c/pthread_pause.c

   1 //Filename: pthread_pause.c
   2 //Author: Tomas 'Harvie' Mudrunka 2021
   3 //Build: CFLAGS=-lpthread make pthread_pause; ./pthread_pause
   4 //Test: valgrind --tool=helgrind ./pthread_pause
   5
   6 //I've wrote this code as excercise to solve following stack overflow question:
   7 // https://stackoverflow.com/questions/9397068/how-to-pause-a-pthread-any-time-i-want/68119116#68119116
   8
   9 #include <signal.h>
  10 #include <pthread.h>
  11 //#include <pthread_extra.h>
  12 #include <stdio.h>
  13 #include <stdlib.h>
  14 #include <assert.h>
  15 #include <unistd.h>
  16 #include <errno.h>
  17 #include <sys/resource.h>
  18
  19 #define PTHREAD_XSIG_STOP (SIGRTMIN+0)
  20 #define PTHREAD_XSIG_CONT (SIGRTMIN+1)
  21 #define PTHREAD_XSIGRTMIN (SIGRTMIN+2) //First unused RT signal
  22
  23 pthread_t main_thread;
  24
  25 void pthread_pause_handler(int signal) {
  26     //Do nothing when there are more signals pending (to cleanup the queue)
  27     sigset_t pending;
  28     sigpending(&pending);
  29     if(sigismember(&pending, PTHREAD_XSIG_STOP)) return;
  30     if(sigismember(&pending, PTHREAD_XSIG_CONT)) return;
  31
  32     if(signal == PTHREAD_XSIG_STOP) {
  33         sigset_t sigset;
  34         sigfillset(&sigset);
  35         sigdelset(&sigset, PTHREAD_XSIG_STOP);
  36         sigdelset(&sigset, PTHREAD_XSIG_CONT);
  37         sigsuspend(&sigset); //Wait for next signal
  38     } else return;
  39 }
  40
  41 void pthread_pause_enable() {
  42     //Having signal queue too deep might not be necessary
  43     //It can be limited using RLIMIT_SIGPENDING
  44     //You can get runtime SigQ stats using following command:
  45     //grep -i sig /proc/$(pgrep binary)/status
  46     struct rlimit sigq = {.rlim_cur = 32, .rlim_max=32};
  47     setrlimit(RLIMIT_SIGPENDING, &sigq);
  48
  49     //Register signal handlers
  50     signal(PTHREAD_XSIG_STOP, pthread_pause_handler);
  51     signal(PTHREAD_XSIG_CONT, pthread_pause_handler);
  52
  53     //UnBlock signals
  54     sigset_t sigset;
  55     sigemptyset(&sigset);
  56     sigaddset(&sigset, PTHREAD_XSIG_STOP);
  57     sigaddset(&sigset, PTHREAD_XSIG_CONT);
  58     pthread_sigmask(SIG_UNBLOCK, &sigset, NULL);
  59 }
  60
  61 void pthread_pause_disable() {
  62         //This is important for when you want to do some signal unsafe stuff
  63         //Eg.: locking mutex, calling printf() which has internal mutex, etc...
  64         //After unlocking mutex, you can enable pause again.
  65
  66         //Block signals
  67         sigset_t sigset;
  68         sigemptyset(&sigset);
  69         sigaddset(&sigset, PTHREAD_XSIG_STOP);
  70         sigaddset(&sigset, PTHREAD_XSIG_CONT);
  71         pthread_sigmask(SIG_BLOCK, &sigset, NULL);
  72 }
  73
  74
  75 int pthread_pause(pthread_t thread) {
  76         //If signal queue is full, we keep retrying
  77         while(pthread_kill(thread, PTHREAD_XSIG_STOP) == EAGAIN) usleep(1000);
  78         return 0;
  79 }
  80
  81 int pthread_unpause(pthread_t thread) {
  82         //If signal queue is full, we keep retrying
  83         while(pthread_kill(thread, PTHREAD_XSIG_CONT) == EAGAIN) usleep(1000);
  84         return 0;
  85 }
  86
  87 void *thread_test() {
  88     //Whole process dies if you kill thread immediately before it is pausable
  89     //pthread_pause_enable();
  90     while(1) {
  91         usleep(1000*300);
  92         //Printf() is not async signal safe (because it holds internal mutex),
  93         //you should call it only with pause disabled!
  94         //Will throw helgrind warnings anyway, not sure why...
  95         //See: man 7 signal-safety
  96         pthread_pause_disable();
  97         printf("Running!\n");
  98         pthread_pause_enable();
  99         //pthread_pause(main_thread);
 100         //pthread_unpause(main_thread);
 101     }
 102 }
 103
 104 int main() {
 105     pthread_t t;
 106     main_thread = pthread_self();
 107     pthread_pause_enable(); //Will get inherited by all threads from now on
 108     //you need to call pthread_pause_enable (or disable) before creating threads,
 109     //otherwise first signal will kill whole process
 110     pthread_create(&t, NULL, thread_test, NULL);
 111
 112     while(1) {
 113         pthread_pause(t);
 114         printf("PAUSED\n");
 115         sleep(3);
 116
 117         printf("UNPAUSED\n");
 118         pthread_unpause(t);
 119         sleep(1);
 120     }
 121
 122     pthread_join(t, NULL);
 123     printf("DIEDED!\n");
 124 }