There is a code that I can't understand if there is any part of the code that causes exiting of the loop without exiting the whole program.
Here is the code:
/* per-packet event loop */
while (true)
{
perf_push (PERF_EVENT_LOOP);
/* wait on tun/socket list */
multi_get_timeout (&multi, &multi.top.c2.timeval);
status = multi_tcp_wait (&multi.top, multi.mtcp);
MULTI_CHECK_SIG (&multi);
/* check on status of coarse timers */
multi_process_per_second_timers (&multi);
/* timeout? */
if (status > 0)
{
/* process the I/O which triggered select */
multi_tcp_process_io (&multi);
MULTI_CHECK_SIG (&multi);
}
else if (status == 0)
{
multi_tcp_action (&multi, NULL, TA_TIMEOUT, false);
}
perf_pop ();
}
/* shut down management interface */
uninit_management_callback_multi (&multi);
Is the last line reachable?
Have a look at the definition of the macro MULTI_CHECK_SIG. I googled for MULTI_CHECK_SIG and found a definition in terms of another macro EVENT_LOOP_CHECK_SIGNAL which contained a break statement.
The loop looks like it has no ending. This is often the case in embedded systems programming. The loop is then simply ended by disconnecting the power supply. Hardware cannot stop working, so there will always be a loop that has no ending condition.
In Linux (and other operating systems) you could also terminate the program by implementing IPC signal functions.
Related
I am trying to invoke external program with some input and retrieve the output from it within a program.
It will be look like;
(some input) | (external program) | (retrieve output)
I first thought about using a popen() but it seems like, it is not possible because the pipe is not bidirectional.
Is there any easy way to handle this kind of stuff in linux?
I can try making a temp file but it will be great if it can be handled clearly without accessing the disk.
Any Solution? Thanks.
On linux you can use pipe function: Open two new pipes, one for each direction, then create a child process using fork, afterwards, you typically close the file descriptors not in use (read end on parent, write end on child of the pipe for parent sending to child and vice versa for the other pipe) and then start your application using execve or one of its front ends.
If you dup2 the pipes' file descriptors to the standard console file handles (STDIN_FILENO/STDOUT_FILENO; each process separately), you should even be able to use std::cin/std::cout for communicating with the other process (you might want to do so only for the child, as you might want to keep your console in parent). I have no tested this, though, so that's left to you.
When done, you'd yet wait or waitpid for your child process to terminate. Might look like similar to the following piece of code:
int pipeP2C[2], pipeC2P[2];
// (names: short for pipe for X (writing) to Y with P == parent, C == child)
if(pipe(pipeP2C) != 0 || pipe(pipeC2P) != 0)
{
// error
// TODO: appropriate handling
}
else
{
int pid = fork();
if(pid < 0)
{
// error
// TODO: appropriate handling
}
else if(pid > 0)
{
// parent
// close unused ends:
close(pipeP2C[0]); // read end
close(pipeC2P[1]); // write end
// use pipes to communicate with child...
int status;
waitpid(pid, &status, 0);
// cleanup or do whatever you want to do afterwards...
}
else
{
// child
close(pipeP2C[1]); // write end
close(pipeC2P[0]); // read end
dup2(pipeP2C[0], STDIN_FILENO);
dup2(pipeC2P[1], STDOUT_FILENO);
// you should be able now to close the two remaining
// pipe file desciptors as well as you dup'ed them already
// (confirmed that it is working)
close(pipeP2C[0]);
close(pipeC2P[1]);
execve(/*...*/); // won't return - but you should now be able to
// use stdin/stdout to communicate with parent
}
}
Below is the example of abort functionality implementation using signals as given in "Advanced Programming in Unix". Few doubts in the below code -
void abort(void) /* POSIX-style abort() function */
{
sigset_t mask;
struct sigaction action;
/*
* Caller can't ignore SIGABRT, if so reset to default.
*/
sigaction(SIGABRT, NULL, &action);
if (action.sa_handler == SIG_IGN) {
action.sa_handler = SIG_DFL;
sigaction(SIGABRT, &action, NULL);
}
if (action.sa_handler == SIG_DFL)
fflush(NULL); /* flush all open stdio streams */
/*
* Caller can't block SIGABRT; make sure it's unblocked.
*/
sigfillset(&mask);
sigdelset(&mask, SIGABRT); /* mask has only SIGABRT turned off */
sigprocmask(SIG_SETMASK, &mask, NULL);
kill(getpid(), SIGABRT); /* send the signal */ **STEP 1**
/*
* If we're here, process caught SIGABRT and returned.
*/
fflush(NULL); /* flush all open stdio streams */
action.sa_handler = SIG_DFL;
sigaction(SIGABRT, &action, NULL); /* reset to default */
sigprocmask(SIG_SETMASK, &mask, NULL); /* just in case ... */
kill(getpid(), SIGABRT); /* and one more time */ **STEP 2**
exit(1); /* this should never be executed ... */
}
Question
a. When we send the first kill with SIGABRT (marked by Step 1), why are we expecting the code to continue to next line? (see the comment - 'If we're here, process caught SIGABRT and returned' )
b. Why do we need to deliver kill signal again (in Step 2) and then exit(1) is not supposed to be hit. (refer comment in the code)
Most programs don't do anything particular with SIGABRT.
But some weird programs could install their own signal handler on SIGABRT, and the abort function should still work, even for them.
So most programs -those not catching SIGABRT- won't go past step 1 (because the default behavior for SIGABRT is to dump core, according to signal(7) ...).
The few programs who do catch SIGABRT will go till step 2 (if calling your abort). At that point, the default behavior of SIGABRT has been reinstalled. So the program dump core at step 2. And the final exit(1) cannot be reached.
I need a code construction for my project which waits for some time, but when there is an interrupt (e.g. incoming udp packets) it leaves this loop, does something, and after this restart the waiting.
How can I implement this? My first idea is using while(wait(2000)), but wait is a void construct...
Thank you!
I would put the loop inside a function
void awesomeFunction() {
bool loop = true;
while (loop) {
wait(2000);
...
...
if (conditionMet)
loop = false;
}
}
Then i would put this function inside another loop
while (programRunning) {
awesomeFunction();
/* Loop ended, do stuff... */
}
There are a few things I am not clear about from the question. Is this a multi-threaded application, where one thread handles (say) the UDP packets, and the other waits for the event, or is this single-threaded? You also didn't mention what operating system this is, which is relevant. So I am going to assume Linux, or something that supports the poll API, or something similar (like select).
Let's assume a single threaded application that waits for UDP packets. The main idea is that once you have the socket's file descriptor, you have an infinite loop on a call to poll. For instance:
#include <poll.h>
// ...
void handle_packets() {
// m_fd was created with `socket` and `bind` or `connect`.
struct pollfd pfd = {.fd = m_fd, .events = POLLIN};
int timeout;
timeout = -1; // Wait indefinitely
// timeout = 2000; // Wait for 2 seconds
while (true) {
pfd.revents = 0;
poll(&pfd, 1, timeout);
if ((pfd.revents & POLLIN) != 0) {
handle_single_packet(); // Method to actually read and handle the packet
}
if ((pfd.revents & (POLLERR | POLLHUP)) != 0) {
break; // return on error or hangup
}
}
}
A simple example of select can be found here.
If you are looking at a multi-threaded application, trying to communicate between the two threads, then there are several options. Two of which are:
Use the same mechanism above. The file descriptor is the result of a call to pipe. The thread sleeping gets the read end of the pipe. The thread waking get the write end, and writes a character when it's time to wake up.
Use C++'s std::condition_variable. It is documented here, with a complete example. This solution depends on your context, e.g., whether you have a variable that you can wait on, or what has to be done.
Other interrupts can also be caught in this way. Signals, for instance, have a signalfd. Timer events have timerfd. This depends a lot on what you need, and in what environment you are running. For instance, timerfd is Linux-specific.
In the main program main loop, I'm listening on a EMS topic by calling tibemsMsgConsumer_Receive. Meanwhile, I want to exit the program at specific time, say 5PM. How can I implement this?
I tried to use the following code but it doesn't work properly in the case there is no message received.
Is there a way I can exit the program when 'while' loop is stuck there?
while (1)
{
status = tibemsMsgConsumer_Receive(m_CmbsSpreadMatrixSubscriber, &msg);
if (status == TIBEMS_OK)
{
DoSomething();
}
if (getRunTime("hour").c_str()) >= 18)
{
exit(0);
}
}
Use tibemsMsgConsumer_ReceiveTimeout() and set an appropriate timeout to check your exit condition repeatedly.
From the description on that page:
This function consumes the next message from the consumer’s destination. When the destination does not have any messages ready, this function blocks:
If a message arrives at the destination, this call immediately consumes that message and returns.
If the (non-zero) timeout elapses before a message arrives, this call returns TIBEMS_TIMEOUT.
If another thread closes the consumer, this call returns TIBEMS_INTR.
before starting the main loop listening on message, I start a thread.
boost::thread aThread(&threadFunc);
and in the thread function I simply count time and exit the program. Not sure if
it's safe and right or not...
void threadFunc()
{
while (true)
{
wait(60);
if (atoi(getRunTime("hour").c_str()) >= 18)
{
Log("Now it's 6PM, let's stop and get back tomorrow.");
exit(0);
}
}
}
I keep running into this problem of trying to run a thread with the following properties:
runs in an infinite loop, checking some external resource, e.g. data from the network or a device,
gets updates from its resource promptly,
exits promptly when asked to,
uses the CPU efficiently.
First approach
One solution I have seen for this is something like the following:
void class::run()
{
while(!exit_flag)
{
if (resource_ready)
use_resource();
}
}
This satisfies points 1, 2 and 3, but being a busy waiting loop, uses 100% CPU.
Second approach
A potential fix for this is to put a sleep statement in:
void class::run()
{
while(!exit_flag)
{
if (resource_ready)
use_resource();
else
sleep(a_short_while);
}
}
We now don't hammer the CPU, so we address 1 and 4, but we could wait up to a_short_while unnecessarily when the resource is ready or we are asked to quit.
Third approach
A third option is to do a blocking read on the resource:
void class::run()
{
while(!exit_flag)
{
obtain_resource();
use_resource();
}
}
This will satisfy 1, 2, and 4 elegantly, but now we can't ask the thread to quit if the resource does not become available.
Question
The best approach seems to be the second one, with a short sleep, so long as the tradeoff between CPU usage and responsiveness can be achieved.
However, this still seems suboptimal, and inelegant to me. This seems like it would be a common problem to solve. Is there a more elegant way to solve it? Is there an approach which can address all four of those requirements?
This depends on the specifics of the resources the thread is accessing, but basically to do it efficiently with minimal latency, the resources need to provide an API for either doing an interruptible blocking wait.
On POSIX systems, you can use the select(2) or poll(2) system calls to do that, if the resources you're using are files or file descriptors (including sockets). To allow the wait to be preempted, you also create a dummy pipe which you can write to.
For example, here's how you might wait for a file descriptor or socket to become ready or for the code to be interrupted:
// Dummy pipe used for sending interrupt message
int interrupt_pipe[2];
int should_exit = 0;
void class::run()
{
// Set up the interrupt pipe
if (pipe(interrupt_pipe) != 0)
; // Handle error
int fd = ...; // File descriptor or socket etc.
while (!should_exit)
{
// Set up a file descriptor set with fd and the read end of the dummy
// pipe in it
fd_set fds;
FD_CLR(&fds);
FD_SET(fd, &fds);
FD_SET(interrupt_pipe[1], &fds);
int maxfd = max(fd, interrupt_pipe[1]);
// Wait until one of the file descriptors is ready to be read
int num_ready = select(maxfd + 1, &fds, NULL, NULL, NULL);
if (num_ready == -1)
; // Handle error
if (FD_ISSET(fd, &fds))
{
// fd can now be read/recv'ed from without blocking
read(fd, ...);
}
}
}
void class::interrupt()
{
should_exit = 1;
// Send a dummy message to the pipe to wake up the select() call
char msg = 0;
write(interrupt_pipe[0], &msg, 1);
}
class::~class()
{
// Clean up pipe etc.
close(interrupt_pipe[0]);
close(interrupt_pipe[1]);
}
If you're on Windows, the select() function still works for sockets, but only for sockets, so you should install use WaitForMultipleObjects to wait on a resource handle and an event handle. For example:
// Event used for sending interrupt message
HANDLE interrupt_event;
int should_exit = 0;
void class::run()
{
// Set up the interrupt event as an auto-reset event
interrupt_event = CreateEvent(NULL, FALSE, FALSE, NULL);
if (interrupt_event == NULL)
; // Handle error
HANDLE resource = ...; // File or resource handle etc.
while (!should_exit)
{
// Wait until one of the handles becomes signaled
HANDLE handles[2] = {resource, interrupt_event};
int which_ready = WaitForMultipleObjects(2, handles, FALSE, INFINITE);
if (which_ready == WAIT_FAILED)
; // Handle error
else if (which_ready == WAIT_OBJECT_0))
{
// resource can now be read from without blocking
ReadFile(resource, ...);
}
}
}
void class::interrupt()
{
// Signal the event to wake up the waiting thread
should_exit = 1;
SetEvent(interrupt_event);
}
class::~class()
{
// Clean up event etc.
CloseHandle(interrupt_event);
}
You get a efficient solution if your obtain_ressource() function supports a timeout value:
while(!exit_flag)
{
obtain_resource_with_timeout(a_short_while);
if (resource_ready)
use_resource();
}
This effectively combines the sleep() with the obtain_ressurce() call.
Check out the manpage for nanosleep:
If the nanosleep() function returns because it has been interrupted by a signal, the function returns a value of -1 and sets errno to indicate the interruption.
In other words, you can interrupt sleeping threads by sending a signal (the sleep manpage says something similar). This means you can use your 2nd approach, and use an interrupt to immediately wake the thread if it's sleeping.
Use the Gang of Four Observer Pattern:
http://home.comcast.net/~codewrangler/tech_info/patterns_code.html#Observer
Callback, don't block.
Self-Pipe trick can be used here.
http://cr.yp.to/docs/selfpipe.html
Assuming that you are reading the data from file descriptor.
Create a pipe and select() for readability on the pipe input as well as on the resource you are interested.
Then when data comes on resource, the thread wakes up and does the processing. Else it sleeps.
To terminate the thread send it a signal and in signal handler, write something on the pipe (I would say something which will never come from the resource you are interested in, something like NULL for illustrating the point). The select call returns and thread on reading the input knows that it got the poison pill and it is time to exit and calls pthread_exit().
EDIT: Better way will be just to see that the data came on the pipe and hence just exit rather than checking the value which came on that pipe.
The Win32 API uses more or less this approach:
someThreadLoop( ... )
{
MSG msg;
int retVal;
while( (retVal = ::GetMessage( &msg, TaskContext::winHandle_, 0, 0 )) > 0 )
{
::TranslateMessage( &msg );
::DispatchMessage( &msg );
}
}
GetMessage itself blocks until any type of message is received therefore not using any processing (refer). If a WM_QUIT is received, it returns false, exiting the thread function gracefully. This is a variant of the producer/consumer mentioned elsewhere.
You can use any variant of a producer/consumer, and the pattern is often similar. One could argue that one would want to split the responsibility concerning quitting and obtaining of a resource, but OTOH quitting could depend on obtaining a resource too (or could be regarded as one of the resources - but a special one). I would at least abstract the producer consumer pattern and have various implementations thereof.
Therefore:
AbstractConsumer:
void AbstractConsumer::threadHandler()
{
do
{
try
{
process( dequeNextCommand() );
}
catch( const base_except& ex )
{
log( ex );
if( ex.isCritical() ){ throw; }
//else we don't want loop to exit...
}
catch( const std::exception& ex )
{
log( ex );
throw;
}
}
while( !terminated() );
}
virtual void /*AbstractConsumer::*/process( std::unique_ptr<Command>&& command ) = 0;
//Note:
// Either may or may not block until resource arrives, but typically blocks on
// a queue that is signalled as soon as a resource is available.
virtual std::unique_ptr<Command> /*AbstractConsumer::*/dequeNextCommand() = 0;
virtual bool /*AbstractConsumer::*/terminated() const = 0;
I usually encapsulate command to execute a function in the context of the consumer, but the pattern in the consumer is always the same.
Any (welln at least, most) approaches mentioned above will do the following: thread is created, then it's blocked wwiting for resource, then it's deleted.
If you're worried about efficiency, this is not a best approach when waiting for IO. On Windows at least, you'll allocate around 1mb of memory in user mode, some in kernel for just one additional thread. What if you have many such resources? Having many waiting threads will also increase context switches and slow down your program. What if resource takes longer to be available and many requests are made? You may end up with tons of waiting threads.
Now, the solution to it (again, on Windows, but I'm sure there should be something similar on other OSes) is using threadpool (the one provided by Windows). On Windows this will not only create limited amount of threads, it'll be able to detect when thread is waiting for IO and will stwal thread from there and reuse it for other operations while waitting.
See http://msdn.microsoft.com/en-us/library/windows/desktop/ms686766(v=vs.85).aspx
Also, for more fine-grained control bit still having ability give up thread when waiting for IO, see IO completion ports (I think they'll anyway use threadpool inside): http://msdn.microsoft.com/en-us/library/windows/desktop/aa365198(v=vs.85).aspx