I have a program(A) that starts another program(B).
What I want is when every time B receives signal A sends this signal to B and all child processes of B. I don't really know how to implement a few things here:
1). How do I determine that signal was sent to B?
2). How do I save this signal in variable?
3). How do I loop until B is alive?
int main() {
pid_t pid = fork();
int32_t num = 0;
if (pid == 0) {
static char *argv[] = {"main", NULL};
execv(argv[0], argv); //start program B
}
else{
while(/*B is alive*/){
//if program B receives signal
//I want to send this signal to B and all child processes,
//cause B doesn't handle any signals
if (/*B receives signal*/){
//save this signal to num.
kill(pid, num); //???
//send signal to parent
//useless cause it was already send to B?
fp = popen((("pgrep -P ") + string(num)).c_str(), "r");
//pgrep all child processes
std::vector<int> children;
while (fgets(buf, 128, fp) != NULL) //getting child pid
children.push_back(stoi(string(buf)));
for(auto a : children)
kill(a, num); //send signal to child
}
}
}
return 0;
}
I am afraid your question is really too broad and it involves too many topics. I will try anyway to help if possible.
About Signal handling. I usually spwan a separate thread in my program that is just dedicated to signal handling. In this way, I won't "disturb" the main execution.
About how to handle signals, please have a look to this code snippet:
void * threadSignalHandler (){
int err, signo;
for (;;) {
err = sigwait(&mask, &signo);
if (err != 0) {
syslog(LOG_ERR, "sigwait failed");
exit(1);
}
switch (signo) {
case SIGHUP:
//Do your stuff here
break;
case SIGTERM:
//Do your stuff here
break;
default:
syslog(LOG_INFO, "unexpected signal %d\n", signo);
break;
}
}
return(0);
}
Again, as exaplined, I usually spawn a new basic thread and I do it with in this way:
int err;
pthread_t tid;
/*
* Restore SIGHUP default and block all signals.
*/
sa.sa_handler = SIG_DFL;
sigemptyset(&sa.sa_mask);
sa.sa_flags = 0;
if (sigaction(SIGHUP, &sa, NULL) < 0)
err_quit("%s: can′t restore SIGHUP default");
sigfillset(&mask);
if ((err = pthread_sigmask(SIG_BLOCK, &mask, NULL)) != 0)
err_exit(err, "SIG_BLOCK error");
/*
* Create a thread to handle SIGHUP and SIGTERM.
*/
err = pthread_create(&tid, NULL, threadSignalHandler, 0);
if (err != 0)
err_exit(err, "can′t create thread");
So, to answer your 3 questions:
A) Use the code I provided, it is tested and I know it works.
B) Just
modify the thread handler to store the signal received (variable
signo)
C) Please have a look here, there are consolidated ways to do
it, according to posix standards
(Check if process exists given its pid)
I'm trying to make a program that forks once and while the parent waits for the child terminates, this child forks again and then executes two execs. There is a Pipe on the program and I've checked the return values of every dup2() and pipe() on the program -just omitted them here to make it looks more concise-. The problem is that I only get the result of ls -a | sort -r AFTER the program finishes.
The code is:
#include <cstdio>
#include <cstring>
#include <sys/wait.h>
#include <sys/types.h>
#include <unistd.h>
#include <stdlib.h>
#include <errno.h>
int main(int argc, char *argv[]) {
printf("Shell> \n"); fflush(stdout);
pid_t pid1;
pid_t pid2;
int status = 0;
int fd[2];
if(pipe(fd) < 0) {
printf("FATAL ERROR.\n");
}
pid1 = fork();
if(pid1 > 0) { // Parent
waitpid(pid1, &status, 0);
printf("\t\t------PID1 Complete-------\n\n");
}
else { // Child
if(pid1 == 0) {
printf("ON CHILD\n");
pid2 = fork();
if(pid2 > 0) { // Child -> Parent
printf("ON CHILD-Parent\n");
close(fd[1]);
dup2(fd[0], STDIN_FILENO);
waitpid(pid2, &status, 0);
printf("ON CHILD-Parent after wait\n");
execlp("sort", "sort", "-r", NULL);
perror("Problem with execlp\n");
exit(1);
}
else { // Child -> Child
printf("ON CHILD->Child\n");
close(fd[0]);
dup2(fd[1], STDOUT_FILENO);
execlp("ls", "ls", "-a", NULL);
perror("Problem with execvp\n");
exit(1);
}
} // End of if(pid1 == 0)
} // End of Child
printf("\nEnd of program.\n");
return 0;
}
My current output is:
Shell>
ON CHILD
ON CHILD-Parent
ON CHILD->Child
ON CHILD-Parent after wait
I think the problem is on the waits, but I just can't figure out how to make this work. Any ideas? Thanks!
The problem is that you call pipe in the grandparent process. After the grandchild process (ls -a) exits, the parent process (sort -r) blocks indefinitely waiting to read more input from the pipe since some process - the grandparent - holds an open descriptor to the write end of the pipe.
If you close the pipe descriptors in the grandparent process, or better yet move the pipe call into the first forked process, then the sort process will terminate when the last process with an open descriptor for the write end of the pipe exits (DEMO):
int main() {
// Turn off buffering of stdout, to help with debugging
setvbuf(stdout, NULL, _IONBF, 0);
printf("Shell> \n");
pid_t pid1 = fork();
if(pid1 < 0) {
perror("fork failed");
}
if(pid1 > 0) { // Parent
int status;
waitpid(pid1, &status, 0);
printf("\t\t------PID1 Complete (%d) -------\n\n", status);
} else { // Child
printf("ON CHILD\n");
int fd[2];
if(pipe(fd) < 0) {
perror("pipe failed");
return 1;
}
pid_t pid2 = fork();
if(pid2 < 0) {
perror("fork failed");
}
if(pid2 > 0) { // Child -> Parent
printf("ON CHILD-Parent\n");
close(fd[1]);
dup2(fd[0], STDIN_FILENO);
execlp("sort", "sort", "-r", NULL);
perror("Problem with execlp");
return 1;
} else { // Child -> Child
printf("ON CHILD->Child\n");
close(fd[0]);
dup2(fd[1], STDOUT_FILENO);
execlp("ls", "ls", "-a", NULL);
perror("Problem with execvp");
return 1;
}
}
printf("\nEnd of program.\n");
}
The other problem with the program is the one #nategoose commented on: the call to waitpid could lead to a deadlock if the output of "ls -a" is too large to fit in the pipe's buffer. There's no reason to wait, so it should simply be eliminated.
This isn't a real answer, but I have some into that I'd like to share.
To make sure that your output comes out in the order that it should, I'm flushing a lot more than you were. Remember that when you are calling functions like fork(), clone(), vfork(), dup(), dup2(), close(), or any of the exec() family of functions you are doing stuff that is BELOW the C runtime environment, which includes stdio. If you do:
printf("cat");
fork();
fflush(stdout);
You are very likely to get:
catcat
as your output because you've duplicated the stdout structure, including all buffered data, so unless stdio decided that it was time to flush anyway before the end of the printf function, then "cat" is in each process's stdout buffer.
There's also the fact that since data can stay buffered when you run a function in the exec family your data may not be flushed before your program is replaced with the new program. When your program is replaced by ls or sort then any data pending in stdout gets lost forever.
Also, when you use dup you have the another issue since you are swapping the file descriptor out from under stdio so it may not have flushed yet and the data may end up getting flushed to the new file after the dup.
Because of these things you should have a lot more calls to fflush, but I don't think that's your problem here.
I have a problem. My program includes 10 TCP Server at the sam etime. As soon as a request from a client is noticed, the appropriate tcp server socket will accept the connection and handle it in a seperate thread. I know this is not the most efficient way of solving my actual problem but okay..
In main I have a for loop that will call a function in an object called Peer which is StartThread()
for (it= ListOfPeers.begin();it!= ListOfPeers.end();it++)
{
(*it).second->StartThread();
}
Of cause there are some conditions that this loop will be used but i wanted to narrow the code as much down as possible.
The function StartThread will be called in each peer object:
void StartThread()
{
pthread_t threadDoEvent;
pthread_create( &threadDoEvent, NULL, &DoEvent_helper,this);
pthread_detach(threadDoEvent);
}
void *DoEvent_helper( void *ptr ) // Helper to implement thread
{
return ((Peer *)ptr)->DoEvent();
}
DoEvent is the function which will handle the request and connection:
void* DoEvent()
{
unsigned char buffer[1024];
int rc;
int close_conn= FALSE;
int new_sd;
new_sd = accept(Socket, NULL, NULL);
if (new_sd < 0)
{
if (errno != EWOULDBLOCK)
{
perror(" accept() failed");
close_conn = TRUE;
}
}
do
{
rc = recv(new_sd, buffer, sizeof(buffer), 0);
if (rc < 0)
{
if (errno != EWOULDBLOCK)
{
perror(" recv() failed");
close_conn = TRUE;
}
break;
}
if (rc == 0)
{
printf(" Connection closed\n");
close_conn = TRUE;
break;
}
[DO SOMETHING WITH THE BUFFER]
rc = send(new_sd, buffer, len, 0);
if (rc <= 0)
{
perror(" send() failed");
close_conn = TRUE;
break;
}
}while (close_conn==FALSE);
close(new_sd);
}
My Question is why am I receiving an error : recv() failed: Bad file descriptor????
When I am adding a sleep(1) in between pthread_create and pthread_detach, everything is working!
Can somebody explain this circumstances to me? or maybe help me solving my problem?
Thanks!
Well you do try to receive even if accept fails. This leads me to believe that you haven't properly set up the passive Socket file descriptor properly before trying to accept from it.
If you set up the Socket descriptor in the main thread, then it could be that the new thread starts and runs before you do that.
Make sure new_sd is in persistent memory. e.g if it is allocated by a function that launches the thread and exits, then new_sd will be pointing to junk. (at least that was the case for me)
Imagine I have a process that starts several child processes. The parent needs to know when a child exits.
I can use waitpid, but then if/when the parent needs to exit I have no way of telling the thread that is blocked in waitpid to exit gracefully and join it. It's nice to have things clean up themselves, but it may not be that big of a deal.
I can use waitpid with WNOHANG, and then sleep for some arbitrary time to prevent a busy wait. However then I can only know if a child has exited every so often. In my case it may not be super critical that I know when a child exits right away, but I'd like to know ASAP...
I can use a signal handler for SIGCHLD, and in the signal handler do whatever I was going to do when a child exits, or send a message to a different thread to do some action. But using a signal handler obfuscates the flow of the code a little bit.
What I'd really like to do is use waitpid on some timeout, say 5 sec. Since exiting the process isn't a time critical operation, I can lazily signal the thread to exit, while still having it blocked in waitpid the rest of the time, always ready to react. Is there such a call in linux? Of the alternatives, which one is best?
EDIT:
Another method based on the replies would be to block SIGCHLD in all threads with pthread \ _sigmask(). Then in one thread, keep calling sigtimedwait() while looking for SIGCHLD. This means that I can time out on that call and check whether the thread should exit, and if not, remain blocked waiting for the signal. Once a SIGCHLD is delivered to this thread, we can react to it immediately, and in line of the wait thread, without using a signal handler.
Don't mix alarm() with wait(). You can lose error information that way.
Use the self-pipe trick. This turns any signal into a select()able event:
int selfpipe[2];
void selfpipe_sigh(int n)
{
int save_errno = errno;
(void)write(selfpipe[1], "",1);
errno = save_errno;
}
void selfpipe_setup(void)
{
static struct sigaction act;
if (pipe(selfpipe) == -1) { abort(); }
fcntl(selfpipe[0],F_SETFL,fcntl(selfpipe[0],F_GETFL)|O_NONBLOCK);
fcntl(selfpipe[1],F_SETFL,fcntl(selfpipe[1],F_GETFL)|O_NONBLOCK);
memset(&act, 0, sizeof(act));
act.sa_handler = selfpipe_sigh;
sigaction(SIGCHLD, &act, NULL);
}
Then, your waitpid-like function looks like this:
int selfpipe_waitpid(void)
{
static char dummy[4096];
fd_set rfds;
struct timeval tv;
int died = 0, st;
tv.tv_sec = 5;
tv.tv_usec = 0;
FD_ZERO(&rfds);
FD_SET(selfpipe[0], &rfds);
if (select(selfpipe[0]+1, &rfds, NULL, NULL, &tv) > 0) {
while (read(selfpipe[0],dummy,sizeof(dummy)) > 0);
while (waitpid(-1, &st, WNOHANG) != -1) died++;
}
return died;
}
You can see in selfpipe_waitpid() how you can control the timeout and even mix with other select()-based IO.
Fork an intermediate child, which forks the real child and a timeout process and waits for all (both) of its children. When one exits, it'll kill the other one and exit.
pid_t intermediate_pid = fork();
if (intermediate_pid == 0) {
pid_t worker_pid = fork();
if (worker_pid == 0) {
do_work();
_exit(0);
}
pid_t timeout_pid = fork();
if (timeout_pid == 0) {
sleep(timeout_time);
_exit(0);
}
pid_t exited_pid = wait(NULL);
if (exited_pid == worker_pid) {
kill(timeout_pid, SIGKILL);
} else {
kill(worker_pid, SIGKILL); // Or something less violent if you prefer
}
wait(NULL); // Collect the other process
_exit(0); // Or some more informative status
}
waitpid(intermediate_pid, 0, 0);
Surprisingly simple :)
You can even leave out the intermediate child if you're sure no other module in the program is spwaning child processes of its own.
This is an interesting question.
I found sigtimedwait can do it.
EDIT 2016/08/29:
Thanks for Mark Edington's suggestion. I'v tested your example on Ubuntu 16.04, it works as expected.
Note: this only works for child processes. It's a pity that seems no equivalent way of Window's WaitForSingleObject(unrelated_process_handle, timeout) in Linux/Unix to get notified of unrelated process's termination within timeout.
OK, Mark Edington's sample code is here:
/* The program creates a child process and waits for it to finish. If a timeout
* elapses the child is killed. Waiting is done using sigtimedwait(). Race
* condition is avoided by blocking the SIGCHLD signal before fork().
*/
#include <sys/types.h>
#include <sys/wait.h>
#include <signal.h>
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <unistd.h>
#include <errno.h>
static pid_t fork_child (void)
{
int p = fork ();
if (p == -1) {
perror ("fork");
exit (1);
}
if (p == 0) {
puts ("child: sleeping...");
sleep (10);
puts ("child: exiting");
exit (0);
}
return p;
}
int main (int argc, char *argv[])
{
sigset_t mask;
sigset_t orig_mask;
struct timespec timeout;
pid_t pid;
sigemptyset (&mask);
sigaddset (&mask, SIGCHLD);
if (sigprocmask(SIG_BLOCK, &mask, &orig_mask) < 0) {
perror ("sigprocmask");
return 1;
}
pid = fork_child ();
timeout.tv_sec = 5;
timeout.tv_nsec = 0;
do {
if (sigtimedwait(&mask, NULL, &timeout) < 0) {
if (errno == EINTR) {
/* Interrupted by a signal other than SIGCHLD. */
continue;
}
else if (errno == EAGAIN) {
printf ("Timeout, killing child\n");
kill (pid, SIGKILL);
}
else {
perror ("sigtimedwait");
return 1;
}
}
break;
} while (1);
if (waitpid(pid, NULL, 0) < 0) {
perror ("waitpid");
return 1;
}
return 0;
}
If your program runs only on contemporary Linux kernels (5.3 or later), the preferred way is to use pidfd_open (https://lwn.net/Articles/789023/ https://man7.org/linux/man-pages/man2/pidfd_open.2.html).
This system call returns a file descriptor representing a process, and then you can select, poll or epoll it, the same way you wait on other types of file descriptors.
For example,
int fd = pidfd_open(pid, 0);
struct pollfd pfd = {fd, POLLIN, 0};
poll(&pfd, 1, 1000) == 1;
The function can be interrupted with a signal, so you could set a timer before calling waitpid() and it will exit with an EINTR when the timer signal is raised. Edit: It should be as simple as calling alarm(5) before calling waitpid().
I thought that select will return EINTR when SIGCHLD signaled by on of the child.
I belive this should work:
while(1)
{
int retval = select(0, NULL, NULL, NULL, &tv, &mask);
if (retval == -1 && errno == EINTR) // some signal
{
pid_t pid = (waitpid(-1, &st, WNOHANG) == 0);
if (pid != 0) // some child signaled
}
else if (retval == 0)
{
// timeout
break;
}
else // error
}
Note: you can use pselect to override current sigmask and avoid interrupts from unneeded signals.
Instead of calling waitpid() directly, you could call sigtimedwait() with SIGCHLD (which would be sended to the parent process after child exited) and wait it be delived to the current thread, just as the function name suggested, a timeout parameter is supported.
please check the following code snippet for detail
static bool waitpid_with_timeout(pid_t pid, int timeout_ms, int* status) {
sigset_t child_mask, old_mask;
sigemptyset(&child_mask);
sigaddset(&child_mask, SIGCHLD);
if (sigprocmask(SIG_BLOCK, &child_mask, &old_mask) == -1) {
printf("*** sigprocmask failed: %s\n", strerror(errno));
return false;
}
timespec ts;
ts.tv_sec = MSEC_TO_SEC(timeout_ms);
ts.tv_nsec = (timeout_ms % 1000) * 1000000;
int ret = TEMP_FAILURE_RETRY(sigtimedwait(&child_mask, NULL, &ts));
int saved_errno = errno;
// Set the signals back the way they were.
if (sigprocmask(SIG_SETMASK, &old_mask, NULL) == -1) {
printf("*** sigprocmask failed: %s\n", strerror(errno));
if (ret == 0) {
return false;
}
}
if (ret == -1) {
errno = saved_errno;
if (errno == EAGAIN) {
errno = ETIMEDOUT;
} else {
printf("*** sigtimedwait failed: %s\n", strerror(errno));
}
return false;
}
pid_t child_pid = waitpid(pid, status, WNOHANG);
if (child_pid != pid) {
if (child_pid != -1) {
printf("*** Waiting for pid %d, got pid %d instead\n", pid, child_pid);
} else {
printf("*** waitpid failed: %s\n", strerror(errno));
}
return false;
}
return true;
}
Refer: https://android.googlesource.com/platform/frameworks/native/+/master/cmds/dumpstate/DumpstateUtil.cpp#46
If you're going to use signals anyways (as per Steve's suggestion), you can just send the signal manually when you want to exit. This will cause waitpid to return EINTR and the thread can then exit. No need for a periodic alarm/restart.
Due to circumstances I absolutely needed this to run in the main thread and it was not very simple to use the self-pipe trick or eventfd because my epoll loop was running in another thread. So I came up with this by scrounging together other stack overflow handlers. Note that in general it's much safer to do this in other ways but this is simple. If anyone cares to comment about how it's really really bad then I'm all ears.
NOTE: It is absolutely necessary to block signals handling in any thread save for the one you want to run this in. I do this by default as I believe it messy to handle signals in random threads.
static void ctlWaitPidTimeout(pid_t child, useconds_t usec, int *timedOut) {
int rc = -1;
static pthread_mutex_t alarmMutex = PTHREAD_MUTEX_INITIALIZER;
TRACE("ctlWaitPidTimeout: waiting on %lu\n", (unsigned long) child);
/**
* paranoid, in case this was called twice in a row by different
* threads, which could quickly turn very messy.
*/
pthread_mutex_lock(&alarmMutex);
/* set the alarm handler */
struct sigaction alarmSigaction;
struct sigaction oldSigaction;
sigemptyset(&alarmSigaction.sa_mask);
alarmSigaction.sa_flags = 0;
alarmSigaction.sa_handler = ctlAlarmSignalHandler;
sigaction(SIGALRM, &alarmSigaction, &oldSigaction);
/* set alarm, because no alarm is fired when the first argument is 0, 1 is used instead */
ualarm((usec == 0) ? 1 : usec, 0);
/* wait for the child we just killed */
rc = waitpid(child, NULL, 0);
/* if errno == EINTR, the alarm went off, set timedOut to true */
*timedOut = (rc == -1 && errno == EINTR);
/* in case we did not time out, unset the current alarm so it doesn't bother us later */
ualarm(0, 0);
/* restore old signal action */
sigaction(SIGALRM, &oldSigaction, NULL);
pthread_mutex_unlock(&alarmMutex);
TRACE("ctlWaitPidTimeout: timeout wait done, rc = %d, error = '%s'\n", rc, (rc == -1) ? strerror(errno) : "none");
}
static void ctlAlarmSignalHandler(int s) {
TRACE("ctlAlarmSignalHandler: alarm occured, %d\n", s);
}
EDIT: I've since transitioned to using a solution that integrates well with my existing epoll()-based eventloop, using timerfd. I don't really lose any platform-independence since I was using epoll anyway, and I gain extra sleep because I know the unholy combination of multi-threading and UNIX signals won't hurt my program again.
I can use a signal handler for SIGCHLD, and in the signal handler do whatever I was going to do when a child exits, or send a message to a different thread to do some action. But using a signal handler obfuscates the flow of the code a little bit.
In order to avoid race conditions you should avoid doing anything more complex than changing a volatile flag in a signal handler.
I think the best option in your case is to send a signal to the parent. waitpid() will then set errno to EINTR and return. At this point you check for waitpid return value and errno, notice you have been sent a signal and take appropriate action.
If a third party library is acceptable then the libkqueue project emulates kqueue (the *BSD eventing system) and provides basic process monitoring with EVFILT_PROC + NOTE_EXIT.
The main advantages of using kqueue or libkqueue is that it's cross platform, and doesn't have the complexity of signal handling. If your program is utilises async I/O you may also find it a lower friction interface than using something like epoll and the various *fd functions (signalfd, eventfd, pidfd etc...).
#include <stdio.h>
#include <stdint.h>
#include <sys/event.h> /* kqueue header */
#include <sys/types.h> /* for pid_t */
/* Link with -lkqueue */
int waitpid_timeout(pid_t pid, struct timespec *timeout)
{
struct kevent changelist, eventlist;
int kq, ret;
/* Populate a changelist entry (an event we want to be notified of) */
EV_SET(&changelist, pid, EVFILT_PROC, EV_ADD, NOTE_EXIT, 0, NULL);
kq = kqueue();
/* Call kevent with a timeout */
ret = kevent(kq, &changelist, 1, &eventlist, 1, timeout);
/* Kevent returns 0 on timeout, the number of events that occurred, or -1 on error */
switch (ret) {
case -1:
printf("Error %s\n", strerror(errno));
break;
case 0:
printf("Timeout\n");
break;
case 1:
printf("PID %u exited, status %u\n", (unsigned int)eventlist.ident, (unsigned int)eventlist.data);
break;
}
close(kq);
return ret;
}
Behind the scenes on Linux libkqueue uses either pidfd on Linux kernels >= 5.3 or a waiter thread that listens for SIGCHLD and notifies one or more kqueue instances when a process exits. The second approach is not efficient (it scans PIDs that interest has been registered for using waitid), but that doesn't matter unless you're waiting on large numbers of PIDs.
EVFILT_PROC support has been included in kqueue since its inception, and in libkqueue since v2.5.0.