly it's no information at all, just a call to the routine or a telephone call (so when the callback is defined to have no arguments or optional arguments). But anyhow normally the caller has some intention to hand the callback over to the callee, so the defined callback-routine or the one whos phone is ringing (it can also be a computer that gets the incoming call) is doing something the caller initiated when he gave that callback to the callee (so the action goes back to him, whether the target can identify the caller or not). Of course a callaback can be defined in a way that the callee that calls the callback always hands over some information to the callback-routine or to the someone answering the phone, it can be some detailed information about the condition, some information gathered by the calle in between or even a reference to the original caller.
On the other hand trap is a very specific word and not at all similiar or interchangeable with callback. It means about what the official translation/definition of the spoken word trap is. A trap is not called by the callee and is not handed over to a callee. A callee can be trapped. In Your example above the calee is just the remaining procedure following after installing the trap (up to the point where the trap is uninstalled or deactivated). A trap somehow is the opposite of a callback as the callee (or the trap insatlled by callee) traps some condition but from outside of the possible calees context.
Simple example, which doesn't work on my platform (Ruby 2.2, Cygwin):
#!/usr/bin/ruby
backtt = fork { exec('mintty','/usr/bin/zsh','-i') }
Process.detach(backtt)
exit
This tiny program (when started from the shell) is supposed to span a terminal window (mintty) and then get me back to the shell prompt.
However, while it DOES create the mintty window, I don't have a shell prompt afterwards, and I can't type anything in the calling shell.
But when I introduce a small delay before the detach, either using 'sleep', or by printing something on stdout, it works as expected:
#!/usr/bin/ruby
backtt = fork { exec('mintty','/usr/bin/zsh','-i') }
sleep 1
Process.detach(backtt)
exit
Why is this necessary?
BTW, I'm well aware that I could (from the shell) do a
mintty /usr/bin/zsh -i &
directly, or I could use system(...... &) from inside Ruby, but this is not the point here. I'm particularily interested in the fork/exec/detach behaviour in Ruby. Any insights?
Posting as an answer, because it is too long for a comment
Although I am no specialist in Ruby, and do not know Cygwin at all, this situation sounds very familiar to me, coming from C/C++.
This script is too short, so the parent of the parent completes, while the grandchild tries to start.
What would happen if you put the sleep after detach and before exit?
If my theory is correct, it should work too. Your program exits before any (or enough) thread-switching happens.
I call such problems "interrupted hand shaking". Although this is psychology terminology, it describes what happens.
Sleep "gives up the time slice", leading to thread-switching,
Console output (any file I/O) runs into semaphores, also leading to thread switching.
If my idea is correct, it should also work, if you dont "sleep", just count to 1e9 (depending on the speed of computation) because then preemptive multitasking makes even the thread-switch itself not giving up the CPU.
So it is an error in programming (IMHO: race condition is philosophical in that case), but it will get hard to find "who" is responsible. There are many things involved.
According to the documentation:
Process::detach prevents this by setting up a separate Ruby thread whose sole job is to reap the status of the process pid when it terminates.
NB: I can’t reproduce this behaviour on any of available to me operating systems, and I’m posting this as an answer just for the sake of formatting.
Since Process.detach(backtt) transparently creates a thread, I would suggest you to try:
#!/usr/bin/ruby
backtt = fork { exec('mintty','/usr/bin/zsh','-i') }
# ⇓⇓⇓⇓⇓
Process.detach(backtt).join
exit
This is no hack by any mean (as opposite to silly sleep,) since you are likely aware of that the underlying command should return more-or-less immediately. I am not a guru in cygwin, but it might have some specific issues with threads, so, let this thread to be handled.
I'm neither a Ruby nor a Cygwin guy, so what I propose here may not work at all. Anyways: I guess, you're not even hitting a Ruby or Cygwin specific bug here. In a program called "start" I've written in C many years ago, I hit the same issue. Here is a comment from the start of the function void daemonize_now():
/*
* This is a little bit trickier than I expected: If we simply call
* setsid(), it may fail! We have to fork() and exit(), and let our
* child call setsid().
*
* Now the problem: If we fork() and exit() immediatelly, our child
* will be killed before it ever had been run. So we need to sleep a
* little bit. Now the question: How long? I don't know an answer. So
* let us being killed by our child :-)
*/
So, he strategy is this: Let the parent wait on it's child (that can be done immediately before the child actually had a chance to do anything) and then let the child do the detaching part. How? Let it create a new process group (it will be reparented to the init process). That's the setsid() call for, I'm talking about in the comment. It will work something like this (C-Syntax, you should be able to lookup the correct usage for Ruby and apply the needed changes yourself):
parentspid = getpid();
Fork = fork();
if (Fork) {
if (Fork == -1) { // fork() failed
handle error
} else { // parent, Fork is the pid of the child
int tmp; waitpid(0, &tmp, 0);
}
} else { // child
if (setsid() == -1) {
handle error - possibly by doing nothing
and just let the parent wait ...
} else {
kill(parentspid, SIGUSR1);
}
exec(...);
}
You can use any signal, that terminates the process (i.e. SIGKILL). I used SIGUSR1 and installed a signal handler that exit(0)s the parent process, so the caller gets a success message. Only caveat: You get a success even if the exec fails. However, that is a problem that can't really be worked around, since after a successful exec you can't signal your parent anything anymore. And since you don't know when the exec will have failed (if it fails), you're back at the race condition part.
How to execute xterm from XWindow program, insert it into my window, but continue execution both while xterm is active and after it was closed?
In my XWindows (XLib over XCB) application I want to execute xterm -Into <handle>. So that my window contains xterm window in it. Unfortunately something wrong is happening.
pseudo code:
if (fork() == 0) {
pipe = popen('xterm -Into ' + handle);
while (feof(pipe)) gets(pipe);
exit(0);
}
I tired system() and execvp() as well. Every thing is fine until I exit from bash that runs in xterm, then my program exits. I guess that connection to X server is lost because it is shared between parent and child.
UPDATE: here is what is shown on terminal after program exits (or rather crashes).
XIO: fatal IO error 11 (Resource temporarily unavailable) on X server ":0.0"
after 59 requests (59 known processed) with 1 events remaining.
[xcb] Unknown sequence number while processing queue
[xcb] Most likely this is a multi-threaded client and XInitThreads has not been called
[xcb] Aborting, sorry about that.
y: ../../src/xcb_io.c:274: poll_for_event: Assertion `!xcb_xlib_threads_sequence_lost' failed.
Aborted
One possibility is that you are terminating due to the SIGCHLD signal not
being ignored and causing your program to abort.
signal(SIGCHLD, SIG_IGN);
Another is, as you suspect something actively closing the X session. Just
closing the socket itself should not matter but are you using a library that
registers an atexit call it could cause an issue.
Since from your snippet,
it looks like you don't actually care about the stdout of the xterm, a
better way to do it would be to actuall close fd's 0,1,2. Also since it looks
like you don't need to do anything in the child process after xterm
terminates you can use 'exec' rather than 'popen' to fully replace the
child process with that of the xterm including any cleanup handlers that
were left around. Though, I am not sure how pruned your snippet is from what you want to do as obviously the call to 'gets' is not what you want.
to make sure the X connection is closed, you can set its close on exec flag
with the following. (this will work on POSIX systems where the x connection
number is the fd of the server socket)
fcntl(XConnectionNumber(display), F_SETFD, fcntl(XConnectionNumber(display), F_GETFD) | FD_CLOEXEC);
Also note that 'popen' itself forks in the background in addition to your fork, I think you probably want to do an execvp there then use waitpid(... , WNOHANG) to check for the childs termination in your main X11 loop if you care to know when it exited.
What's the best way for a running C or C++ program that's been launched from the command line to put itself into the background, equivalent to if the user had launched from the unix shell with '&' at the end of the command? (But the user didn't.) It's a GUI app and doesn't need any shell I/O, so there's no reason to tie up the shell after launch. But I want a shell command launch to be auto-backgrounded without the '&' (or on Windows).
Ideally, I want a solution that would work on any of Linux, OS X, and Windows. (Or separate solutions that I can select with #ifdef.) It's ok to assume that this should be done right at the beginning of execution, as opposed to somewhere in the middle.
One solution is to have the main program be a script that launches the real binary, carefully putting it into the background. But it seems unsatisfying to need these coupled shell/binary pairs.
Another solution is to immediately launch another executed version (with 'system' or CreateProcess), with the same command line arguments, but putting the child in the background and then having the parent exit. But this seems clunky compared to the process putting itself into background.
Edited after a few answers: Yes, a fork() (or system(), or CreateProcess on Windows) is one way to sort of do this, that I hinted at in my original question. But all of these solutions make a SECOND process that is backgrounded, and then terminate the original process. I was wondering if there was a way to put the EXISTING process into the background. One difference is that if the app was launched from a script that recorded its process id (perhaps for later killing or other purpose), the newly forked or created process will have a different id and so will not be controllable by any launching script, if you see what I'm getting at.
Edit #2:
fork() isn't a good solution for OS X, where the man page for 'fork' says that it's unsafe if certain frameworks or libraries are being used. I tried it, and my app complains loudly at runtime: "The process has forked and you cannot use this CoreFoundation functionality safely. You MUST exec()."
I was intrigued by daemon(), but when I tried it on OS X, it gave the same error message, so I assume that it's just a fancy wrapper for fork() and has the same restrictions.
Excuse the OS X centrism, it just happens to be the system in front of me at the moment. But I am indeed looking for a solution to all three platforms.
My advice: don't do this, at least not under Linux/UNIX.
GUI programs under Linux/UNIX traditionally do not auto-background themselves. While this may occasionally be annoying to newbies, it has a number of advantages:
Makes it easy to capture standard error in case of core dumps / other problems that need debugging.
Makes it easy for a shell script to run the program and wait until it's completed.
Makes it easy for a shell script to run the program in the background and get its process id:
gui-program &
pid=$!
# do something with $pid later, such as check if the program is still running
If your program forks itself, this behavior will break.
"Scriptability" is useful in so many unexpected circumstances, even with GUI programs, that I would hesitate to explicitly break these behaviors.
Windows is another story. AFAIK, Windows programs automatically run in the background--even when invoked from a command shell--unless they explicitly request access to the command window.
On Linux, daemon() is what you're looking for, if I understand you correctly.
The way it's typically done on Unix-like OSes is to fork() at the beginning and exit from the parent. This won't work on Windows, but is much more elegant than launching another process where forking exists.
Three things need doing,
fork
setsid
redirect STDIN, STDOUT and STDERR to /dev/null
This applies to POSIX systems (all the ones you mention claim to be POSIX (but Windows stops at the claiming bit))
On UNIX, you need to fork twice in a row and let the parent die.
A process cannot put itself into the background, because it isn't the one in charge of background vs. foreground. That would be the shell, which is waiting for process exit. If you launch a process with an ampersand "&" at the end, then the shell does not wait for process exit.
But the only way the process can escape the shell is to fork off another child and then let its original self exit back to the waiting shell.
From the shell, you can background a process with Control-Z, then type "bg".
Backgrounding a process is a shell function, not an OS function.
If you want an app to start in the background, the typical trick is to write a shell script to launch it that launches it in the background.
#! /bin/sh
/path/to/myGuiApplication &
To followup on your edited question:
I was wondering if there was a way to put the EXISTING process into the background.
In a Unix-like OS, there really is not a way to do this that I know of. The shell is blocked because it is executing one of the variants of a wait() call, waiting for the child process to exit. There is not a way for the child process to remain running but somehow cause the shell's wait() to return with a "please stop watching me" status. The reason you have the child fork and exit the original is so the shell will return from wait().
Here is some pseudocode for Linux/UNIX:
initialization_code()
if(failure) exit(1)
if( fork() > 0 ) exit(0)
setsid()
setup_signal_handlers()
for(fd=0; fd<NOFILE; fd++) close(fd)
open("/dev/null", O_RDONLY)
open("/dev/null", O_WRONLY)
open("/dev/null", o_WRONLY)
chdir("/")
And congratulations, your program continues as an independent "daemonized" process without a controlling TTY and without any standard input or output.
Now, in Windows you simply build your program as a Win32 application with WinMain() instead of main(), and it runs without a console automatically. If you want to run as a service, you'll have to look that up because I've never written one and I don't really know how they work.
You edited your question, but you may still be missing the point that your question is a syntax error of sorts -- if the process wasn't put in the background to begin with and you want the PID to stay the same, you can't ignore the fact that the program which started the process is waiting on that PID and that is pretty much the definition of being in the foreground.
I think you need to think about why you want to both put something in the background and keep the PID the same. I suggest you probably don't need both of those constraints.
As others mentioned, fork() is how to do it on *nix. You can get fork() on Windows by using MingW or Cygwin libraries. But those will require you to switch to using GCC as your compiler.
In pure Windows world, you'd use CreateProcess (or one of its derivatives CreateProcessAsUser, CreateProcessWithLogonW).
The simplest form of backgrounding is:
if (fork() != 0) exit(0);
In Unix, if you want to background an disassociate from the tty completely, you would do:
Close all descriptors which may access a tty (usually 0, 1, and 2).
if (fork() != 0) exit(0);
setpgroup(0,getpid()); /* Might be necessary to prevent a SIGHUP on shell exit. */
signal(SIGHUP,SIG_IGN); /* just in case, same as using nohup to launch program. */
fd=open("/dev/tty",O_RDWR);
ioctl(fd,TIOCNOTTY,0); /* Disassociates from the terminal */
close(fd);
if (fork() != 0) exit(0); /* just for good measure */
That should fully daemonize your program.
The most common way of doing this under Linux is via forking. The same should work on Mac, as for Windows I'm not 100% sure but I believe they have something similar.
Basically what happens is the process splits itself into two processes, and then the original one exits (returning control to the shell or whatever), and the second process continues to run in the background.
I'm not sure about Windows, but on UNIX-like systems, you can fork() then setsid() the forked process to move it into a new process group that is not connected to a terminal.
Under Windows, the closing thing you're going to get to fork() is loading your program as a Windows service, I think.
Here is a link to an intro article on Windows services...
CodeProject: Simple Windows Service Sample
So, as you say, just fork()ing will not do the trick. What you must do is fork() and then re-exec(), as this code sample does:
#include stdio.h>
#include <unistd.h>
#include <string.h>
#include <CoreFoundation/CoreFoundation.h>
int main(int argc, char **argv)
{
int i, j;
for (i=1; i<argc; i++)
if (strcmp(argv[i], "--daemon") == 0)
{
for (j = i+1; j<argc; j++)
argv[j-1] = argv[j];
argv[argc - 1] = NULL;
if (fork()) return 0;
execv(argv[0], argv);
return 0;
}
sleep(1);
CFRunLoopRun();
CFStringRef hello = CFSTR("Hello, world!");
printf("str: %s\n", CFStringGetCStringPtr(hello, CFStringGetFastestEncoding(hello)));
return 0;
}
The loop is to check for a --daemon argument, and if it is present, remove it before re-execing so an infinite loop is avoided.
I don't think this will work if the binary is put into the path because argv[0] is not necessarily a full path, so it will need to be modified.
/**Deamonize*/
pid_t pid;
pid = fork(); /**father makes a little deamon(son)*/
if(pid>0)
exit(0); /**father dies*/
while(1){
printf("Hello I'm your little deamon %d\n",pid); /**The child deamon goes on*/
sleep(1)
}
/** try 'nohup' in linux(usage: nohup <command> &) */
In Unix, I have learned to do that using fork().
If you want to put a running process into the background, fork it twice.
I was trying the solution.
Only one fork is needed from the parent process.
The most important point is that, after fork, the parent process must die by calling _exit(0); and NOT by calling exit(0);
When _exit(0); is used, the command prompt immediately returns on the shell.
This is the trick.
If you need a script to have the PID of the program, you can still get it after a fork.
When you fork, save the PID of the child in the parent process. When you exit the parent process, either output the PID to STD{OUT,ERR} or simply have a return pid; statement at the end of main(). A calling script can then get the pid of the program, although it requires a certain knowledge of how the program works.