What's the proper way to log a shutdown message when an application (a C++ daemon, in my case) receives a SIGTERM or SIGINT?
According to CERT and the signal(7) manpage, many functions (including, presumably, those used by most logging libraries) aren't safe to call from signal handlers.
Vlad Lazarenko wrote a great blog post earlier this year on this very topic. On Linux it boils down to creating a signal descriptor with signalfd(2) and use an event loop such as poll(2) or epoll_wait(2). Here is Vlad's example reading from the descriptor
#include <sys/signalfd.h>
#include <signal.h>
#include <unistd.h>
#include <stdlib.h>
#include <stdio.h>
#define handle_error(msg) \
do { perror(msg); exit(EXIT_FAILURE); } while (0)
int
main(int argc, char *argv[])
{
sigset_t mask;
int sfd;
struct signalfd_siginfo fdsi;
ssize_t s;
sigemptyset(&mask);
sigaddset(&mask, SIGINT);
sigaddset(&mask, SIGQUIT);
/* Block signals so that they aren't handled
according to their default dispositions */
if (sigprocmask(SIG_BLOCK, &mask, NULL) == -1)
handle_error("sigprocmask");
sfd = signalfd(-1, &mask, 0);
if (sfd == -1)
handle_error("signalfd");
for (;;) {
s = read(sfd, &fdsi, sizeof(struct signalfd_siginfo));
if (s != sizeof(struct signalfd_siginfo))
handle_error("read");
if (fdsi.ssi_signo == SIGINT) {
printf("Got SIGINT\n");
} else if (fdsi.ssi_signo == SIGQUIT) {
printf("Got SIGQUIT\n");
exit(EXIT_SUCCESS);
} else {
printf("Read unexpected signal\n");
}
}
}
This example can easily be extended to integrate into an event loop.
Logging could be done not from handler, but after it:
int received_sigterm = 0;
void
sigterm_handler(int sig)
{
received_sigterm = 1;
}
void
loop(void)
{
for(;;) {
sleep(1);
if (received_sigterm)
log("finish\n");
}
}
int
main()
{
log("start\n");
signal(SIGTERM, sigterm_handler);
loop();
}
The concept is borrowed from openssh-6.1 sources.
Related
I am learning about signals and wrote a simple programs that plays with them.
So i am inputting a number then using fork i create a process.The parent process is supposed to send the number as a signal to the child process,then the child_signal handler is supposed to send back the number squared as a signal.
This is the code.
#include <iostream>
#include <signal.h>
#include <sys/types.h>
#include <unistd.h>
#include <sys/wait.h>
#include <string.h>
#include <errno.h>
using namespace std;
void child_handler(int sig_num){
cout<<"Child recieved a signal"<<endl;
pid_t ppid = getppid();
if(kill(ppid,sig_num*sig_num) == -1){
cout<<"Childs signal handler failed to send a signal "<<endl;
}
cout<<"Sent a sgnal to the parent"<<endl;
return;
}
void parent_handler(int sig_num){
cout<<"Parent recieved a signal "<<endl;
cout<<sig_num<<endl;
return;
}
int main(){
int n;
cin>>n;
pid_t pid = fork();
if(pid != 0){
struct sigaction sa2;
memset(&sa2,0,sizeof(sa2));
sa2.sa_handler = parent_handler;
if(sigaction(n,&sa2,NULL) == -1){
cout<<"Parents sigaction failed "<<endl;
}
if(kill(pid,n) == -1){
cout<<"Kill failed "<<endl;
}
cout<<"Sent a signal to the child"<<endl;
waitpid(pid,0,0);
}
else{
struct sigaction sa1;
memset(&sa1,0,sizeof(sa1));
sa1.sa_handler = child_handler;
if(sigaction(n,&sa1,NULL) == -1){
cout<<"Childs sigaction failed eerno:"<<errno<<endl;
}
sleep(20);
return 0;
}
return 0;
}
The output is this.
Sent a signal to the child.
And it doesn't say anything about sigaction.
In your code a child process can receive a signal before setting a handler.
When the following C program is executed, and SIGUSR1 is sent to the running process repeatedly, the pclose() call will sometimes return 13. 13 corresponds to SIGPIPE on my system.
Why does this happen?
I am using while true; do kill -SIGUSR1 <process-id>; done to send SIGUSR1 to the program. The program is executed on Ubuntu 14.04.
#include <pthread.h>
#include <signal.h>
#include <unistd.h>
#include <stdio.h>
void handler(int i) {}
void* task(void*)
{
FILE *s;
char b [BUFSIZ];
while (1) {
if ((s = popen("echo hello", "r")) == NULL) {
printf("popen() failed\n");
}
while (fgets(b, BUFSIZ, s) != NULL) ;
if (int r = pclose(s)) {
printf("pclose() failed (%d)\n", r);
}
}
return 0;
}
int main(int argc, char **argv)
{
struct sigaction action;
action.sa_handler = handler;
sigemptyset(&action.sa_mask);
action.sa_flags = 0;
sigaction(SIGUSR1, &action, NULL);
pthread_t tid;
pthread_create(&tid, 0, task, NULL);
pthread_join(tid, NULL);
}
This happens when fgets gets interrupted by the signal. The program doesn't read the pipe to the end and closes it. The other program then SIGPIPEs.
The correct pipe reading operation is:
do {
while (fgets(b, BUFSIZ, s) != NULL) ;
} while (errno == EINTR);
i am running valgrind in a bash script to use it for automated testing. I already added the option to return an exit code on error and to trace children.
/usr/bin/valgrind --error-exitcode=1 --trace-children=yes ./test_prog
My programm forks other processes and I can see the output of valgrind running the different processes in the terminal. The problem is, that the exit code option only seems to work when there is an error in the parent process. Because even though there is an error (SIGSEGV) in one of the child processes the exit code of valgrind is still 0, which means it is useless for the automated testing of several processes.
So is there any option, that would make the parent valgrind catch the error in the child and return it? I already looked into the man page. Maybe there would be another solution to this problem, like grepping the output of the children to the terminal for any error messages?
Thanks in advance.
it's important to implement a proper error handling in the code. Compare following two pieces of code.
A:
#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>
#include <time.h>
#include <string.h>
#include <signal.h>
#define BUFSIZE 1024
int incr=0;
int loop=1;
void runTicks(const char *n) {
time_t t;
char buf[BUFSIZE+1];
pid_t pid;
int counter;
pid=getpid();
counter=0;
while(loop) {
sleep(1);
t=time(NULL);
strftime(buf,BUFSIZE,"%Y.%m.%d %H:%M:%S",localtime(&t));
printf("%s[%d] %s\n",n,pid,buf);
counter+=incr;
if(counter>5) memcpy((void *)1,buf,1); /* this line is for causing SEGV */
}
}
void handler(int s) {
if(s==SIGCHLD) {
printf("Received SIGCHLD\n");
loop=0;
}
}
void setHandler() {
struct sigaction sa;
sa.sa_handler=handler;
sigemptyset(&sa.sa_mask);
sa.sa_flags=SA_NOCLDSTOP;
if(sigaction(SIGCHLD,&sa,NULL)!=0) {
printf("Cannot set signal handler, there is no purpose in running the test\n");
exit(0);
}
}
int main() {
pid_t pid;
printf("start\n");
pid=fork();
if(pid==-1) {
printf("fork failed\n");
exit(10);
}
if(pid==0) {
printf("child\n");
incr=1;
usleep(500000);
runTicks("C");
exit(1);
} else {
printf("parent spawned child pid=%d\n",pid);
setHandler();
runTicks("P");
exit(0);
}
}
B:
#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>
#include <time.h>
#include <string.h>
#include <signal.h>
#include <sys/wait.h>
#define BUFSIZE 1024
int incr=0;
int loop=1;
void runTicks(const char *n) {
time_t t;
char buf[BUFSIZE+1];
pid_t pid;
int counter;
pid=getpid();
counter=0;
while(loop) {
sleep(1);
t=time(NULL);
strftime(buf,BUFSIZE,"%Y.%m.%d %H:%M:%S",localtime(&t));
printf("%s[%d] %s\n",n,pid,buf);
counter+=incr;
if(counter>5) memcpy((void *)1,buf,1); /* this line is for causing SEGV */
}
}
void handler(int s) {
if(s==SIGCHLD) {
int status;
printf("Received SIGCHLD\n");
wait(&status);
printf("Exit code from child: %d\n",status);
if(status!=0) exit(status);
loop=0;
}
}
void setHandler() {
struct sigaction sa;
sa.sa_handler=handler;
sigemptyset(&sa.sa_mask);
sa.sa_flags=SA_NOCLDSTOP;
if(sigaction(SIGCHLD,&sa,NULL)!=0) {
printf("Cannot set signal handler, there is no purpose in running the test\n");
exit(0);
}
}
int main() {
pid_t pid;
printf("start\n");
pid=fork();
if(pid==-1) {
printf("fork failed\n");
exit(10);
}
if(pid==0) {
printf("child\n");
incr=1;
usleep(500000);
runTicks("C");
exit(1);
} else {
printf("parent spawned child pid=%d\n",pid);
setHandler();
runTicks("P");
exit(0);
}
}
Run those first without valgrind and compare the exit code of both programs. Then run them under valgrind and enjoy.
Using such construction you even don't need to run it under valgrind, to catch segfaults from child processes.
Parent receives SIGPIPE sending chars to aborted child process through FIFO pipe.
I am trying to avoid this, using select() function. In the attached sample code,
select() retruns OK even after the child at the other end of pipe having been terminated.
Tested in
RedHat EL5 （Linux 2.6.18-194.32.1.el5）
GNU C Library stable release version 2.5
Any help appreciated. Thnak you.
#include <stdio.h>
#include <stdlib.h>
#include <fcntl.h>
#include <signal.h>
#include <sys/stat.h>
#include <unistd.h>
static void sigpipe_fct();
main()
{
struct stat st;
int i, fd_out, fd_in, child;
char buf[1024];
#define p_out "/tmp/pout"
signal(SIGPIPE, sigpipe_fct);
if (stat(p_out, &st) != 0) {
mknod(p_out, S_IFIFO, 0);
chmod(p_out, 0666);
}
/* start receiving process */
if ((child = fork()) == 0) {
if ((fd_in = open(p_out, O_RDONLY)) < 0) {
perror(p_out);
exit(1);
}
while(1) {
i = read(fd_in, buf, sizeof(buf));
fprintf(stderr, "child %d read %.*s\n", getpid(), i, buf);
lseek(fd_in, 0, 0);
}
}
else {
fprintf(stderr,
"reading from %s - exec \"kill -9 %d\" to test\n", p_out, child);
if ((fd_out = open(p_out, O_WRONLY + O_NDELAY)) < 0) { /* output */
perror(p_out);
exit(1);
}
while(1) {
if (SelectChkWrite(fd_out) == fd_out) {
fprintf(stderr, "SelectChkWrite() success write abc\n");
write(fd_out, "abc", 3);
}
else
fprintf(stderr, "SelectChkWrite() failed\n");
sleep(3);
}
}
}
static void sigpipe_fct()
{
fprintf(stderr, "SIGPIPE received\n");
exit(-1);
}
SelectChkWrite(ch)
int ch;
{
#include <sys/select.h>
fd_set writefds;
int i;
FD_ZERO(&writefds);
FD_SET (ch, &writefds);
i = select(ch + 1, NULL, &writefds, NULL, NULL);
if (i == -1)
return(-1);
else if (FD_ISSET(ch, &writefds))
return(ch);
else
return(-1);
}
From the Linux select(3) man page:
A descriptor shall be considered ready for writing when a call to an
output function with O_NONBLOCK clear would not block, whether or not
the function would transfer data successfully.
When the pipe is closed, it won't block, so it is considered "ready" by select.
BTW, having #include <sys/select.h> inside your SelectChkWrite() function is extremely bad form.
Although select() and poll() are both in the POSIX standard, select() is much older and more limited than poll(). In general, I recommend people use poll() by default and only use select() if they have a good reason. (See here for one example.)
This is my first Program....ctrlcsignal.c
enter code here
#include<stdio.h>
#include<unistd.h>
#include<signal.h>
void signal_handler(int sigNo)
{
//if Ctrl+c signal
if(sigNo==SIGINT){
printf("value of SIGINT:-%d\t",SIGINT);
printf("received SIGINT\n");
}
// if some other signal , but this part wont get executed
// as the signal_handler function is registered with SIGINT only
else
{
printf("Some other signal found");
printf("value of other signal:-%d",sigNo);
}
}
int main(void)
{
//registering the signal handler function with a signal
kill(19574,SIGUSR1);
if(signal(SIGINT,signal_handler)==SIG_ERR)
{
printf("\n can't catch SIGINT\n");
}
while(1) //infinite loop
sleep(1); // 1s ,so that the CPU is not busy
return 0;
}
and this my second program....userdefinedsignals.c
enter code here
#include <stdio.h>
#include <unistd.h>
#include <signal.h>
void signal_handler(int sigNo)
{
printf("function entered...");
// check for userdefined Signal SIGUSR1
if (sigNo == SIGUSR1)
{
printf("received SIGUSR1 with value :- %d",SIGUSR1);
}
//checking for KILL Signal
else if (sigNo == SIGKILL)
{
printf("received SIGKILL with value :- %d",SIGKILL);
}
//checking for STOP Signal
else if (sigNo == SIGSTOP)
{
printf("received SIGSTOP with value :- %d",SIGSTOP);
}
// if some other signal , but this part wont get executed
// as the signal_handler function is registered with SIGINT only
else
{
printf("Some other signal found");
printf("value of other signal:-%d",sigNo);
}
}
int main(void)
{
int pid_t;
printf("process id is %d",getpid());
//registering the signal handler function with a signal
if(signal(SIGUSR1,signal_handler) == SIG_ERR)
{
printf("\n can't catch SIGSIGUSR1\n");
}
if(signal(SIGKILL,signal_handler)==SIG_ERR)
{
printf("\n can't catch SIGKILL\n");
}
if(signal(SIGSTOP,signal_handler)==SIG_ERR)
{
printf("\n can't catch SIGSTOP\n");
}
while(1) //infinite loop
sleep(1); // 1s ,so that the CPU is not busy
return 0;
}
I get the pid of the second process ... suppose xxxx
then i use the following command...
enter code here
kill -USR1 xxxx
but it shows nothing ....
also then i tried by calling the following function int the first program...but no use..
enter code herekill(xxxx,SIGUSR1);
HELP ME..!!!!
Works here.
#include <stdio.h>
#include <unistd.h>
#include <signal.h>
#include <stdarg.h> /* vsnprintf() */
#include <signal.h> /* signal */
void myprintf(FILE *fp, char *fmt, ...)
{
char buff[512];
int rc,fd;
va_list argh;
va_start (argh, fmt);
rc = vsnprintf(buff, sizeof buff, fmt, argh);
if (rc < 0 || rc >= sizeof buff) {
rc = sprintf(buff, "Argh!: %d:\n", rc);
}
if (!fp) fp = stderr;
fd = fileno(fp);
if (fd < 0) return;
if (rc > 0) write(fd, buff, rc);
return;
}
void signal_handler(int sigNo)
{
switch (sigNo ) {
case SIGUSR1:
myprintf(NULL, "received SIGUSR1 with value :- %d\n", SIGUSR1);
break;
case SIGKILL:
myprintf(NULL, "received SIGKILL with value :- %d\n", SIGKILL);
break;
case SIGSTOP:
myprintf(NULL, "received SIGSTOP with value :- %d\n", SIGSTOP);
break;
default:
myprintf(NULL, "Some other signal occured: %d\n", sigNo);
break;
}
return;
}
int main(void)
{
pid_t mypid;
mypid = getpid();
printf("process id is %d\n", (int) mypid);
if(signal(SIGUSR1,signal_handler) == SIG_ERR)
{ printf("\n can't catch SIGSIGUSR1\n"); }
if(signal(SIGKILL,signal_handler)==SIG_ERR)
{ printf("\n can't catch SIGKILL\n"); }
if(signal(SIGSTOP,signal_handler)==SIG_ERR)
{ printf("\n can't catch SIGSTOP\n"); }
if(signal(SIGCONT,signal_handler)==SIG_ERR)
{ printf("\n can't catch SIGCONT\n"); }
while(1) {
sleep(1);
}
return 0;
}
You're catching the signal all right, but not seeing the message because you don't terminate lines properly, and the standard output stream on your system is line buffered (assuming your program runs in a terminal).
Standard C defines three levels of buffering for output streams:
unbuffered, where output is transmitted immediately
line buffered, where output is transmitted when a newline character is encountered
fully buffered, where output is transmitted when an internal buffer fills
(This is a simplification - see a C reference or the Standard for details).
Consider:
#include <stdio.h>
#include <unistd.h>
int main(void)
{
printf("Hello");
pause();
}
This produces no output in a terminal. Fix it by terminating the line:
printf("Hello\n");
This will produce the expected output in a terminal.
If stdout is not connected to a terminal - for example, you redirect to a file - then the stream becomes fully buffered. This:
./a.out > foo
Ctrl-C
cat foo
produces no output, even with the newline character added. Here you need an explicit flush to transmit the output before the buffer is full.
#include <stdio.h>
#include <unistd.h>
int main(void)
{
printf("Hello\n");
fflush(stdout);
pause();
}
This produces output even when redirected to a file.