Operating Systems Practical Session 3, Signals 1

Signals are a way of sending simple messages to processes. Used to notify a process of important events. processes kernel. Signals can be sent by other processes, or by the kernel. Signals are defined in POSIX. Signals can be found in Linux but not in XV6, you can add them yourself! 2

Blocking and Ignoring Blocking: The signal is not delivered to the process. It remains pending until the block is removed. Ignoring: The signal is discarded by the kernel without any action being taken. The execution of the process continues even if non- meaningful (i.e. ignoring SIGFPE or SIGSEGV).  According to POSIX, the behavior of a process is undefined after it ignores a SIGFPE, SIGILL, or SIGSEGV signal that was not generated by kill or raise. 3

Signals-Examples SIGSEGV – Segmentation Faults SIGFPE – Floating Point Error SIGSTOP – Causes process to suspend itself SIGCONT – Causes a suspended process to resume execution  A list of signals in Linux: courses/Building/files/signals.pdf courses/Building/files/signals.pdf 4

Kernel Signals Data-Structures (Linux) 5

Signal Table Each process has a signal table Each signal has an entry in the table Each signal has an indicator whether to ignore the signal or not (SIG_IGN) Each signal has a column of what to do upon receiving the signal (if not ignoring it) 6 HandlerFLAGS (IGN, BLK)Sig_Num 1 2

Default Signal Handlers Five default actions: – Ignore: ignores the signal; no action taken. – Exit: forces the process to exit. – Core: forces the process to exit and create a core file. – Stop: stops the process. – Continue: resume execution of a stopped process.  Some functions are not safe to call from within a signal handler, such as printf, malloc, etc. A useful technique to overcome this is to use a signal handler to set a flag and then check that flag from the main program and print a message if required. Further reading: reent/index.htmlhttp:// reent/index.html 7

Pending Signals Each process has a pending signals bit array Each bit represents one signal When a signal is received the kernel will set the corresponding bit After signal handled, the kernel will clear the bit 8 …SIGINTSIGHUP 10

Interfacing with Signals (Linux) 9

Scheme of signal processing 10 User Mode Kernel Mode Normal program flow do_signal() handle_signal() setup_frame() Signal handler Return code on the stack system_call() sys_sigreturn() restore_sigcontext() An event which traps to kernel

Sending Signals Signals can be sent: – Using system calls – Kernel Notifications 11

System call Signals Kill(pid_t pid,int sig) Usage example: #include /* standard unix functions, like getpid() */ #include /* various type definitions, like pid_t */ #include /* signal name macros, and the kill() prototype */ /* first, find my own process ID */ pid_t my_pid = getpid(); /* now that I got my PID, send myself the STOP signal. */ kill(my_pid, SIGSTOP); 12

Keyboard Signals (sent by shell via system call) Ctrl–C – Sends a SIGINT signal. By default this causes the process to terminate. Ctrl-\ - Sends a SIGQUIT signal. Causes the process to terminate. Ctrl-Z – Sends a SIGTSTP signal. By default this causes the process to suspend execution.  Note: not all keyboard signals are supported in all shells. 13

Command line Signals (sent by shell via system call) kill kill – Sends the specified signal to the specified PID. A Negative PID specifies a whole process group. – Kill -9 sends a SIGKILL which terminates the process. killall killall – can be used to send multiple signals to all processes running specific commands. – Example: killall -9 java fg fg – Resumes the execution of a suspended process (sends a SIGCONT). 14

Kernel Notification Signals -Floating point exception (SIGFPE) -Division by zero -Segmentation Fault (SIGSEGV) 15

alarm The system call alarm unsigned alarm(unsigned seconds); Requests the system to generate a SIGALRM for the process after seconds time have elapsed. Processor scheduling delays may prevent the process from handling the signal as soon as it is generated. If seconds is 0, a pending alarm request, if any, is canceled. Alarm requests are not stacked; only one SIGALRM generation can be scheduled in this manner. If the SIGALRM signal has not yet been generated, the call shall result in rescheduling the time at which the SIGALRM signal is generated. 16

Signal Handlers Each signal has a default action. For example: – SIGTERM – Terminate process. – SIGFPE (floating point exception) – dump core and exit. The default action can be changed by the process using the signal * / sigaction system call.  It is highly recommended to refrain from using the signal call in your code, as we will see later. Nonetheless it is important to be familiar with it since it appears in many legacy programs. 17

Signal Handlers Two signals cannot be ignored or have their associated action changed: – SIGKILL – SIGSTOP Fork will not change the signal handlers but clears the pending signals When calling execvp() all signals are set to their default action. The bit that specifies whether to ignore the signal or not is preserved. Why? 18

Manipulation of Signals sighandler_t signal(int signum, sighandler_t handler) Registers a new signal handler for the signal with number signum. The signal handler is set to sighandler which may be a user specified function, or either SIG_IGN or SIG_DFL. If the corresponding handler is set to SIG_IGN or SIG_DFL, then the signal is ignored or set do default action accordingly. Return Value: previous value of the signal handler, or SIG_ERR on error. Deprecated, do not use! 19

Manipulation of Signals reset On some systems (e.g. System V Unix), if the handler is set to a function sighandler and a signal is received, then first the handler is reset to SIG_DFL, and next sighandler is called. This may result in portability issues, or unwanted signal handling. One solution to this problem is demonstrated in the “ouch” signal handler function: void ouch(int sig) { printf(“OUCH! - I got signal %d\n”, sig); signal(SIGINT, ouch); } What is the problem with this solution? 20

Reacting to Signals Signals are processed (by the kernel) after a process finished running in kernel mode, just before returning to user mode: – Upon returning from a system call. – Upon returning from a interrupt (e.g., interrupt sent by the hardware clock). 21

Signals: Synchronous VS. Asynchronous Programs are synchronous: executed line by line Signals can be synchronous or asynchronous – Synchronous – Synchronous: occur as a direct result of the executing instruction stream. Examples: dividing by zero, segmentation fault, etc. – Asynchronous – Asynchronous: external (and in some cases unrelated) to the current execution context. A mechanism for an inter-process communication. Example: receiving a termination signal from a different process. 22

Signal Priority Each pending signal is marked by a bit in a 32 bit word. Therefore there can only be one signal pending of each type. A process can’t know which signal came first. The process executes the signals starting at the lowest numbered signal. POSIX 2001 also defines a set of Real-Time Signals which behave differently: – Multiple instances may be queued – Provide richer information (may be accompanied by an integer) – Delivered in guaranteed order – Use SIGRTMIN+n up to SIGRTMAX to refer to these signals (32 in Linux) 23

Waiting for signals int pause(void); any Causes the calling process (or thread) to sleep until a (any) signal is delivered that either terminates the process or causes the invocation of a signal-catching function. int sigsuspend(const sigset_t *mask); Temporarily replaces the signal mask of the process, and suspends the process until a signal not belonging to the waiting mask arrives. particular Allows waiting for a particular signal. 24

Alternative to the signal syscall 25

sigaction Manipulation of Signals- sigaction int sigaction(int signum, const struct sigaction *act, struct sigaction *oldact); A more sophisticated (and safe) way of manipulating signals. Doesn’t restore (by default) the signal handler to default when delivering a signal. signum is the number of the signal. act is a pointer to a struct containing much information including the new signal handler. oldact if not null will receive the old signal handler. For more details and another example see: 26

sigaction Manipulation of Signals- sigaction The sigaction structure is defined as something like: struct sigaction { void (*sa_handler)(int); sigset_t sa_mask; int sa_flags; void (*sa_restorer)(void); }; sa_handler specifies the action to be associated with signum and may be SIG_DFL, SIG_IGN, or a pointer to a signal handling function. sa_mask specifies a mask of signals which should be blocked during execution of the signal handler. In addition, the signal which triggered the handler will be blocked, unless the SA_NODEFER flag is used. sa_flags specifies a set of flags which modify the behavior of the signal. 27

sigprocmask Manipulation of Signals- sigprocmask int sigprocmask(int how, const sigset_t *set, sigset_t *oldset); how The sigprocmask call is used to change the list of currently blocked signals. The behaviour of the call is dependent on the value of how, as follows: – SIG_BLOCK The set of blocked signals is the union of the current set and the set argument. – SIG_UNBLOCK The signals in set are removed from the current set of blocked signals. It is legal to attempt to unblock a signal which is not blocked. – SIG_SETMASK The set of blocked signals is set to the argument set. 28

sigprocmask Manipulation of Signals- sigprocmask sigset_t is a basic data structure used to represent a signal set. Initialization of sigset_t should be done using: sigemptyset, sigfillset, sigaddset, … A variable of type sigset_t should not be manipulated manually (for portability)! An example of usage can be found at: sigprocmask sigprocmask 29

sigpending Manipulation of Signals- sigpending int sigpending(sigset_t *set); Returns the set of signals that are pending for delivery to the calling thread (i.e., the signals which have been raised while blocked). The mask of pending signals is returned in set. 30

Examples 31

Example 1 #include /* standard I/O functions */ #include /* standard unix functions, like getpid() */ #include /* various type definitions, like pid_t*/ #include /* signal name macros, and the signal() prototype */ /* first, here is the signal handler */ void catch_int(int sig_num){ /* reassign the signal handler again to catch_int, for next time */ signal(SIGINT, catch_int); /* and print the message */ printf("Don't do that\n"); } int main(){ /* set the INT (Ctrl-C) signal handler to 'catch_int' */ signal(SIGINT, catch_int); /* now, lets get into an infinite loop of doing nothing */ while (true) { pause(); } } Causes the process to halt execution until it receives any signal. 32

Example 2 int cpid[5]; // holds the pids of the children int j; // index to cpid // function to activate when a signal is caught int sigCatcher() { signal(SIGINT, sigCatcher); // re-assign the signal catcher printf("PID %d caught one\n", getpid()); if(j > -1) kill(cpid[j], SIGINT); // send signal to next child in cpid } 33

Example 2-Continued int main() { int i; int zombie; int status; int pid; signal(SIGINT, sigCatcher); // sets a handler for INT signal … 34

Example 2-Continued for(i=0; i<5; i++){ if((pid=fork()) == 0){ // create new child printf("PID %d ready\n", getpid()); j = i-1; pause(); // wait for signal exit(0); // end process (become a zombie) } else // Only father updates the cpid array. cpid[i] = pid; } sleep(2); // allow children time to enter pause kill(cpid[4], SIGINT); // send signal to first child sleep(2); // wait for children to become zombies for(i=0; i<5; i++){ zombie = wait(&status); // collect zombies printf("%d is dead\n", zombie); } exit(0); } 35

Output PID ready PID ready PID ready PID ready PID ready PID caught one PID caught one PID caught one PID caught one PID caught one is dead is dead is dead is dead is dead 36

HOMEWORK 37

Question from midterm 2004 תלמיד קיבל משימה לכתוב תכנית שמטרתה להריץ תכנית נתונה ( ברשותו רק הקובץ הבינארי ) prompt ע " י שימוש ב -fork ו -execvp. בנוסף נדרש התלמיד למנוע מן המשתמש " להרוג " את התכנית ע " י הקשת ctrl-c (SIGINT) ( שים לב כי התכנית prompt אינה מסתיימת לעולם ). מצורף פתרון שהוצע ע " י התלמיד (my_prog.c) וכן התכנית prompt. א-תאר במדויק את פלט התכנית כאשר הקלט הנו : Good luck in the ^c midterm exam. ב-האם הפתרון המוצע עונה על הגדרת התרגיל ? ג-אם תשובתך ל - ב ' היא לא, כיצד היית משנה את התכנית my_prog.c ( ניתן להוסיף / לשנות שורה או שתיים בקוד לכל היותר )? 38

Question from midterm 2004 #include… void cntl_c_handler(int dummy){ signal(SIGINT, cntl_c_handler); } main (int argc, char **argv){ int waited; int stat; argv[0] = “prompt”; signal (SIGINT, cntl_c_handler); if (fork() == 0) { // son execvp(“prompt”,argv[0]); } else { // father waited = wait(&stat); printf(“My son (%d) has terminated \n”,waited); } my_prog.c 39

Question from midterm 2004 main(int argc, char** argv){ char buf[20]; while(1) { printf(“Type something: “); gets(buf); printf(“\nYou typed: %s\n”,buf); } prompt.c (זכרו כי קוד זה אינו ניתן לשינוי ע"י התלמיד) 40

Sample execution of code א-תאר במדויק את פלט התכנית כאשר הקלט הנו : Good luck in the ^c midterm exam. Type something: Good luck You typed: Good luck Type something: in the ^c My son 139 has terminated 41

Code is incorrect האם הפתרון המוצע עונה על הגדרת התרגיל ? Execvp doesn’t preserve signal handlers. Therefore prompt.c doesn’t ignore ^c. This means that the process can be terminated. 42

Code correction אם תשובתך ל - ב ' היא לא, כיצד היית משנה את התכנית my_prog.c ( ניתן להוסיף / לשנות שורה או שתיים בקוד לכל היותר )? 1.Change signal (SIGINT, cntl_c_handler); in my_prog.c With signal (SIGINT, SIG_IGN); 2.Add if (fork()==0){ signal (SIGINT, SIG_IGN); execvp(“prompt”,argv[0]); 43

Question from midterm 2012 נתון קטע הקוד הבא : void sigchld_handler( int s) { printf(“S”); } int main(){ signal(SIGCHLD, sigchld_handler); signal_block(SIGCHLD); if (fork() != 0 ) { printf(“A”); signal_unblock(SIGCHLD); printf(“B”); wait (); printf(“C”); } else { printf(“D”); } 44

Question from midterm 2012 ידוע כי הפקודות signal_block וכן signal_unblock חוסמות ומשחררות חסימה לסיגנלים. שרטטו גרף מכוון המתאר את הפלטים האפשריים לקוד זה. כל צומת בגרף תסמל הדפסה וכל קשת מכוונת תייצג יחס סדר מתחייב בין הדפסות. לדוגמא, אם עפ " י קוד מסוים ידוע כי יודפסו X, Y ו – Z וכי ההדפסה של X תופיע בהכרח לפני ההדפסה של Y ( אך Z יכול להופיע לפני או אחרי כל אחת מן ההדפסות האחרות ), יתקבל הגרף הבא : 45 X YZ

Question from midterm 2012 הגרף שיתקבל מהקוד : void sigchld_handler( int s) { printf(“S”); } int main(){ signal(SIGCHLD, sigchld_handler); signal_block(SIGCHLD); if (fork() != 0 ) { printf(“A”); signal_unblock(SIGCHLD); printf(“B”); wait (); printf(“C”); } else { printf(“D”); } 46 A S D C B

More Information _intr()&source=bl&ots=JCjEvqiVM- &sig=z8CtaNgkFpa1MPQaCWjJuU5tq4g&hl=en&ei=zf3zSZsvjJOwBs- UxYkB&sa=X&oi=book_result&ct=result&resnum=22#PPA159,M1 _intr()&source=bl&ots=JCjEvqiVM- &sig=z8CtaNgkFpa1MPQaCWjJuU5tq4g&hl=en&ei=zf3zSZsvjJOwBs- UxYkB&sa=X&oi=book_result&ct=result&resnum=22#PPA159,M1 man signal, sigaction… man kill… Process groups:

CODE EXAMPLES 48

sigaction code example 49 #include sig_atomic_t sigusr1_count = 0; void handler (int signal_number){ ++sigusr1_count; } int main (int argc, char ** argv){ struct sigaction sa; memset (&sa, 0, sizeof (sa)); sa.sa_handler = &handler; sigaction (SIGUSR1, &sa, NULL); /* Do some lengthy stuff here. */ /*... */ printf (“SIGUSR1 was raised %d times\n”, sigusr1_count); return 0; } Back

sigprocmask code example /** This program blocks SIGTERM signal for 10 seconds using sigprocmask(2) * After that the signal is unblocked and the queued signal is handled. */ #include static int got_signal = 0; static void hdl (int sig) { got_signal = 1; } int main (int argc, char *argv[]) { sigset_t mask; sigset_t orig_mask; struct sigaction act; memset (&act, 0, sizeof(act)); act.sa_handler = hdl; 50

sigprocmask code example if (sigaction(SIGTERM, &act, 0)) { perror ("sigaction"); return 1; } sigemptyset (&mask); sigaddset (&mask, SIGTERM); if (sigprocmask(SIG_BLOCK, &mask, &orig_mask) < 0) { perror ("sigprocmask"); return 1; } sleep (10); if (sigprocmask(SIG_SETMASK, &orig_mask, NULL) < 0) { perror ("sigprocmask"); return 1; } sleep (1); if (got_signal) puts ("Got signal"); return 0; } 51 Back