-
-
Save aspyct/3462238 to your computer and use it in GitHub Desktop.
/** | |
* More info? | |
* a.dotreppe@aspyct.org | |
* http://aspyct.org | |
* | |
* Hope it helps :) | |
*/ | |
#include <stdio.h> | |
#include <stdlib.h> | |
#include <signal.h> // sigaction(), sigsuspend(), sig*() | |
#include <unistd.h> // alarm() | |
void handle_signal(int signal); | |
void handle_sigalrm(int signal); | |
void do_sleep(int seconds); | |
/* Usage example | |
* | |
* First, compile and run this program: | |
* $ gcc signal.c | |
* $ ./a.out | |
* | |
* It will print out its pid. Use it from another terminal to send signals | |
* $ kill -HUP <pid> | |
* $ kill -USR1 <pid> | |
* $ kill -ALRM <pid> | |
* | |
* Exit the process with ^C ( = SIGINT) or SIGKILL, SIGTERM | |
*/ | |
int main() { | |
struct sigaction sa; | |
// Print pid, so that we can send signals from other shells | |
printf("My pid is: %d\n", getpid()); | |
// Setup the sighub handler | |
sa.sa_handler = &handle_signal; | |
// Restart the system call, if at all possible | |
sa.sa_flags = SA_RESTART; | |
// Block every signal during the handler | |
sigfillset(&sa.sa_mask); | |
// Intercept SIGHUP and SIGINT | |
if (sigaction(SIGHUP, &sa, NULL) == -1) { | |
perror("Error: cannot handle SIGHUP"); // Should not happen | |
} | |
if (sigaction(SIGUSR1, &sa, NULL) == -1) { | |
perror("Error: cannot handle SIGUSR1"); // Should not happen | |
} | |
// Will always fail, SIGKILL is intended to force kill your process | |
if (sigaction(SIGKILL, &sa, NULL) == -1) { | |
perror("Cannot handle SIGKILL"); // Will always happen | |
printf("You can never handle SIGKILL anyway...\n"); | |
} | |
if (sigaction(SIGINT, &sa, NULL) == -1) { | |
perror("Error: cannot handle SIGINT"); // Should not happen | |
} | |
for (;;) { | |
printf("\nSleeping for ~3 seconds\n"); | |
do_sleep(3); // Later to be replaced with a SIGALRM | |
} | |
} | |
void handle_signal(int signal) { | |
const char *signal_name; | |
sigset_t pending; | |
// Find out which signal we're handling | |
switch (signal) { | |
case SIGHUP: | |
signal_name = "SIGHUP"; | |
break; | |
case SIGUSR1: | |
signal_name = "SIGUSR1"; | |
break; | |
case SIGINT: | |
printf("Caught SIGINT, exiting now\n"); | |
exit(0); | |
default: | |
fprintf(stderr, "Caught wrong signal: %d\n", signal); | |
return; | |
} | |
/* | |
* Please note that printf et al. are NOT safe to use in signal handlers. | |
* Look for async safe functions. | |
* http://man7.org/linux/man-pages/man7/signal-safety.7.html | |
*/ | |
printf("Caught %s, sleeping for ~3 seconds\n" | |
"Try sending another SIGHUP / SIGINT / SIGALRM " | |
"(or more) meanwhile\n", signal_name); | |
/* | |
* All signals are blocked during this handler | |
* (because of the `sigfillset(&sa.sa_mask);` in `main()`). | |
* But, at least on OSX, if you send 2 other SIGHUP, | |
* only one will be delivered: signals are not queued | |
* However, if you send HUP, INT, HUP, | |
* you'll see that both INT and HUP are queued | |
* Even more, on my system, HUP has priority over INT | |
* | |
* There are two types of signals: standard signals, | |
* and realtime signals. Realtime signals are SIGRTMIN (34) | |
* and above. | |
* While there can be only one traditional signal waiting at | |
* a time for each type (i.e. one HUP, one INT etc), there | |
* However, there can be multiple realtime signals queued | |
* for each type (e.g. 3 SIGRTMIN, 4 SIGRTMAX...) | |
* | |
* See the other file for a list of all the signals available. | |
*/ | |
do_sleep(3); | |
printf("Done sleeping for %s\n", signal_name); | |
// So what did you send me while I was asleep? | |
sigpending(&pending); | |
if (sigismember(&pending, SIGHUP)) { | |
printf("A SIGHUP is waiting\n"); | |
} | |
if (sigismember(&pending, SIGUSR1)) { | |
printf("A SIGUSR1 is waiting\n"); | |
} | |
printf("Done handling %s\n\n", signal_name); | |
} | |
void handle_sigalrm(int signal) { | |
if (signal != SIGALRM) { | |
fprintf(stderr, "Caught wrong signal: %d\n", signal); | |
} | |
printf("Got sigalrm, do_sleep() will end\n"); | |
} | |
void do_sleep(int seconds) { | |
struct sigaction sa; | |
sigset_t mask; | |
sa.sa_handler = &handle_sigalrm; // Intercept and ignore SIGALRM | |
sa.sa_flags = SA_RESETHAND; // Remove the handler after first signal | |
sigfillset(&sa.sa_mask); | |
sigaction(SIGALRM, &sa, NULL); | |
// Get the current signal mask | |
sigprocmask(0, NULL, &mask); | |
// Unblock SIGALRM | |
sigdelset(&mask, SIGALRM); | |
// Wait with this mask | |
alarm(seconds); | |
sigsuspend(&mask); | |
printf("sigsuspend() returned\n"); | |
} |
For reference, here is a list of all the signals on Linux. | |
The signals from SIGRTMIN and above are real time signals. | |
$ uname -a | |
Linux localhost 4.9.0-6-amd64 #1 SMP Debian 4.9.82-1+deb9u3 (2018-03-02) x86_64 GNU/Linux | |
$ kill -l | |
1) SIGHUP 2) SIGINT 3) SIGQUIT 4) SIGILL 5) SIGTRAP | |
6) SIGABRT 7) SIGBUS 8) SIGFPE 9) SIGKILL 10) SIGUSR1 | |
11) SIGSEGV 12) SIGUSR2 13) SIGPIPE 14) SIGALRM 15) SIGTERM | |
16) SIGSTKFLT 17) SIGCHLD 18) SIGCONT 19) SIGSTOP 20) SIGTSTP | |
21) SIGTTIN 22) SIGTTOU 23) SIGURG 24) SIGXCPU 25) SIGXFSZ | |
26) SIGVTALRM 27) SIGPROF 28) SIGWINCH 29) SIGIO 30) SIGPWR | |
31) SIGSYS 34) SIGRTMIN 35) SIGRTMIN+1 36) SIGRTMIN+2 37) SIGRTMIN+3 | |
38) SIGRTMIN+4 39) SIGRTMIN+5 40) SIGRTMIN+6 41) SIGRTMIN+7 42) SIGRTMIN+8 | |
43) SIGRTMIN+9 44) SIGRTMIN+10 45) SIGRTMIN+11 46) SIGRTMIN+12 47) SIGRTMIN+13 | |
48) SIGRTMIN+14 49) SIGRTMIN+15 50) SIGRTMAX-14 51) SIGRTMAX-13 52) SIGRTMAX-12 | |
53) SIGRTMAX-11 54) SIGRTMAX-10 55) SIGRTMAX-9 56) SIGRTMAX-8 57) SIGRTMAX-7 | |
58) SIGRTMAX-6 59) SIGRTMAX-5 60) SIGRTMAX-4 61) SIGRTMAX-3 62) SIGRTMAX-2 | |
63) SIGRTMAX-1 64) SIGRTMAX |
Thanks.
Thanks a lot.
Instead of calling printf() from within your signal handler (bad idea) you could implement a FIFO of messages to be written and check that (calling printf() on non-empty) from your main event loop. (Basically any case where you're going to handle signals in a non-terminating fashion entails implementing one or more event loops to handle the interruption by signals and resume whatever was being done by main()).
Thanks. 👍
Thanks, man, this is useful for me
BH1SCW
Thanks 👍
I must be missing an important concept here. After reading these comments and the signal man pages it seems multiple people say that using printf in a signal handler is a bad idea, but not why. Could someone elaborate?
@kohbo the man page you want is: http://man7.org/linux/man-pages/man7/signal-safety.7.html.
Thanks a lot it really helped to understand the signal handling by reading your code.I now understand much better how it is done.
It is very good example! Thanks!
Please, add before codes line (for correct execute all functions):
#define _GNU_SOURCE
Hi @AlexGin. Care elaborating why we need this _GNU_SOURCE
?
thanks a lot bro very helpful! replace printf with this line:
const char buf[ ] = "your message\n";
write(STDOUT_FILENO, buf, strlen(buf));