I need some help on C program - it is a reverse shell (https://github.com/arturgontijo/remoteShell/blob/master/reverseShell.c) I made few changes, like put that all in a loop and some sleep pattern + put some argument to pass directly IP and PORT now that thing works very good it's stable (problem that cannot autocomplete stuff with TAB I don't really care) BUT what I really care is that this thing will break if on target machine I press CTRL+C the program just exits itself. Now I used this example to block CTRL+C calls:
/* Signal Handler for SIGINT */
void sigintHandler(int sig_num)
{
/* Reset handler to catch SIGINT next time.
Refer http://en.cppreference.com/w/c/program/signal */
signal(SIGINT, sigintHandler);
printf("\n Cannot be terminated using Ctrl+C \n");
fflush(stdout);
}
signal(SIGINT, sigintHandler);
I got this example online and put it on my loop as well, but still from client pressing ctrl+C breaks program. I wonder dup2() is responsible for that or something because on simple C program this actually worked fine.
You can use the sigetops family of functions to manipulate the signals sent into your application.
So for your example you could use:
#include <signal.h>
#include <unistd.h>
int main(int argc, char **argv)
{
sigset_t block_set;
sigemptyset(&block_set);
sigaddset(&block_set, SIGINT);
sigprocmask(SIG_BLOCK, &block_set, NULL);
while(1) {
sleep(1);
}
}
Running Example: https://repl.it/repls/RelevantImaginarySearchservice
You can unblock the signal at a later time by calling
sigprocmask(SIG_UNBLOCK, &block_set, NULL);
Related
I have a question about handling a signal.
Assume that if we recieve SIGINT signal, we should print "Recieved Signal". If within ten seconds the handler recieves another signal, it should print "Shutting Down" then exit with status 1.
I made my code like this:
#include <stdio.h>
#include <signal.h>
#include <unistd.h>
void handler(int);
void secondhandler(int);
void alrmhandler(int);
void alrmhandler (int alrmsig)
{
alarm(0);
}
void secondhandler(int sig)
{
/* after recieving second signal prints shutting down and exit */
printf("Shutting Down\n");
exit(1);
}
void handler ( int sig )
{
/* recieve first SIGINT signal */
printf ("Recieved Signal\n");
/* handle for the alarm function */
signal(SIGALRM, alrmhandler);
/* start 10s alarm */
alarm(10);
/* catch second SIGINT signal within 10s*/
signal(SIGINT, secondhandler);
}
int main( void )
{
signal(SIGINT, handler);
printf( "Hello World!\n" );
for ( ;; )
{
/* infinite loop */
}
return 0;
}
I tried to compile it with dev c++, but it failed. Because SIGALRM undeclared(first use in this function).
Anyway, what I want to know is if this code is right. I actually kinda not sure with the alrmhandler(). should I ignore the SIGALRM?
If you are on a Windows platform, the only signals you will be able to send are : SIGABRT, SIGFPE, SIGILL, SIGINT, SIGSEGV, or SIGTERM.
You write:
what I want to know is if this code is right.
Not entirely. printf() is not async-signal-safe, and so should not be called from within a signal handler unless you are very sure it is safe to do so. It is not safe to do so within the code you provide.
The alarm() technique is, generally, race-prone. Your ten second alarm might expire in the middle of your secondhandler() function. To guard against this, you might mask out signals to compensate with a more sophisticated signal manipulation function.
There are more elegant/flexible ways of implementing the timeout you desire, but that's perhaps a question better suited for codereview.stackexchange.com.
I have a main application that spawns a seperate thread to process messages off a queue. I have an issue on AIX when I hit CTRL-C as it seems to make some "connection handles" in the thread become invalid. I do have a shutdown hook in the main program catching the SIGINT but on AIX it seems to somehow send a signal to the thread as well...although that is not really possible from what I hear...
Essentially I would like to know if I want the MAIN application to handle ALL signals I am interested in and have the thread/s NEVER handle any signals...is that "good practice"?
If so how can I NOT use "sigwait" in the thread...in fact I do not want any "signal code" in the thread/s...they must simply not receive any signals at all.
I have emptied out all the signals:
sigemptyset(&set);
And have set the SIG_BLOCK
s = pthread_sigmask(SIG_BLOCK, &set, NULL);
So here is a dummy test programe:
#include <pthread.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <signal.h>
#include <errno.h>
#define handle_error_en(en, msg) do { errno = en; perror(msg); exit(EXIT_FAILURE); } while (0)
static void * threadMainLoop(){
//Here I do not want the thread to use "sigwait"....
while(running == TRUE){
//do some thread work and never have any signals come in
}
}
void shutdownHook(int sig){
printf("\nCtrl-C pressed....shutdown hook in main...\n");
}
void signalErrorHandler(int signum){
printf("\nSignal error handler in main...\n");
}
int main(int argc, char *argv[]){
pthread_t thread;
sigset_t set;
int s;
//Catch the following signals in the MAIN thread
(void) signal(SIGINT, shutdownHook);
(void) signal(SIGSEGV, signalErrorHandler);
(void) signal(SIGBUS, signalErrorHandler);
(void) signal(SIGILL, signalErrorHandler);
(void) signal(SIGTERM, signalErrorHandler);
(void) signal(SIGABRT, signalErrorHandler);
sigemptyset(&set); //BLOCK all signals
s = pthread_sigmask(SIG_BLOCK, &set, NULL);
if (s != 0)
handle_error_en(s, "pthread_sigmask");
s = pthread_create(&thread, NULL, &threadMainLoop, (void *) NULL);
if (s != 0)
handle_error_en(s, "pthread_create");
pause();
}
If I just create a thread and have, for example, the SIGINT signal handler in the MAIN thread but do NOT has the SIG_BLOCK set for the thread and the user hits CTRL-C....does the thread get affected at all even though the signal handler in the main thread runs? That seems to be what I am seeing on AIX ;-(
Thanks for the help, much appreciated
Lynton
With s = pthread_sigmask(SIG_BLOCK, &set, NULL); , you're not blocking anything.
Use:
sigfillset(&set);
sets = pthread_sigmask(SIG_SETMASK, &set, NULL);
If you want to block every signal, or explicitly add the signals you want to block to the set if you're using SIG_BLOCK.
After you've created the threads, you need to restore the signal mask, otherwise no threads will catch any signal.
However, looking at your previous question, it might be that the thread catching the signal doesn't handle being interrupted. That is, if you're blocked doing a syscall, and a signal arrives, that syscall gets aborted. Some operating systems defaults to automatically call the system call again, some returns an error and sets errno to EINTR, which the application must handle - and bad things might happen if that's not handled.
Instead, install your signal handlers with sigaction() instead of signal() , and set the SA_RESTART flag, which will cause system calls to automatically restart in case it got aborted by a signal.
Still wrong design.
Do not use CTRL+C to stop an application in a controlled manner.
Use a correctly designed controller app that will be accessible over CORBA, RMI, or some other method to interact with the user and control the background app.
Have fun guys...
I have to code a multithreaded(say 2 threads) program where each of these threads do a different task. Also, these threads must keep running infinitely in the background once started. Here is what I have done. Can somebody please give me some feedback if the method is good and if you see some problems. Also, I would like to know how to shut the threads in a systematic way once I terminate the execution say with Ctrl+C.
The main function creates two threads and let them run infinitely as below.
Here is the skeleton:
void *func1();
void *func2();
int main(int argc, char *argv[])
{
pthread_t th1,th2;
pthread_create(&th1, NULL, func1, NULL);
pthread_create(&th2, NULL, func2, NULL);
fflush (stdout);
for(;;){
}
exit(0); //never reached
}
void *func1()
{
while(1){
//do something
}
}
void *func2()
{
while(1){
//do something
}
}
Thanks.
Edited code using inputs from the answers:
Am I exiting the threads properly?
#include <stdlib.h> /* exit() */
#include <stdio.h> /* standard in and output*/
#include <pthread.h>
#include <unistd.h>
#include <time.h>
#include <sys/time.h>
#include <sys/types.h>
#include <signal.h>
#include <semaphore.h>
sem_t end;
void *func1();
void *func2();
void ThreadTermHandler(int signo){
if (signo == SIGINT) {
printf("Ctrl+C detected !!! \n");
sem_post(&end);
}
}
void *func1()
{
int value;
for(;;){
sem_getvalue(&end, &value);
while(!value){
printf("in thread 1 \n");
}
}
return 0;
}
void *func2()
{
int value;
for(;;){
sem_getvalue(&end, &value);
while(!value){
printf("value = %d\n", value);
}
}
return 0;
}
int main(int argc, char *argv[])
{
sem_init(&end, 0, 0);
pthread_t th1,th2;
int value = -2;
pthread_create(&th1, NULL, func1, NULL);
pthread_create(&th2, NULL, func2, NULL);
struct sigaction sa;
sigemptyset(&sa.sa_mask);
sa.sa_flags = SA_SIGINFO;
sa.sa_sigaction = ThreadTermHandler;
// Establish a handler to catch CTRL+c and use it for exiting.
if (sigaction(SIGINT, &sa, NULL) == -1) {
perror("sigaction for Thread Termination failed");
exit( EXIT_FAILURE );
}
/* Wait for SIGINT. */
while (sem_wait(&end)!=0){}
//{
printf("Terminating Threads.. \n");
sem_post(&end);
sem_getvalue(&end, &value);
/* SIGINT received, cancel threads. */
pthread_cancel(th1);
pthread_cancel(th2);
/* Join threads. */
pthread_join(th1, NULL);
pthread_join(th2, NULL);
//}
exit(0);
}
There are mainly two approaches for thread termination.
Use a cancellation point. The thread will terminate when requested to cancel and it reaches a cancellation point, thus ending execution in a controlled fashion;
Use a signal. Have the threads install a signal handler which provides a mechanism for termination (setting a flag and reacting to EINTR).
Both approaches has caveats. Refer to Kill Thread in Pthread Library for more details.
In your case, it seems a good opportunity to use cancellation points. I will work with a commented example. The error-checking has been omitted for clarity.
#define _POSIX_C_SOURCE 200809L
#include <pthread.h>
#include <signal.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
void sigint(int signo) {
(void)signo;
}
void *thread(void *argument) {
(void)argument;
for (;;) {
// Do something useful.
printf("Thread %u running.\n", *(unsigned int*)argument);
// sleep() is a cancellation point in this example.
sleep(1);
}
return NULL;
}
int main(void) {
// Block the SIGINT signal. The threads will inherit the signal mask.
// This will avoid them catching SIGINT instead of this thread.
sigset_t sigset, oldset;
sigemptyset(&sigset);
sigaddset(&sigset, SIGINT);
pthread_sigmask(SIG_BLOCK, &sigset, &oldset);
// Spawn the two threads.
pthread_t thread1, thread2;
pthread_create(&thread1, NULL, thread, &(unsigned int){1});
pthread_create(&thread2, NULL, thread, &(unsigned int){2});
// Install the signal handler for SIGINT.
struct sigaction s;
s.sa_handler = sigint;
sigemptyset(&s.sa_mask);
s.sa_flags = 0;
sigaction(SIGINT, &s, NULL);
// Restore the old signal mask only for this thread.
pthread_sigmask(SIG_SETMASK, &oldset, NULL);
// Wait for SIGINT to arrive.
pause();
// Cancel both threads.
pthread_cancel(thread1);
pthread_cancel(thread2);
// Join both threads.
pthread_join(thread1, NULL);
pthread_join(thread2, NULL);
// Done.
puts("Terminated.");
return EXIT_SUCCESS;
}
The need for blocking/unblocking signals is that if you send SIGINT to the process, any thread may be able to catch it. You do so before spawning the threads to avoid having them doing it by themselves and needing to synchronize with the parent. After the threads are created, you restore the mask and install a handler.
Cancellation points can be tricky if the threads allocates a lot of resources; in that case, you will have to use pthread_cleanup_push() and pthread_cleanup_pop(), which are a mess. But the approach is feasible and rather elegant if used properly.
The answer depends a lot on what you want to do when the user presses CtrlC.
If your worker threads are not modifying data that needs to be saved on exit, you don't need to do anything. The default action of SIGINT is to terminate the process, and that includes all threads that make up the process.
If your threads do need to perform cleanup, however, you've got some work to do. There are two separate issues you need to consider:
How you handle the signal and get the message to threads that they need to terminate.
How your threads receive and handle the request to terminate.
First of all, signal handlers are a pain. Unless you're very careful, you have to assume most library functions are not legal to call from a signal handler. Fortunately, sem_post is specified to be async-signal-safe, and can meet your requirements perfectly:
At the beginning of your program, initialize a semaphore with sem_init(&exit_sem, 0, 0);
Install a signal handler for SIGINT (and any other termination signals you want to handle, like SIGTERM) that performs sem_post(&exit_sem); and returns.
Replace the for(;;); in the main thread with while (sem_wait(&exit_sem)!=0).
After sem_wait succeeds, the main thread should inform all other threads that they should exit, then wait for them all to exit.
The above can also be accomplished without semaphores using signal masks and sigwaitinfo, but I prefer the semaphore approach because it doesn't require you to learn lots of complicated signal semantics.
Now, there are several ways you could handle informing the worker threads that it's time to quit. Some options I see:
Having them check sem_getvalue(&exit_sem) periodically and cleanup and exit if it returns a nonzero value. Note however that this will not work if the thread is blocked indefinitely, for example in a call to read or write.
Use pthread_cancel, and carefully place cancellation handlers (pthread_cleanup_push) all over the place.
Use pthread_cancel, but also use pthread_setcancelstate to disable cancellation during most of your code, and only re-enable it when you're going to perform blocking IO operations. This way you need only put the cleanup handlers just in the places where cancellation is enabled.
Learn advanced signal semantics, and setup an additional signal and interrupting signal handler which you send to all threads via pthread_kill which will cause blocking syscalls to return with an EINTR error. Then your threads can act on this and exit the normal C way via a string of failure returns all the way back up the the start function.
I would not recommend approach 4 for beginners, because it's hard to get right, but for advanced C programmers it may be the best because it allows you to use the existing C idiom of reporting exceptional conditions via return values rather than "exceptions".
Also note that with pthread_cancel, you will need to periodically call pthread_testcancel if you are not calling any other functions which are cancellation points. Otherwise the cancellation request will never be acted upon.
This is a bad idea:
for(;;){
}
because your main thread will execute unnecessary CPU instructions.
If you need to wait in the main thread, use pthread_join as answered in this question: Multiple threads in C program
What you have done works, I see no obvious problems with it (except that you are ignoring the return value of pthread_create). Unfortunately, stopping threads is more involved than you might think. The fact that you want to use signals is another complication. Here's what you could do.
In the "children" threads, use pthread_sigmask to block signals
In the main thread, use sigsuspend to wait for a signal
Once you receive the signal, cancel (pthread_cancel) the children threads
Your main thread could look something like this:
/* Wait for SIGINT. */
sigsuspend(&mask);
/* SIGINT received, cancel threads. */
pthread_cancel(th1);
pthread_cancel(th2);
/* Join threads. */
pthread_join(th1, NULL);
pthread_join(th2, NULL);
Obviously, you should read more about pthread_cancel and cancellation points. You could also install a cleanup handler. And of course, check every return value.
Looked at your updated coded and it still does not look right.
Signal handling must be done in only one thread. Signals targeted for a process (such as SIGINT) get delivered to any thread that does not have that signal blocked. In other words, there is no guarantee that given the three threads you have it is going to be the main thread that receives SIGINT. In multi-threaded programs the best practise is too block all signals before creating any threads, and once all threads have been created unblock the signals in the main thread only (normally it is the main thread that is in the best position to handle signals). See Signal Concepts and Signalling in a Multi-Threaded Process for more.
pthread_cancel is best avoided, there no reason to ever use it. To stop the threads you should somehow communicate to them that they should terminate and wait till they have terminated voluntarily. Normally, the threads will have some sort of event loop, so it should be relatively straightforward to send the other thread an event.
Wouldn't it be much easier to just call pthread_cancel and use pthread_cleanup_push in the thread function to potentially clean up the data that was dynamically allocated by the thread or do any termination tasks that was required before the thread stops.
So the idea would be:
write the code to handle signals
when you do ctrl+c ... the handling function is called
this function cancels the thread
each thread which was created set a thread cleanup function using pthread_cleanup_push
when the tread is cancelled the pthread_cleanup_push's function is called
join all threads before exiting
It seems like a simple and natural solution.
static void cleanup_handler(void *arg)
{
printf("Called clean-up handler\n");
}
static void *threadFunc(void *data)
{
ThreadData *td = (ThreadData*)(data);
pthread_cleanup_push(cleanup_handler, (void*)something);
while (1) {
pthread_testcancel(); /* A cancellation point */
...
}
pthread_cleanup_pop(cleanup_pop_arg);
return NULL;
}
You don't need the foor loop in the main. A th1->join(); th2->join(); will suffice as a wait condition since the threads never end.
To stop the threads you could use a global shared var like bool stop = false;, then when catching the signal (Ctrl+Z is a signal in UNIX), set stop = true aborting the threads, since you are waiting with join() the main program will also exit.
example
void *func1(){
while(!stop){
//do something
}
}
I am writing a program that repeatedly does an operation till Ctrl+C is hit by the user in the Linux terminal. I am programming in C. Any ideas how i can implement this.
I have tested my program by using "for" loops with a condition but now i want to make it run as long as Ctrl+C is hit by the user and interrupted.
What I was thinking was of writing a do while loop like the following
do{
/Computation/
}
while(Ctrl+C is not hit)
But i dont know how to check for the Ctrl+C input from the user.
Any suggestions will be appreciated.
Thanks
A signal trap will do the trick more cleanly than spudd86's suggestion. This example can be improved significantly, though.
#include <signal.h>
/* ... */
int execute;
void trap(int signal){ execute = 0; }
int main() {
/* ... */
signal(SIGINT, &trap);
execute = 1;
while(execute){
/* ... */
}
signal(SIGINT, SIG_DFL);
/* ... */
}
Ctrl+C will send your program a signal (SIGINT), if you DON'T set up a handler for it will cause your program to exit. So all you have to do is write
do {
//computation
} while(1);
and that will do what you want
EDIT: To do what you REALLY wanted without needing a signal handler, and avoiding doing things that might result in the compiler's optimizations breaking your code you can do this:
#include <signal.h>
/* ... */
int main() {
sigset_t set, oldset;
sigemptyset(&set);
/* ... */
sigaddset(&set, SIGINT);
sigprocmask(SIG_BLOCK, &set, &oldset);
do {
sigset_t pending_set;
/* ... */
sigpending(&pending_set);
} while(!sigismember(&pending_set, SIGINT));
sigprocmask(SIG_SETMASK, &oldset, NULL);
/* ... */
}
When Ctrl-C is hit, a signal is sent to the program. The default behavior is that the program is terminated.
This means that you can let your program run forever, looping endlessly. When the user hits Ctrl-C, the program will abort.
How can I implement signal Handling for Ctrl-C and Ctrl-D in C....So If Ctrl-C is pressed then the program will ignore and try to get the input from the user again...If Ctrl-D is pressed then the program will terminate...
My program follows:
int main(){
char msg[400];
while(1){
printf("Enter: ");
fgets(msg,400,stdin);
printf("%s\n",msg);
}
}
Thanks,
Dave
When dealing with POSIX signals, you have two means at your disposal. First, the easy (but discouraged) way, signal(). Second, the more elegant, current but complex way, sigaction(). Please use sigaction() unless you find that it isn't available on some platform that you need to work on.
This chapter of the glibc manual explains differences between the two and gives good example code on how to use both. It also lists the signals that can be handled, recommends how they should be handled and goes more in depth on how to tell how any given signal is (or is not) currently being handled. That's way more code than I'd want to paste into an answer here, hence the links.
It really is worth the hour or two it would take you to read the links and work through the examples. Signal handling (especially in programs that daemonize) is extremely important. A good program should handle all fatal signals that can be handled (i.e. SIGHUP) and explicitly ignore signals that it might not be using (i.e. SIGUSR1 / SIGUSR2).
It also won't hurt to study the difference between normal and real time signals, at least up to the understanding of how the kernel merges the prior and not the latter.
Once you work through it, you'll probably feel inclined to write up an easy to modify set of functions to handle your signals and re-use that code over and over again.
Sorry for not giving a quick and dirty code snippet to show you how to solve your immediate need, but this isn't a quick and dirty topic :)
Firstly, Ctrl+D is an EOF indicator which you cannot trap, when a program is waiting for input, hitting Ctrl+D signifies end of file and to expect no more input. On the other hand, using Ctrl+C to terminate a program - that is SIGINT, which can be trapped by doing this:
#include <stdio.h>
#include <signal.h>
#include <stdlib.h>
#include <stdarg.h>
static void signal_handler(int);
static void cleanup(void);
void init_signals(void);
void panic(const char *, ...);
struct sigaction sigact;
char *progname;
int main(int argc, char **argv){
char *s;
progname = *(argv);
atexit(cleanup);
init_signals();
// do the work
exit(0);
}
void init_signals(void){
sigact.sa_handler = signal_handler;
sigemptyset(&sigact.sa_mask);
sigact.sa_flags = 0;
sigaction(SIGINT, &sigact, (struct sigaction *)NULL);
}
static void signal_handler(int sig){
if (sig == SIGINT) panic("Caught signal for Ctrl+C\n");
}
void panic(const char *fmt, ...){
char buf[50];
va_list argptr;
va_start(argptr, fmt);
vsprintf(buf, fmt, argptr);
va_end(argptr);
fprintf(stderr, buf);
exit(-1);
}
void cleanup(void){
sigemptyset(&sigact.sa_mask);
/* Do any cleaning up chores here */
}
In your example it seems you don't need CTRL-C handlind at all. A "signal(SIGINT,SIG_IGN)" seems enough for you, unless your application must handle a SIGINT coming from some other source.
CTRL-D doesn't usually generates signals, it simply communicates the EOF condition.
You can in general control the behavior of your terminal (we are talking about console input, it isn't?) by using the termios library (also here). You can enable, redefine or disable the "interrupt" character (CTRL-C), the EOF one and many other ones (XON, XOFF, modem control...)
Regards
This is a program for handling signal when pressed Ctrl+c
The syntax for signal function is : signal(signal name, function name);
#include<stdio.h>
#include<signal.h> // for handling signal
void signal_handler()
{
printf("Signal Handled here\n");
}
main()
{
printf("In main function..\n");
// SIGINT is signal name create when Ctrl+c will pressed
signal(SIGINT,signal_handler);
sleep(15);
printf("In main after called from signal_handle \n");
}
#include<signal.h>
#include<unistd.h>
#include<stdio.h>
void signal_catch()
{
printf("hi,Your signal catched Here");
}
int main()
{
signal(SIGINT,signal_catch);
//press ctrl+c
sleep(10);
return 0;
}//end main
//if you want to simply ignore ctrl+c interrupt use following code in main
int main()
{
signal(SIGINT,SIG_IGN);
sleep(100);
return 0;
}//end main
//this program wont accept ctrl+c interrupt for 100 seconds.