So basically I have 4-5 functions in my program. It is a lot of lines of codes and reading and writing from a file and it could end in an infinite loop (worse case) and I would like to terminate my program if it goes beyond lets say 20 seconds. below code is not working, my program hangs and the OS terminates it for me instead of the program terminating itself. I think the main problem I am having is the alarm is set in the main and when the alarm time limit is reached the process is executing in another function and this is causing the program to shut without closing files and killing child processes. This is what I have for now:
volatile sig_atomic_t keep_going = 1;
/* The signal handler just clears the flag and re-enables itself. */
void
catch_alarm (int sig)
{
printf("Alarm went off");
exit(EXIT_SUCCESS);
}
void function1
{}
void forkingfunction()
{
or(i=0;i<size;i++,temp++)
{
pid_t pID = vfork();
if (pID == 0) // child
{
printf("\nchild pid %d\n",getpid());
//open some files and read and write
function1();
exit(EXIT_SUCCESS);
kill(pID,SIGKILL);
}
}
else if (pID < 0) // failed to fork
{
perror("Failed to fork:");
}
}
void function2
{
function1();
}
int main()
{
int options
while(options){
switch (options)
{
case 1:
case 2:
}
}
signal (SIGALRM, catch_alarm);
alarm (0.1);//testing for 0.1 seconds
function1();
return 0;
}
there is only a certain set of function which can be executed safely from a signal handler. And exit is not one of them. Neither is printf.
You might able to use the _exit() function instead (with underscore in front). However it will only exit the top-most process, leaving the children running.
You can still kill everything using kill(0, signal), as here.
void catch_alarm (int sig) {
kill(0, SIGTERM);
}
Here is an example of a working poc code:
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <signal.h>
void catch_alarm (int sig) {
kill (0,SIGTERM);
}
void forkingfunction()
{
int i;
for(i=0;i<4;i++,i++) {
pid_t pID = fork();
if (pID == 0) {
sleep(5);
printf("\nchild pid %d\n",getpid());
exit(EXIT_SUCCESS);
}
else if (pID < 0) {
perror("Failed to fork:");
}
}
}
int main()
{
signal (SIGALRM, catch_alarm);
alarm (1);//testing for 0.1 seconds
forkingfunction();
sleep(10);
printf("\nnormal exit\n");
return 0;
}
So after spending more than 24 hrs trying to fix this. The solution was actually simple. Keep a global array of PID of children alive +1 for parent PID. before sending the kill(PID, SIGTERM) we have to mention a handler for SIGTERM which sleeps till the process is killed. Thus a graceful exit.
Related
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <signal.h>
static void sigusr(int iSig) //SIGUSER are the user defined signals
{
if (iSig == SIGUSR1)
{
printf("Received SIGUSR1 signal, going to sleep for 2 seconds\n");
sleep(2);
}
}
int main ()
{
int pid;
signal(SIGUSR1, sigusr);
pid = fork();
if (pid > 0) //Parent process created
{
for(int i=0; i<=1000;i++)
{
printf("%d\n",i);
usleep(70);
}
}
else //Child process created
{
sleep(5);
kill(pid,SIGUSR1);
exit(0);
}
}
Create 2 processes, a parent and a child using fork().
The parent prints the value of ‘i’ from 0 to 1000 and then exits.
Meanwhile the child process sleeps for 5 seconds after it is created,
sends a SIGUSR1 signal to the parent and then exits.
The parent should catch that signal, print on standard output “Received SIGUSR1 signal,
going to sleep for 2 seconds”, sleep for two seconds and then continueprinting the numbers.
But i am unable to continue the process after child process termination.
For me everything works fine if I increase usleep time, without it parent process terminates before child process send signal.
The problem is with kill call, else statement is executed only in child process, so it means that pid value is 0, kill with pid 0 sends signal to whole group, in this case to parent and a child, you should change it to
kill(getppid(), SIGUSR1);
In addition to the #complikator 's answer, you shoudl print and sleep outside the signal handler.
There are remaining questions like "main finishes before the signal is received", but this is really depending on you use case...
would look like this:
#include <stdbool.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <signal.h>
#include <sys/types.h>
#include <sys/wait.h>
volatile static bool called = false;
static void sigusr(int iSig)
{
called = true;
}
void check_signaled(void) {
if (called) {
called = false;
printf("Received SIGUSR1 signal, going to sleep for 2 seconds\n");
sleep(2);
}
}
int main(void)
{
int pid;
pid = fork();
if (pid > 0) //Parent process created
{
signal(SIGUSR1, sigusr);
for(int i=0; i<=1000;i++)
{
check_signaled(); /* if signal come while iterating */
printf("%d\n",i);
usleep(70);
}
wait(NULL); /* wait child completion */
check_signaled(); /* signal may happen "too late" */
}
else //Child process created
{
sleep(1);
kill(getppid(),SIGUSR1);
}
}
I'm trying to learn how to handle signals. In my program I have an array of pids of earlier created subprocesess. No I want to every couple seconds send a sigtstp signal to one of them. He just have to send sigchld to parent process and exit. Parent process should print an exit code of exited process and create next one in the place of exit one. Everything works fine in first loop but it hangs in second. So on output get:
loop
slept
forking
in to array
loop
Zakonczyl sie potomek 3934 z kodem 0.
So it's seems that sleep works in first loop but not in second. Or just main process didn't get back control after handling signal but this should't happen. So I have no idea whats may be wrong here.
while(1) {
printf("loop\n");
sleep(5);
printf("slept\n");
int r = rand() % n;
if(kill(process_tab[r],SIGTSTP) < 0) {
printf("Error while sending sigtstp signal.\n");
} else {
printf("forking\n");
if((child = fork()) < 0) {
printf("Fork failed.\n");
} else if(child == 0) {//to sie dzieje w procesie
if(signal(SIGTSTP,&catch_sigtstp)) {
printf("Error while setting signal handler.\n");
_exit(EXIT_FAILURE);
}
while(1) {
}
} else { //to sie dzieje w parencie
process_tab[r] = child;
printf("in to array\n");
}
}
}
And here are handlers.
void catch_sigtstp(int signal) {
kill(ppid,SIGCHLD);
_exit(EXIT_SUCCESS);
}
void catch_sigchld (int signal) {
int status;
pid_t child = wait(&status);
printf("Zakonczyl sie potomek %d z kodem %d.\n",child,status);
}
Add fflush after printf.
printf("Something\n");
fflush(stdout);
Otherwise you may not get the output as stdio is buffered by default.
Edit: Issues of handler
It is pretty unsafe to use printf function in signal handler, as it is not reentrant. Also, the catch_sigchild function can be modified:
void catch_sigchld (int signal) {
int status;
pid_t child;
while ((child = waitpid(-1, &status, WNOHANG)) > 0)
{
// may be something else?
// ...printf("Zakonczyl sie potomek %d z kodem %d.\n",child,status);
}
}
The reason is that one signal can be delivered for multiple dead children.
Edit: blocking signal when printing.
To avoid deadlock inside stdio, you should block the signal:
sigset_t set;
sigemptyset(&set);
sigaddset(&set, SIGCHILD);
...
sigprocmask(SIG_BLOCK, &set, NULL);
printf("my output");
sigprocmask(SIG_UNBLOCK, &set, NULL);
...
Edit: as #Barmar has pointed, you parent process will receive SIGCHILD signal twice: once from your child'd signal handler, and one from OS.
To fix, it might be sufficient to remove your own signal source:
void catch_sigtstp(int signal) {
// kill(ppid,SIGCHLD); //< This one causes two signals per one child
_exit(EXIT_SUCCESS);
}
I'm doing fork in my main program,and doing exec in the child process which will run another program. Now i want to terminate the child(i.e., the program invoked by exec) and return back to the main program(or parent program). how could i achieve this.. I tried with ctrl+c but its killing parent process and child also.please help me.
/*This is main.c*/
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/types.h>
#include <signal.h>
#include <sys/wait.h>
void sig_int(void);
void sig_term(void);
pid_t pid,ppid;
int main(char argc,char **argv){
int n;
char ch;
printf("***********Application to start or stop services**********\n");
do
{
printf("Enter 1 to start service no.1\n");
printf("Enter 2 to start service no.2\n");
printf("Enter 3 to start service no.3\n");
scanf("%d",&n);
if(fork() == 0)
{
switch(n)
{
case 1: printf("starting service no. 1..\n");
printf("checking whether the given service is already running...\n");
// system("./det.sh ./test")
pid = getpid();
printf("child process pid = %d\n",pid);
// signal(SIGINT,(void *)sig_int);
// signal(SIGTERM,(void *)sig_term);
//execl("/var/vR_main","vR_main",argv[1],argv[2],argv[3],argv[4],NULL);
execl("./test","test",0,0);//will run test.c
break;
case 2: printf("starting service no. 2..\n");
break;
case 3: printf("starting service no. 3..\n");
break;
}
}
else
{
int status;
wait(&status);
if (WIFEXITED(status))
printf("CHILD exited with %d\n", WEXITSTATUS(status));
if (WIFSIGNALED(status))
printf("signaled by %d\n", WTERMSIG(status));
if (WIFSTOPPED(status))
printf("stopped by %d\n", WSTOPSIG(status));
// sleep(2);
ppid = getpid();
printf("%d\n",ppid);
// wait();
printf("\nDo you want to continue...y/n:");
scanf(" %c",&ch);
}
}while(ch == 'y');
return 0;
}
void sig_int(void)
{
printf("caught signal\n");
kill(pid,SIGKILL);
// signal(SIGINT,SIG_DFL);
// exit(0);
}
void sig_term(void)
{
printf("killing the process\n");
signal(SIGINT,SIG_DFL);
// exit(0);
}
/*This is test.c*/
#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>
#include <unistd.h>
#include <signal.h>
void sig_int(void);
void sig_term(void);
pid_t pid;
int main()
{
// int a=10,b=40,c=50,max;
pid = getpid();
printf("exec pid = %d\n",pid);
while (1)
{
signal(SIGINT,(void *)sig_int);
signal(SIGTERM,(void *)sig_term);
}
// max=a>b?a>c?a:c:b>c?b:c;
// printf("%d\n",max);
}
void sig_int(void)
{
printf("caught signal\n");
// signal(SIGINT,SIG_DFL);
kill(pid,SIGKILL);
// exit(0);
}
void sig_term(void)
{
printf("killing the process\n");
signal(SIGINT,SIG_DFL);
// exit(0);
}
Now I want to kill "test application" (invoked by exec),and return to the parent process or the "else block" to continue the program.
You need to do the following:
Do a kill(pid, SIGTERM) first - this gives the child process an opportunity to terminate gracefully
Wait a period of time (use sleep). The period of time depends on the time the child process takes to close down gracefully.
Use waitpid(pid, &status, WNOHANG) checking the return value. If the process has not aborted do step 4
Do a kill(pid, SIGKILL) then harvest the zombie by doing waitpid(pid, &status, 0).
These steps ensure that you give the child process to have a signal handler to close down and also ensures that you have no zombie processes.
Either in or outside your program, it is possible to use kill. By including <signal.h>, you can kill a process with a given PID (use the fork return value to do this).
#include <signal.h>
int pid;
switch (pid = fork())
{
case -1:
/* some stuff */
break;
case 0:
/* some stuff */
break;
default:
/* some stuff */
kill(pid, SIGTERM);
}
It is also possible to use kill command in the shell. To find the PID of your child process, you can run ps command.
man kill
The kill() function shall send a signal to a process or a group of processes specified by pid. The signal to be sent is specified by sig and is either one from the list given in <signal.h> or 0. If sig is 0 (the null signal), error checking is performed but no signal is actually sent. The null signal can be used to check the validity of pid.
POSIX defines the kill(2) system call for this:
kill(pid, SIGKILL);
I have the following code which create a child fork. And I want to kill the child before it finish its execution in the parent. how to do it?
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
int i;
main (int ac, char **av)
{
int pid;
i = 1;
if ((pid = fork()) == 0) {
/* child */
while (1) {
printf ("I m child\n");
sleep (1);
}
}
else {
/* Error */
perror ("fork");
exit (1);
}
sleep (10)
// TODO here: how to add code to kill child??
}
See kill system call. Usually a good idea to use SIGTERM first to give the process an opportunity to die gratefully before using SIGKILL.
EDIT
Forgot you need to use waitpid to get the return status of that process and prevent zombie processes.
A FURTHER EDIT
You can use the following code:
kill(pid, SIGTERM);
bool died = false;
for (int loop; !died && loop < 5 /*For example */; ++loop)
{
int status;
pid_t id;
sleep(1);
if (waitpid(pid, &status, WNOHANG) == pid) died = true;
}
if (!died) kill(pid, SIGKILL);
It will give the process 5 seconds to die gracefully
Send a signal.
#include <sys/types.h>
#include <signal.h>
kill(pid, SIGKILL);
/* or */
kill(pid, SIGTERM);
The second form preferable, among other, if you'll handle signals by yourself.
Issue kill(pid, SIGKILL) from out of the parent.
I'm having trouble with a process forking exercise. I want to fork a child process and have it hang after announcing it has been forked, and wait for a signal to terminate, after which the parent process must announce it is terminating and then exit.
I can get the processes forked and have the parent wait for the hanging child to be killed by the signal, but it seems to kill the parent as well. I tried killing the child process specifically by its PID, but with no success.
Thanks for any help!
Code:
#include <stdio.h>
#include <signal.h>
#include <unistd.h>
#include <stdlib.h>
#include <sys/wait.h>
#include <sys/types.h>
void catchInt (int signum)
{
printf("\nMy sincerest apologies, master\n");
/*kill(0, SIGINT);*/
exit(0);
}
void ignoreInt (int signum)
{
wait(NULL);
}
int main () {
pid_t pid;
/* fork process */
pid = fork();
if (pid < 0) /* error handler */
{
fprintf(stderr, "Fork Failed");
exit(-1);
}
else if (pid == 0) /* child */
{
printf("Child reporting in\n");
signal(SIGINT, catchInt);
for ( ;; )
pause();
}
else /* parent */
{
/* parent will wait for the child to complete */
signal(SIGINT, ignoreInt);
wait(NULL);
printf("You're welcome\n");
exit(0);
}
}
Even assuming you fix the code so it compiles (you've not defined tempPID), there are problems:
You set the child to go to sleep until a signal arrives.
You set the parent to wait until the child dies.
So, you have a state where neither process is going to do anything more.
You probably need the parent to send a signal to the child:
kill(pid, SIGINT);
It is not clear that you need the parent to set a signal handler.
You probably want the child to set a signal handler.
You probably don't want the infinite loop in the child.
Oh, and void main() is incorrect - int main() or int main(void) or int main(int argc, char **argv) are the approved declarations for main().
And it is tidier if you return a value (0) from main(). The C99 standard does permit you to drop off the end of main() and will treat that as returning zero, but only if the function is properly declared as returning an int.
The header for wait() and relatives in POSIX is <sys/wait.h>.
And, because I'm a sucker, here's code that compiles and might even do what you want:
#include <stdio.h>
#include <signal.h>
#include <unistd.h> /* getpid() */
#include <stdlib.h>
#include <sys/wait.h>
void catchInt(int signum)
{
printf("Child's PID is %d\n", (int)getpid());
printf("My sincerest apologies, master\n");
exit(1);
}
int main()
{
pid_t pid = fork();
if (pid < 0) /* error handler */
{
fprintf(stderr, "Fork Failed");
exit(-1);
}
else if (pid == 0) /* child */
{
printf("Child reporting in\n");
signal(SIGINT, catchInt);
pause();
}
else /* parent */
{
sleep(1);
kill(pid, SIGINT);
wait(NULL);
printf("You're welcome\n");
}
return(0);
}
Just figured out what I was doing wrong, I should have realized SIGINT is sent to every process, and so the parent was simply being sent an unhandled SIGINT, causing it to exit. Thanks for all the help (my apologies on the sloppy coding, I really shouldn't wait until the program is completed to clean that up), the code's been edited above and works as intended.
Thanks again.