Operating Systems Practical Session 3 Signals 1 Signals

Operating Systems Practical Session 3, Signals 1

Signals • Signals are a way of sending simple messages to processes/ threads. • Used to notify a process of important events. • Signals can be sent by other processes/ threads, or by the kernel. threads • Signals are defined in POSIX. • Signals can be found in Linux but not in XV 6, you can add them yourself! 2

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). 3

Signals: Synchronous VS. Asynchronous • Programs are synchronous: executed line by line • Signals can be synchronous or asynchronous – Synchronous: occur as a direct result of the executing Synchronous instruction stream. Examples: dividing by zero, segmentation fault, etc. – Asynchronous: Asynchronous external to (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. 4

Signals-Examples • • SIGSEGV – Segmentation Faults SIGFPE – Floating Point Error SIGSTOP – Causes process to suspend itself SIGCONT – Causes a suspended process to resume execution Which are synchronous? Ø A list of signals in Linux: http: //www. ucs. cam. ac. uk/docs/course-notes/unixcourses/Building/files/signals. pdf 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) Sig_Num 1 2 6 SIG_IGN Handler

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 nonmeaningful (i. e. ignoring SIGFPE or SIGSEGV). Ø 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. 7

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. Ø 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. 8

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. Ø 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: http: //www. ibm. com/developerworks/linux/library/lreent/index. html 9

Signal Handlers • Two signals cannot be ignored or have their associated action changed: – SIGKILL – SIGSTOP (Don’t confuse with SIGTSTP, which is sent when a user ^z in the shell. The default actions of both signals are similar, but the latter can be modified). • 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? 10

Scheme of signal processing User Mode Normal program flow Kernel Mode An event which traps to kernel do_signal() handle_signal() setup_frame() Signal handler Return code on the stack 11 system_call() sys_sigreturn() restore_sigcontext()

trapasm. S Scheme of signal processing User Mode Normal program flow # vectors. S sends all traps here. . globl alltraps: # Build trap frame. pushl %ds Kernel Mode pushl %es pushl %fs pushl %gs An event which traps to kernel do_signal() pushal handle_signal() setup_frame() # Set up data and per-cpu segments. movw $(SEG_KDATA<<3), %ax movw %ax, %ds movw %ax, %es Signal movw $(SEG_KCPU<<3), %ax movw %ax, %fs handler movw %ax, %gs Return code on the stack 12 # Call trap(tf), where tf=%esp system_call() pushl %esp call trap sys_sigreturn() . restore_sigcontext(). .

Scheme of signal processing User Mode Normal program trapasm. S flow Kernel Mode An event which traps to kernel . . . do_signal() handle_signal() setup_frame() addl $4, %esp Signal handler # Return falls through to trapret. . globl trapret: popal Return popl %gs popl %fs code on popl %es the stack popl %ds addl $0 x 8, %esp # trapno and errcode iret 13 system_call() sys_sigreturn() restore_sigcontext()

Scheme of signal processing User Mode Normal program flow Kernel Mode An event which traps to kernel ? do_signal() handle_signal() setup_frame() Signal handler Return code on the stack 14 system_call() sys_sigreturn() restore_sigcontext()

Scheme of signal processing User Mode Kernel Mode proc. c Normal An event which traps to kernel void program register_handler(sighandler_t sighandler) flow { char* addr = uva 2 ka(proc->pgdir, (char*)proc->tf->esp); if ((proc->tf->esp & 0 x. FFF) == 0) panic("esp_offset == 0"); do_signal() handle_signal() setup_frame() Signal /* open a new frame */ handler *(int*)(addr + ((proc->tf->esp - 4) & 0 x. FFF)) = proc->tf->eip; proc->tf->esp -= 4; /* update eip */ Return proc->tf->eip = (uint)sighandler; code on } the stack 15 system_call() sys_sigreturn() restore_sigcontext()

Scheme of signal processing User Mode Normal program flow Kernel Mode An event which traps to kernel Signal handler Return code on the stack 16 do_signal() handle_signal() setup_frame() ? system_call() sys_sigreturn() restore_sigcontext()

Sending Signals • Signals can be sent: – From the keyboard – From the command line via the shell – Using system calls 17

Keyboard Signals • 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. Ø Note: not all keyboard signals are supported in all shells. 18

Command line Signals • kill <signal> <PID> – Sends the specified signal to kill the specified PID. A Negative PID specifies a whole process group. – Kill -9 <PID> sends a SIGKILL which terminates the process. • killall <args> <commands> – 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). 19

System call Signals Kill(pid_t pid, int sig) Usage example: #include <unistd. h> /* standard unix functions, like getpid() */ #include <sys/types. h> /* various type definitions, like pid_t */ #include <signal. h> /* 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); 20

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: – – 21 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)

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! 22

Manipulation of Signals • 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 %dn”, sig); signal(SIGINT, ouch); } • What is the problem with this solution? 23

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: http: //www. opengroup. org/onlinepubs/009695399/functions/sigaction. html 24

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. 25

Manipulation of Signals- sigprocmask int sigprocmask(int how, const sigset_t *set, sigset_t *oldset); 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: how – 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. 26

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: http: //www. linuxprogrammingblog. com/code-examples/blocking-signals-withsigprocmask 27

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. 28

Waiting for signals int pause(void); Causes the calling process (or thread) to sleep until a (any) signal is any 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. Allows waiting for a particular signal. particular 29

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. 30

Example 1 #include <stdio. h> /* standard I/O functions */ #include <unistd. h> /* standard unix functions, like getpid() */ #include <sys/types. h> /* various type definitions, like pid_t*/ #include <signal. h> /* 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 thatn"); } 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 */ Causes the process to halt while (true) { execution until it receives pause(); any signal. } } 31

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 sig. Catcher() { signal(SIGINT, sig. Catcher); // re-assign the signal catcher printf("PID %d caught onen", getpid()); if(j > -1) kill(cpid[j], SIGINT); // send signal to next child in cpid } 32

Example 2 -Continued int main() { int i; int zombie; int status; int pid; signal(SIGINT, sig. Catcher); // sets a handler for INT signal … 33

Example 2 -Continued for(i=0; i<5; i++){ if((pid=fork()) == 0){ printf("PID %d readyn", getpid()); j = i-1; pause(); exit(0); } else cpid[i] = pid; } sleep(2); kill(cpid[4], SIGINT); sleep(2); for(i=0; i<5; i++){ zombie = wait(&status); printf("%d is deadn", zombie); } exit(0); } 34 // create new child // wait for signal // end process (become a zombie) // Only father updates the cpid array. // allow children time to enter pause // send signal to first child // wait for children to become zombies // collect zombies

Output PID 22899 ready PID 22900 ready PID 22901 ready PID 22902 ready PID 22903 caught one PID 22902 caught one PID 22901 caught one PID 22900 caught one PID 22899 caught one 22903 is dead 22901 is dead 22902 is dead 22899 is dead 22900 is dead 35

Security Issues • Not all processes can send signals to all processes. • Only the kernel and super user can send signals to all processes. • Normal processes can only send signals to processes owned by the same user. 36

Process ID and Group ID • Each process has an ID (pid). ID • A process group is a collection of related processes. • Each process has a process-group identifier (pgid) • One process in the group is the group leader and all member’s group ID is equal to the leaders pid. The group leader is the process group's initial member. • A signal can be sent to a single process or to a process group. • Used by the shell to control different tasks executed by it. 37

Process Group ID int getpid() – return the process’s PID. int getpgrp() – return the process’s PGID. setpgrp() – set this process’s PGID to be equal to his PID. setpgrp(int pid 1, int pid 2) – set process’s pid 1 PGID to be equal to pid 2’s PID. 38

MIDTERM QUESTION (APPENDIX) 39

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); } } 41 my_prog. c

Question from midterm 2004 main(int argc, char** argv){ char buf[20]; while(1) { printf(“Type something: “); gets(buf); printf(“n. You typed: %sn”, buf); } } 42 prompt. c )זכרו כי קוד זה אינו ניתן ( לשינוי ע"י התלמיד

Sample execution of code : תאר במדויק את פלט התכנית כאשר הקלט הנו - א 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 43

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. 44

Code correction כיצד היית משנה את התכנית , ב' היא לא - אם תשובתך ל לשנות שורה או שתיים בקוד לכל / )ניתן להוסיף my_prog. c ? ( 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]); 45

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

Question from midterm 2012 : הגרף שיתקבל מהקוד void sigchld_handler(int s) { } printf(“S”); int main(){ } 48 signal(SIGCHLD, sigchld_handler); signal_block(SIGCHLD); if (fork() != 0) { printf(“A”); signal_unblock(SIGCHLD); printf(“B”); wait (); printf(“C”); } else { printf(“D”); } A D B S C