Sogang University
Exceptional Control Flow: Signals and Nonlocal Jumps
CSE4100: System Programming
Youngjae Kim (PhD)
Copyright By PowCoder代写 加微信 powcoder
https://sites.google.com/site/youkim/home
Distributed Computing and Operating Systems Laboratory (DISCOS)
https://discos.sogang.ac.kr
Office: R911, E-mail:
and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition
Sogang University
ECF Exists at All Levels of a System ¢ Exceptions
§ Hardware timer and kernel software ¢ Signals
§ Kernel software and application software ¢ Nonlocal jumps
§ Application code
§ Hardware and operating system kernel software ¢ Process Context Switch
Previous Lecture
This Lecture
Textbook and supplemental slides
Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition
Sogang University
¢ Nonlocal jumps
Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition
University
Linux Process Hierarchy
e.g. httpd
Grandchild
[0] init [1]
Login shell Child
Login shell Child
Note: you can view the hierarchy using the Linux pstree command
Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition
Grandchild
Sogang University
Shell Programs
¢ A shell is an application program that runs programs on behalf of the user.
§ csh/tcsh § bash
Original Unix shell ( , AT&T Bell Labs, 1977) BSD Unix C shell
“Bourne-Again” Shell (default Linux shell)
Execution is a sequence of
read/evaluate steps
int main()
char cmdline[MAXLINE]; /* command line */
/* evaluate */
eval(cmdline);
while (1) {
/* read */
printf(“> “);
Fgets(cmdline, MAXLINE, stdin);
if (feof(stdin))
Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition
University
Simple Shell eval Function
void eval(char *cmdline)
char *argv[MAXARGS]; /* Argument list execve() */
char buf[MAXLINE]; int bg;
pid_t pid;
/* Holds modified command line */
/* Should the job run in bg or fg? */ /* Process id */
strcpy(buf, cmdline);
bg = parseline(buf, argv); if (argv[0] == NULL)
return; /* Ignore empty lines */
if (!builtin_command(argv)) {
if ((pid = Fork()) == 0) { /* Child runs user job */
if (execve(argv[0], argv, environ) < 0) { printf("%s: Command not found.\n", argv[0]);
exit(0); }
/* Parent waits for foreground job to terminate */
if (!bg) {
int status;
if (waitpid(pid, &status, 0) < 0)
} unix_error("waitfg: waitpid error");
elseprintf("%d %s", pid, cmdline); }return;
Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition
Sogang University
Problem with Simple Shell Example
¢ Our example shell correctly waits for and reaps foreground jobs
¢ But what about background jobs?
§ Will become zombies when they terminate
§ Will never be reaped because shell (typically) will not terminate
§ Will create a memory leak that could run the kernel out of memory
Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition
University
ECF to the Rescue!
¢ Solution: Exceptional control flow
§ The kernel will interrupt regular processing to alert us when a background process completes
§ In Unix, the alert mechanism is called a signal
Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition
University
¢ Nonlocal jumps
Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition
University
¢ A signal is a small message that notifies a process that an event of some type has occurred in the system
Akin to exceptions and interrupts
Sent from the kernel (sometimes at the request of another process) to a process
Signal type is identified by small integer ID’s (1-30)
Only information in a signal is its ID and the fact that it arrived
SIGINT SIGSEGV SIGCHLD
Terminate Terminate Ignore
User typed ctrl-c Segmentation violation Child stopped or terminated
Default Action Corresponding Event
Terminate Kill program (cannot override or ignore)
14 SIGALRM
Terminate Timer signal
Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition
Sogang University
Signal Concepts: Sending a Signal
¢ Kernel sends (delivers) a signal to a destination process by updating some state in the context of the destination process
¢ Kernel sends a signal for one of the following reasons:
§ Kernel has detected a system event such as divide-by-zero (SIGFPE)
or the termination of a child process (SIGCHLD)
§ Another process has invoked the kill system call to explicitly request the kernel to send a signal to the destination process
Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition
University
Signal Concepts: Receiving a Signal
¢ A destination process receives a signal when it is forced by the kernel to react in some way to the delivery of the signal
¢ Some possible ways to react:
§ Ignore the signal (do nothing)
§ Terminate the process (with optional core dump)
§ Catch the signal by executing a user-level function called signal handler
§ Akin to a hardware exception handler being called in response to an asynchronous interrupt:
(1) Signal received by process
Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition
(2) Control passes to signal handler
(4) Signal handler returns to
next instruction
(3) Signal handler runs
Sogang University
Concepts: Pending and Blocked Signals
¢ A signal is pending if sent but not yet received
§ There can be at most one pending signal of any particular type § Important: Signals are not queued
§ If a process has a pending signal of type k, then subsequent signals of type k that are sent to that process are discarded
¢ A process can block the receipt of certain signals
§ Blocked signals can be delivered, but will not be received until the
signal is unblocked
¢ A pending signal is received at most once Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition
Sogang University
Signal Concepts: Pending/Blocked Bits
¢ Kernel maintains pending and blocked bit vectors in the context of each process
§ pending: represents the set of pending signals
§ Kernel sets bit k in pending when a signal of type k is
§ Kernel clears bit k in pending when a signal of type k is received
§ blocked: represents the set of blocked signals
§ Can be set and cleared by using the sigprocmask function
§ Also referred to as the signal mask.
Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition
University
Sending Signals: Process Groups
¢ Every process belongs to exactly one process group
pid=10 pgid=10
pid=20 pgid=20
pid=21 pgid=20
Foreground process group 20
pid=22 pgid=20
Fore- ground job
Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition
Back- ground job #1
Background process group 32
pid=32 pgid=32
Return process group of current process
Change process group of a process (see text for details)
Back- ground job #2
Background process group 40
pid=40 pgid=40
Sogang University
Signals with /bin/kill Program
¢ /bin/kill program sends arbitrary signal to a process or process group
¢ Examples
§ /bin/kill –9 24818
Send SIGKILL to process 24818
§ /bin/kill –9 – 24817
Send SIGKILL to every process in process group 24817
linux> ./forks 16
Child1: pid=24818 pgrp=24817 Child2: pid=24819 pgrp=24817
24788 pts/2
24820 pts/2
linux> /bin/kill -9 -24817 linux> ps
00:00:00 tcsh
24818 pts/2 00:00:02 forks
24819 pts/2 00:00:02 forks
Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition
24788 pts/2
24823 pts/2
00:00:00 tcsh
00:00:00 ps
00:00:00 ps
Sogang University
Sending Signals from the Keyboard
¢ Typing ctrl-c (ctrl-z) causes the kernel to send a SIGINT (SIGTSTP) to every job in the foreground process group.
§ SIGINT – default action is to terminate each process
§ SIGTSTP – default action is to stop (suspend) each process
pgid=10 Shell
Fore- pgid=20 ground
Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition
Back- ground job #1
Background process group 32
pid=32 pgid=32
Back- ground job #2
Background process group 40
pid=40 pgid=40
pid=21 pgid=20
pid=22 pgid=20
Foreground process group 20
Sogang University
of ctrl-c and ctrl-z
STAT (process state) Legend:
First letter:
S: sleeping T: stopped R: running
Second letter:
s: session leader
+: foreground proc group
See “man ps” for more details
bluefish> ./forks 17
Child: pid=28108 pgrp=28107 Parent: pid=28107 pgrp=28107
bluefish> ps w
PID TTY STAT
27699 pts/8 Ss
28107 pts/8 T
28108 pts/8 T
28109 pts/8 R+
bluefish> fg
./forks 17
bluefish> ps w
PID TTY STAT
27699 pts/8 Ss
28110 pts/8 R+
TIME COMMAND
0:00 -tcsh
0:01 ./forks 17
0:01 ./forks 17
TIME COMMAND
0:00 -tcsh
Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition
Sogang University
Sending Signals with kill Function
void fork12()
pid_t pid[N];
(i = 0; i < N; i++) {
printf("Killing process %d\n", pid[i]); kill(pid[i], SIGINT);
child_status;
(i = 0; i < N; i++)
if ((pid[i] = fork()) == 0) {
/* Child: Infinite Loop */
while(1) ;
(i = 0; i < N; i++) {
pid_t wpid = wait(&child_status); if (WIFEXITED(child_status))
printf("Child %d terminated with exit status %d\n",
wpid, WEXITSTATUS(child_status)); printf("Child %d terminated abnormally\n", wpid);
and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition
Sogang University
#define N 5
and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition
Sogang University
Receiving Signals
¢ Suppose kernel is returning from an exception handler
and is ready to pass control to process p Process A Process B
context switch context switch
kernel code
kernel code
Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition
Sogang University
¢ Suppose kernel is returning from an exception handler and is
ready to pass control to process p
¢ Kernel computes pnb = pending & ~blocked
§ The set of pending nonblocked signals for process p
¢ If (pnb == 0)
§ Pass control to next instruction in the logical flow for p
§ Choose least nonzero bit k in pnb and force process p to receive signal k
§ The receipt of the signal triggers some action by p
§ Repeat for all nonzero k in pnb
§ Pass control to next instruction in logical flow for p
Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition
Sogang University
Default Actions
¢ Each signal type has a predefined default action, which is one of:
§ The process terminates
§ The process stops until restarted by a SIGCONT signal § The process ignores the signal
Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition
University
Installing Signal Handlers
¢ The signal function modifies the default action associated with the receipt of signal signum:
§ handler_t *signal(int signum, handler_t *handler)
¢ Different values for handler:
§ SIG_IGN: ignore signals of type signum
§ SIG_DFL: revert to the default action on receipt of signals of type signum § Otherwise, handler is the address of a user-level signal handler
§ Called when process receives signal of type signum
§ Referred to as “installing” the handler
§ Executing handler is called “catching” or “handling” the signal
§ When the handler executes its return statement, control passes back to instruction in the control flow of the process that was interrupted by receipt of the signal
Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition
University
Signal Handling Example
void sigint_handler(int sig) /* SIGINT handler */
printf("So you think you can stop the bomb with ctrl-c, do you?\n");
printf("Well...");
fflush(stdout);
printf("OK. :-)\n");
int main() {
/* Install the SIGINT handler */
if (signal(SIGINT, sigint_handler) == SIG_ERR)
unix_error("signal error");
/* Wait for the receipt of a signal */
Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition
Sogang University
and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition
Sogang University
Signals Handlers as Concurrent Flows
¢ A signal handler is a separate logical flow (not process) that runs concurrently with the main program
Process A Process A
while (1) handler(){ ;...
and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition
Sogang University
Another View of Signal Handlers as Concurrent Flows
user code (main)
kernel code
user code (main)
kernel code
user code (handler)
kernel code
user code (main)
Signal delivered to process A
Signal received by process A
context switch context switch
Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition
Sogang University
Nested Signal Handlers
¢ Handlers can be interrupted by other handlers
Main program (2) Control passes
(1) Program catches signal s
(7) Main program resumes
to handler S
(3) Program catches signal t
(6) returns to main program
(4) Control passes to handler T
(5) returns to handler S
Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition
Sogang University
Blocking and Unblocking Signals
¢ Implicit blocking mechanism
§ Kernel blocks any pending signals of type currently being handled. § E.g., A SIGINT handler can’t be interrupted by another SIGINT
¢ Explicit blocking and unblocking mechanism § sigprocmask function
¢ Supporting functions
§ sigemptyset – Create empty set
§ sigfillset – Add every signal number to set § sigaddset – Add signal number to set
§ sigdelset – Delete signal number from set
Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition
University
Temporarily Blocking Signals
sigset_t mask, prev_mask;
Sigemptyset(&mask);
Sigaddset(&mask, SIGINT);
/* Block SIGINT and save previous blocked set */
Sigprocmask(SIG_BLOCK, &mask, &prev_mask);
/* Code region that will not be interrupted by SIGINT */
/* Restore previous blocked set, unblocking SIGINT */
Sigprocmask(SIG_SETMASK, &prev_mask, NULL);
Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition
Sogang University
Safe Signal Handling
¢ Handlers are tricky because they are concurrent with main program and share the same global data structures.
§ Shared data structures can become corrupted.
¢ We’ll explore concurrency issues later in the term.
¢ For now here are some guidelines to help you avoid trouble.
Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition
University
Guidelines for Writing Safe Handlers
¢ G0: Keep your handlers as simple as possible § e.g., Set a global flag and return
¢ G1: Call only async-signal-safe functions in your handlers § printf, sprintf, malloc, and exit are not safe!
¢ G2: Save and restore errno on entry and exit
§ So that other handlers don’t overwrite your value of errno
¢ G3: Protect accesses to shared data structures by temporarily blocking all signals.
§ To prevent possible corruption
¢ G4: Declare global variables as volatile
§ To prevent compiler from storing them in a register
¢ G5: Declare global flags as volatile sig_atomic_t
§ flag: variable that is only read or written (e.g. flag = 1, not flag++)
§ Flag declared this way does not need to be protected like other globals
Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition
University
Async-Signal-Safety
¢ Function is async-signal-safe if either reentrant or non- interruptible by signals.
¢ Posix guarantees 117 functions to be async-signal-safe § Popular functions on the list:
§ _exit, write, wait, waitpid, sleep, kill
§ Popular functions that are not on the list:
§ printf, sprintf, malloc, exit
§ Unfortunate fact: write is the only async-signal-safe output function
Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition
University
Safely Generating Formatted Output
¢ Use the reentrant SIO (Safe I/O library) from csapp.c in your handlers.
§ ssize_t sio_puts(char s[]) /* Put string */
§ ssize_t sio_putl(long v) /* Put long */
§ void sio_error(char s[]) /* Put msg & exit */
void sigint_handler(int sig) /* Safe SIGINT handler */ {
Sio_puts("So you think you can stop the bomb with ctrl- c, do you?\n");
sleep(2); Sio_puts("Well..."); sleep(1); Sio_puts("OK. :-)\n"); _exit(0);
sigintsafe.c
Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition
University
Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition
Sogang University
Signal Handling
¢ Pending signals are
not queued
§For each signal type, one bit indicates whether or not signal is pending...
§...thus at most one pending signal of any particular type.
¢ You can’t use signals to count events, such as children terminating.
int ccount = 0;
void child_handler(int sig) {
int olderrno = errno; pid_t pid;
if ((pid = wait(NULL)) < 0)
Sio_error("wait error"); ccount--;
Sio_puts("Handler reaped child "); Sio_putl((long)pid);
Sio_puts(" \n");
errno = olderrno; }
void fork14() { pid_t pid[N];
ccount = N;
Signal(SIGCHLD, child_handler);
for (i = 0; i < N; i++) {
if ((pid[i] = Fork()) == 0) {
exit(0); /* Child exits */ }
while (ccount > 0) /* Parent spins */
Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition
whaleshark> ./forks 14 Handler reaped child 23240 Handler reaped child 23241
Sogang University
Correct Signal Handling
¢ Must wait for all terminated child processes
§ Put wait in a loop to reap all terminated children
void child_handler2(int sig)
int olderrno = errno;
pid_t pid;
while ((pid = wait(NULL)) > 0) {
Sio_puts(“Handler reaped child “);
Sio_putl((long)pid);
} Sio_puts(” \n”);
if (errno != ECHILD)
Sio_error(“wait error”);
errno = olderrno;
Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition
whaleshark> ./forks 15 Handler reaped child 23246 Handler reaped child 23247 Handler reaped child 23248 Handler reaped child 23249 Handler reaped child 23250 whaleshark>
University
Portable Signal Handling
¢ Ugh! Different versions of Unix can have different signal handling semantics
§ Some older systems restore action to default after catching signal § Some interrupted system calls can return with errno == EINTR
§ Some systems don’t block signals of the type being handled
¢ Solution:sigaction
handler_t *Signal(int signum, handler_t *handler) {
struct sigaction action, old_action;
action.sa_handler = handler;
sigemptyset(&action.sa_mask); /* Block sigs of type being handled */ action.sa_flags
程序代写 CS代考 加微信: powcoder QQ: 1823890830 Email: powcoder@163.com