I started to learn pipes and FIFO and i don t understand exactly what happens in the background in this situation.Why nothing is printed in the console? No other process opens the "abc" FIFO
`int r,w,n=7;
r=open("abc",O_RDONLY);
n--;
w=open("abc",O_WRONLY);
n--;
printf("%d",n);`
As https://stackoverflow.com/a/23435538/139985 explains, when you open a FIFO, the open call will block until the system gets a corresponding open on the the other side of same FIFO.
In your example, you have a single-threaded C program that attempts to open both sides one after the other. That won't work. The program will just lock up.
If you redesign your program to use two threads and open the read and write ends of the FIFO in different threads, that should work. One thread will block in open until the other thread calls open.
However, using a FIFO to communicate between two threads in the same application is (to say the least) inefficient.
Related
I was programming in C doing system calls, and I was wondering the following:
What's an example of where you'd want a process to ignore alarm signals, say if the signal was sent because of a lost packet in intra-network processes?
Many important daemons are very picky about the signals they will respond to; they often install a handler for SIGHUP to re-read their configuration file, use one of SIGUSR1 or SIGUSR2 to indicate the need to close and re-open their log files for log-rotation, and handle SIGINT, SIGQUIT, SIGTERM, etc., in some sort of graceful way.
Everything else should be ignored so that accidental signals do not cause the program to do funny things. The signals that are part of the program's interface should work exactly as designed -- and the other signals should do as little harm as possible.
I recently had the privilege of setting $| = 1; inside my Perl script to help it talk faster with another application across a pipe.
I'm curious as to why this is not the default setting. In other words, what do I lose out on if my buffer gets flushed straightaway?
Writing to a file descriptor is done via system calls, and system calls are slow.
Buffering a stream and flushing it only once some amount of data has been written is a way to save some system calls.
Benchmark it and you will understand.
Buffered depends on the device type of the output handle: ttys are line-buffered; pipes and sockets are pipe-buffered; disks are block-buffered.
This is just basic programming. It’s not a Perl thing.
The fewer times the I/O buffer is flushed, the faster your code is in general (since it doesn't have to make a system call as often). So your code spends more time waiting for I/O by enabling auto-flush.
In a purely network I/O-driven application, that obviously makes more sense. However, in the most common use cases, line-buffered I/O (Perl's default for TTYs) allows the program to flush the buffer less often and spend more time doing CPU work. The average user wouldn't notice the difference on a terminal or in a file.
I have a Perl script that forks.
Each fork runs an external program, parses the output, and converts the output to a Storable file.
The Storable files are then read in by the parent and the total data from each of the children are analyzed before proceeding onto a repeat of the previous fork or else the parent stops.
What exactly happens when I issue a ^C while some of the children are still running the external program? The parent perl script was called in the foreground and, I presume, remained in the foreground despite the forking.
Is the SIGINT passed to all children, that is, the parent, the parent's children, and the external program called by the children??
UPDATE:
I should add, it appears that when I issue the SIGINIT, the external program called by the children of my script seem to acknowledge the signal and terminate. But the children, or perhaps the parent program, carry on. This is all unclear to me.
UPDATE 2:
With respect to tchrist's comment, the external program is called with Perl's system() command.
In fact, tchrist's comment also seems to contain the explanation I was looking for. After some more debugging, based on the behavior of my program, it appears that, indeed, SIGINT is being passed from the parent to all children and from all children to all of their children (the external program).
Thus, what appears to be happening, based on tchrist's comment, is that CTRL-C is killing the external program which causes the children to move out of the system() command - and nothing more.
Although I had my children check the exit status of what was called in system(), I was assuming that a CTRL-C would kill everything from the parent down, rather than lead to the creation of more rounds of processing, which is what was happening!!!
SOLUTION (to my problem):
I need to just create a signal handler for SIGINT in the parent. The signal handler would then send SIGTERM to each of the children (which I presume would also send a SIGTERM to the children's children), and then cause the parent to exit gracefully. Although this somewhat obvious solution likely would have fixed things, I wanted to understand my misconception about the behavior of SIGINT with respect to forking in Perl.
Perl’s builtin system function works just like the C system(3) function from the standard C library as far as signals are concerned. If you are using Perl’s version of system() or pipe open or backticks, then the parent — the one calling system rather than the one called by it — will IGNORE any SIGINT and SIGQUIT while the children are running. If you’ve you rolled your own using some variant of the fork?wait:exec trio, then you must think about these matters yourself.
Consider what happens when you use system("vi somefile") and hit ^C during a long search in vi: only vi takes a (nonfatal) SIGINT; the parent ignores it. This is correct behavior. That’s why C works this way, and that’s why Perl works this way.
The thing you have to remember is that just because a ^C sends a SIGINT to all processes in the foregrounded process group (even those of differing effective UID or GID), that does not mean that it causes all those processes to exit. A ^C is only a SIGINT, meant to interrupt a process, not a SIGKILL, meant to terminate with no questions asked.
There are many sorts of program that it would be wrong to just kill off with no warning; an editor is just one such example. A mailer might be another. Be exceedingly careful about this.
Many sorts of programs selectively ignore, trap, or block (means delay delivery of) various sorts of signals. Only the default behavior of SIGINT is to cause the process to exit. You can find out whether this happened, and in fact which signal caused it to happen (amongst other things), with this sort of code on traditional operating systems:
if ($wait_status = system("whatever")) {
$sig_killed = $wait_status & 127;
$did_coredump = $wait_status & 128;
$exit_status = $wait_status >> 8;
# now do something based on that...
}
Note carefully that a ^C’d vi, for example, will not have a wait status word indicating it died from an untrapped SIGINT, since there wasn’t one: it caught it.
Sometimes your kids will go and have kids of their own behind your back. Messy but true. I have therefore been known, on occasion, to genocide all progeny known and unknown this way:
# scope to temporize (save+restore) any previous value of $SIG{HUP}
{
local $SIG{HUP} = "IGNORE";
kill HUP => -$$; # the killpg(getpid(), SIGHUP) syscall
}
That of course doesn’t work with SIGKILL or SIGSTOP, which are not amenable to being IGNOREd like that.
Another matter you might want to be careful of is that before the 5.8 release, signal handling in Perl has not historically been a reliably safe operation. It is now, but this is a version-dependent issue. If you haven’t yet done so, then you should definitely read up on deferred signals in the perlipc manpage, and perhaps also on the PERL_SIGNALS envariable in the perlrun manpage.
When you hit ^C in a terminal window (or any terminal, for that matter), it will send a SIGINT to the foreground process GROUP in that terminal. Now when you start a program from the command line, its generally in its own process group, and that becomes the foreground process group. By default, when you fork a child, it will be in the same process group as the parent, so by default, the parent (top level program invoked from the command line), all its children, children's children, etc, as well as any external programs invoked by any of these (which are all just children) will all be in that same process group, so will all receive the SIGINT signal.
However, if any of those children or programs call setpgrp or setpgid or setsid, or any other call that causes the process to be a in a new progress group, those processes (and any children they start after leaving the foreground process group) will NOT receive the SIGINT.
In addition, when a process receives a SIGINT, it might not terminate -- it might be ignoring the signal, or it might catch it and do something completely different. The default signal handler for SIGINT terminates the process, but that's just the default which can be overridden.
edit
From your update, it sounds like everything is remaining in the same process group, so the signals are being delivered to everyone, as the grandchildren (the external programs) are exiting, but the children are catching and ignoring the SIGINTs. By tchrist's comment, it sounds like this is the default behavior for perl.
If you kill the parent process, the children (and external programs running) will still run until they terminate one way or another. This can be avoided if the parent has a signal handler that catches the SIGINT, and then kills the process group (often parent's pid). That way, once the parent gets the SIGINT, it will kill all of it's children.
So to answer your question, it all depends on the implementation of the signal handler. Based on your update, it seems that the parent indeed kills it's children and then instead of terminating itself, it goes back to doing something else (think of it as a reset instead of a complete shutdown/start combination).
I'm writing a Perl script that runs 4 simultaneous, identical processes with different input parameters (see background here - the rest of my question will make much more sense after reading that).
I am making a system() call to a program that generates data (XFOIL, again see above link). My single-core version of this program looks like this:
eval{
local $SIG{ALRM} = sub{die "TIMEOUT"};
alarm 250;
system("xfoil <command_list >xfoil_output");
alarm 0;
};
if ($#){
# read the output log and run timeout stuff...
system('killall xfoil') # Kill the hung XFOIL. now it's a zombie.
}
Essentially, XFOIL should take only about 100 seconds to run - so after 250 seconds the program is hanging (presumably waiting for user input that it's never going to get).
The problem now is, if I do a killall in the multi-core version of my program, I'm going to kill 3 other instances of XFOIL, and those processes are generating data. So I need to kill only the hung instance, and this requires getting a PID.
I don't know very much about forks and such. From what I can tell so far, I would run an exec('xfoil') inside the child process that I fork. But the PID of the exec() will be different than the PID of the child process (or is it? It's a separate process so I'd assume it is, but again I've no experience with this..), so this still doesn't help when I want to forcefully kill the process since I won't have the PID anyway. How do I go about doing this?
Thanks a ton for your help!
If you want the PID, fork the process yourself instead of using system. The system command is mostly designed as a "fire and forget" tool. If you want to interact with the process, use something else. See, for instance, the perlipc documentation.
I think you've already looked at Parallel::ForkManager based on answers to your question How can I make my Perl script use multiple cores for child processes?
There are so many possible errors in the POSIX environment. Why do some of them (like writing to an unconnected socket in particular) get special treatment in the form of signals?
This is by design, so that simple programs producing text (e.g. find, grep, cat) used in a pipeline would die when their consumer dies. That is, if you're running a chain like find | grep | sed | head, head will exit as soon as it reads enough lines. That will kill sed with SIGPIPE, which will kill grep with SIGPIPE, which will kill find with SEGPIPE. If there were no SIGPIPE, naively written programs would continue running and producing content that nobody needs.
If you don't want to get SIGPIPE in your program, just ignore it with a call to signal(). After that, syscalls like write() that hit a broken pipe will return with errno=EPIPE instead.
See this SO answer for a detailed explanation of why writing a closed descriptor / socket generates SIGPIPE.
Why is writing a closed TCP socket worse than reading one?
SIGPIPE isn't specific to sockets — as the name would suggest, it is also sent when you try to write to a pipe (anonymous or named) as well. I guess the reason for having separate error-handling behaviour is that broken pipes shouldn't always be treated as an error (whereas, for example, trying to write to a file that doesn't exist should always be treated as an error).
Consider the program less. This program reads input from stdin (unless a filename is specified) and only shows part of it at a time. If the user scrolls down, it will try to read more input from stdin, and display that. Since it doesn't read all the input at once, the pipe will be broken if the user quits (e.g. by pressing q) before the input has all been read. This isn't really a problem, though, so the program that's writing down the pipe should handle it gracefully.
it's up to the design.
at the beginning people use signal to control events notification which were sent to the user space, and later it is not necessary because there're more popular skeletons such as polling which don't require a system caller to make a signal handler.