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.
Related
I am trying to implement some of the features present in the shell including quitting only when a user enters quit and not on Ctrl+C. Below is a simplified version of the code that I am tried.
Code 1: without calling loop() in a signal handler.
void loop(){
while(1){
char a[20];
printf("Enter Command : " );
scanf("%s",a);
printf("%s\n", a);
}
}
void sigintHandler(int sig_num)
{
signal(SIGINT, sigintHandler);
printf("\n");
}
int main(int argc, char const *argv[]) {
signal(SIGINT, sigintHandler);
loop();
return 0;
}
The output of Code 1:
As can be seen on a third input, I go to a new line and continued right from where I left the loop. I want to instead start the loop again. So, I did the following modification by calling loop in signal handler itself.
void loop(){
while(1){
char a[20];
printf("Enter Command : " );
scanf("%s",a);
printf("%s\n", a);
}
}
void sigintHandler(int sig_num)
{
signal(SIGINT, sigintHandler);
printf("\n");
loop();
}
int main(int argc, char const *argv[]) {
signal(SIGINT, sigintHandler);
loop();
return 0;
}
Output for code 2:
As can be seen that when I clicked first-time Ctrl+C (on input line 3), it works properly and I can continue. But when I click Ctrl+C second time I don't go to a new line and I have to press enter for a program to execute.
I went through this question but it doesn't seem to apply for my case. This is my first time using signals and system calls so my question may come as silly. But it will be a great help if someone can help me to get to a proper way of implementing signals. Thank you.
Jonathan Leffler provided helpful hints (though not sufficient, at least on some popular operating system):
Look at what scanf() returns.
Don't use loop() in the signal handler
Don't use signal() — use sigaction(). In your case, this has two advantages:
Restoring the signal action to the default state on call of the signal handler can be avoided, so you don't have to change the action again in the handler.
Restarting the read() system call (inside scanf()) can be avoided, so that scanf() returns and you can react to the interrupt in the first place.
So, the signal handler can be just
void sigintHandler(int sig_num)
{
printf("\n"); // or perhaps better write(1, "\n", 1)
}
The signal(SIGINT, sigintHandler); in main() can be replaced with
sigaction(SIGINT, &(struct sigaction){ .sa_handler = sigintHandler }, NULL);
and the scanf("%s",a); with
if (scanf("%s", a) == EOF)
{
if (errno == EINTR) continue; // read operation interrupted by signal
return;
}
EOF is returned on interrupt as well as on end of file, so the cases are to be distinguished through errno. Besides, it's nice if your program provides a way to exit it, thus the return.
I am trying to learn Signals. I know invalid memory access will cause segfault. So, I register a signal handler for SIGSEGV signal.
#include <stdio.h>
#include <signal.h>
void sighandler(int signum)
{
printf("%s\n", __func__);
}
int main()
{
int *a = NULL;
signal(SIGSEGV, sighandler);
*a = 5;
return 0;
}
Running this code, I am continuously getting SIGSEGV Signals. I thought i should only get the signal once. Can you guys explain why I am getting signals continuously
After the SEGV handler finishes, the instruction that triggered re-executes. Since you didn't do anything to prevent the next execution from faulting, you get SEGV again, ad infinitum.
See more in this answer.
The signal handler is returning to instruction that triggered it namely *a = 5 which is causing it to loop.
You have several problems including the use of printf inside a signal handler.
There are safe and not-safe ways of dealing with this
NOTES
Using signal(2) is not recommended for signal handling in general.
Handling SIGSEGV is even more complicated because of the way the signal semantics work. Quoting from the man page:
The only portable use of signal() is to set a signal's disposition to SIG_DFL or SIG_IGN. The semantics when using signal()
to establish a signal handler vary across
systems (and POSIX.1 explicitly permits this variation); do not use it for this purpose.
POSIX.1 solved the portability mess by specifying sigaction(2), which provides explicit control of the semantics when a
signal handler is invoked; use that interface instead of signal().
So the first thing you should do is use sigaction.
Next, handling SIGSEGV is a weird beast:
How to write a signal handler to catch SIGSEGV?
and
Does linux allow any system call to be made from signal handlers?
have good answers and get into specific details. There are external links in some of the answers given there.
How to do this using signal(2)
Well :-) let's say you want to use signal(2) and you want to play with this in a weird way....
You can use sigjmpset and siglongjmp.
sigjmpset marks a point where siglongjmp should jump to. The first time sigjmpset is called (to set the point) it returns 0. When siglongjmp jumps to it, (which means it gets called again as a result of the long jump), it returns 1.
Which means we can do this:
#include <stdio.h>
#include <signal.h>
#include <unistd.h>
#include <setjmp.h>
sigjmp_buf env;
int sigsav;
void sighandler(int signum)
{
const char msg[] = "Skipping signal\n";
write(2, msg, sizeof(msg));
siglongjmp(env, sigsav);
}
int main()
{
int *a = NULL;
signal(SIGSEGV, sighandler);
if(!sigsetjmp(env, sigsav)) {
printf("setting value of a\n");
*a = 5;
}
else {
printf("returned to sigsetjmp, but now we skip it!\n");
}
return 0;
}
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);
I have a signal handling snippet but it is somehow malfunctioning on my Mac and virtual Linux box at koding.com but on my office Linux PC it is working..Can someone please tell me why..
#include <stdio.h>
#include <signal.h>
#include <unistd.h>
void my_isr(int n){
printf("Hello World");
signal(SIGINT, SIG_DFL);
}
int main(){
signal(SIGINT, my_isr);
printf("pid = %d\n", getpid());
while(1);
return 0;
}
When I am pressing Ctrl+C it is not printing Hello World on the first time but it is re-modifying the SIGINT signal action & hence it is exiting the program when I press Ctrl+C second time. Can someone explain me why?
You are not allowed to call every function in a signal handler.
Read signal(7). Only async signal safe functions can be called (directly or indirectly) from a signal handler, and printf is not such a function. If you really want to reliably "print" something from inside a signal handler (which I don't recommend), you can only use the low-level write(2) syscall (it is async signal safe).
So you've got undefined behavior. This explains why it is so bad.
The recommended way is to set a volatile sigatomic_t flag in your signal handler, and to test it outside of it (e.g. in your while loop...).
And you forgot to call fflush(3). You might be more lucky by ending your printf format string with \n since stdout is line-buffered!
Of course, changing your printf inside your signal handler is still UB, even with a \n, but very often it would appear to work.
Here is a conforming version of your program....
#include <signal.h>
#include <unistd.h>
#include <stdio.h>
volatile sig_atomic_t got_signal;
void my_sigint_handler (int signum) {
if (signum == SIGINT) // this is always true!
got_signal = 1;
#define INTERRUPT_MESSAGE "Interrupted!\n"
write(STDOUT_FILENO, INTERRUPT_MESSAGE, strlen(INTERRUPT_MESSAGE));
};
int main(int argc, char**argv) {
struct sigaction act_int;
memset (&act_int, 0, sizeof(act_int));
act_int.sa_handler = my_sigint_handler;
if (sigaction(SIGINT, &act_int, NULL)) {
perror("sigaction"); exit(EXIT_FAILURE);
};
printf ("start %s pid %d\n", argv[0], (int)getpid());
while (!got_signal) {
};
printf ("ended %s after signal\n", argv[0]);
return 0;
}
A useful (and permissible) trick could be to write(2) a single byte -inside your signal handler- on a pipe(7) to self (you set up that pipe using pipe(2) early at program initialization), and in your event loop poll(2) the read end of that pipe.
printf is the culprit just use counter in handler and print outside handler its value it will work.
use sigaction instead of signal
Hope you can help me to resolve this problem.
For school I have to transform Ctrl+C to a command which doesn't shut down the shell, but he reminds through printf() that I must type exit to close the shell. I don't even know where to start.
Thank a lot.
Here's a trivial implementation of handling SIGINT using sigaction which will work on posix systems. Left out error checking for brevity. The linked manual should explain about sigaction.
Basically the program loops through an infinite loop and break if user types exit. Using write as you can't use printf in signal handler. See signal manual for a list of functions that can be used safely in a signal handler.
#include<stdio.h>
#include<signal.h>
#include<string.h>
#include<stdlib.h>
char s[]="Type 'exit' to terminate\n";
void int_handler (int signum)
{
write(fileno(stdin), s, sizeof s - 1);
}
int main (void)
{
char str[256];
struct sigaction sh;
sh.sa_handler = int_handler;
sigemptyset (&sh.sa_mask);
sh.sa_flags = 0;
sigaction (SIGINT, &sh, NULL);
printf("%s", s);
while(1) {
fgets(str, sizeof str, stdin);
char *p = strchr(str, '\n');
if(p) *p = 0;
if(!strcmp(str, "exit")) {
printf("Exiting on request...");
break;
}
}
return 0;
}
As you're talking about doing it from the shell, you probably want:
$ trap "echo Please type \'exit\' to close the shell." SIGINT
<Ctrl-C>
Please type 'exit' to close the shell.
$
This specifies a command to execute when the listed signal is trapped (the trap command can also trap other signals; SIGINT is the one generated by Ctrl-C). The \' protects the quote from being interpreted by the shell.
Ctrl+C sends an interrupt signal (SIGINT) to the running process.You can use signal() to catch SIGINT like this:
#include<stdio.h>
#include<signal.h>
void sigint_handler(int sig)
{
printf("Type exit to close the shell!\n");
}
int main()
{
signal(SIGINT, sigint_handler);
/*Your code should replace the while loop.*/
while(1)
{
printf("Running!\n");
getchar();
}
return 0 ;
}