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Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition
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14
–
513
18
–
613
Carnegie Mellon
Exceptional Control Flow: Signals and Nonlocal Jumps
15-213/18-213/14-513/15-513/18-613: Introduction to Computer Systems 20th Lecture, November 5, 2020
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Review from last lecture
Exceptions
▪ Events that require nonstandard control flow
▪ Generated externally (interrupts) or internally (traps and faults)
Processes
▪ At any given time, system has multiple active processes
▪ Only one can execute at a time on any single core
▪ Each process appears to have total control of
processor + private memory space
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Review (cont.)
Spawning processes ▪ Call fork
▪ One call, two returns
Process completion ▪ Call exit
▪ One call, no return
Reaping and waiting for processes
▪ Call wait or waitpid
Loading and running programs ▪ Call execve (or variant)
▪ One call, (normally) no return
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execve: Loading and Running Programs
int execve(char *filename, char *argv[], char *envp[])
Loads and runs in the current process: ▪Executable filefilename
▪ Can be object file or script file beginning with #!interpreter (e.g., #!/bin/bash)
▪ …with argument list argv
▪ By convention argv[0]==filename
▪ …and environment variable list envp
▪ “name=value” strings (e.g., USER=droh) ▪ getenv, putenv, printenv
Overwrites code, data, and stack
▪ Retains PID, open files and signal context
Called once and never returns
▪ …except if there is an error
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ECF Exists at All Levels of a System Exceptions
▪ Hardware and operating system kernel software Process Context Switch
▪ Hardware timer and kernel software Signals
▪ Kernel software and application software Nonlocal jumps
▪ Application code
Previous Lecture
This Lecture
Textbook and supplemental slides
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Today Shells
Signals
CSAPP 8.4.6 CSAPP 8.5
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Linux Process Hierarchy
Daemon e.g. httpd
Child
Grandchild
[0] init [1]
Login shell … Child
Grandchild
Login shell Child
Note: you can view the hierarchy using the Linux pstree command
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Shell Programs
A shell is an application program that runs programs on behalf
of the user.
▪ sh
▪ csh/tcsh ▪ bash
Original Unix shell (Stephen Bourne, AT&T Bell Labs, 1977) BSD Unix C shell
Simple shell
▪ Described in the textbook, starting at p. 753 ▪ Implementation of a very elementary shell ▪ Purpose
“Bourne-Again” Shell
(default Linux shell)
▪ Understand what happens when you type commands
▪ Understand use and operation of process control operations
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Simple Shell Example
linux> ./shellex
> /bin/ls -l csapp.c
-rw-r–r– 1 bryant users 23053 Jun 15 2015 csapp.c > /bin/ps
PID TTY
31542 pts/2
32017 pts/2
32019 pts/2
> /bin/sleep 10 & 32031 /bin/sleep 10 & > /bin/ps
PID TTY
31542 pts/2
32024 pts/2
32030 pts/2
32031 pts/2
32033 pts/2
> quit
TIME CMD
00:00:01 tcsh
00:00:00 emacs
00:00:00 shellex
00:00:00 sleep
00:00:00 ps
Sleep is running in background
TIME CMD
00:00:01 tcsh
Must give full pathnames for programs
00:00:00 shellex
00:00:00 ps
Run program in background
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Simple Shell Implementation Basic loop
▪ Read line from command line
▪ Execute the requested operation
▪ Built-in command (only one implemented is quit) ▪ Load and execute program from file
int main(int argc, char** argv)
{
char cmdline[MAXLINE]; /* command line */
while (1) {
/* read */
printf(“> “);
Fgets(cmdline, MAXLINE, stdin);
if (feof(stdin))
exit(0);
/* evaluate */
eval(cmdline);
}
…
shellex.c
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Execution is a sequence of read/evaluate steps
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Simple Shell eval Function
void eval(char *cmdline)
{
char *argv[MAXARGS]; /* Argument list execve() */
}
shellex.c
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;
if (!builtin_c
if ((pid = */
if (ex pr
ex }
}
/* Parent waits for foreground job to terminate */
if (!bg) {
int status;
if (waitpid(pid, &status, 0) < 0)
}
} else
unix_error("waitfg: waitpid error");
printf("%d %s", pid, cmdline);
return;
Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition
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/* Ignore empty lines */
parselinewill parse ‘buf’ into ommand(argv)) {
‘argv’ and return whether or not
Fork()) == 0) { /* Child runs user job
ecve(argv[0], argv, environ) < 0) {
intf("%s: Command not found.\n", argv[0]);
input line ended in ‘&’
it(0);
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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)
}
shellex.c
return; /* Ignore empty lines */ 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");
}
else
printf("%d %s", pid, cmdline);
return;
}
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Simple Shell eval Function
void eval(char *cmdline)
{
char *argv[MAXARGS]; /* Argument list execve() */
}
shellex.c
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); }
}
If it is a ‘built in’ command, then
handle it here in this program. Otherwise fork/exec the program
/* Parent waits for foreground job to terminate */
if (!bg) {
int status;
}
else
if (waitpid(pid, &status, 0) < 0) unix_error("waitfg: waitpid error");
printf("%d %s", pid, cmdline);
return;
}
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specified in argv[0]
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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); }
}
Create child
}
shellex.c
/* Parent waits for foreground job to terminate */
if (!bg) {
int status;
if (waitpid(pid, &status, 0) < 0) unix_error("waitfg: waitpid error");
}
else
printf("%d %s", pid, cmdline);
return;
}
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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");
else
} return;
}
shellex.c
}
Start argv[0].
Remember execve only returns on
printf("%d %s", pid, cmdline); error.
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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");
}
else return;
If running child in
}
printf("%d %s", pid, cmdline); foreground, wait until
it is done. Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition
}
shellex.c
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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");
}
else{
printf("%d %s", pid, cmdline); }
}
return; }
Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition
shellex.c 21
If running child in background, print pid and continue doing other stuff.
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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");
}
else
printf("%d %s", pid, cmdline);
return;
}
shellex.c
}
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Oops. There is a problem with this code.
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Problem with Simple Shell Example
Shell designed to run indefinitely
▪ Should not accumulate unneeded resources
▪ Memory
▪ Child processes ▪ File descriptors
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
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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
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Today
Shells Signals
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(partial) Taxonomy
Handled in kernel
Handled in user process
ECF
Asynchronous
Synchronous
Faults
Interrupts
Signals
Traps
Aborts
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Signals
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
ID Name
Default Action Corresponding Event
2 SIGINT 11 SIGSEGV 17 SIGCHLD
Terminate Terminate Ignore
User typed ctrl-c Segmentation violation Child stopped or terminated
9 SIGKILL
Terminate Kill program (cannot override or ignore)
14 SIGALRM
Terminate Timer signal
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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
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Signal Concepts: Sending a Signal
User level
Process A
Process C
Process B
kernel
Pending for A
Pending for B
Pending for C
Blocked for A
Blocked for B
Blocked for C
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Signal Concepts: Sending a Signal
User level
Process A
Process C
Process B
kernel
Pending for A
Pending for B
Pending for C
Blocked for A
Blocked for B
Blocked for C
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Signal Concepts: Sending a Signal
User level
Process A
Process C
Process B
kernel
Pending for A
Pending for B
1
Pending for C
Blocked for A
Blocked for B
Blocked for C
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Signal Concepts: Sending a Signal
User level
Process A
Process C
Process B
kernel
Pending for A
Pending for B
1
Pending for C
Blocked for A
Blocked for B
Blocked for C
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Signal Concepts: Sending a Signal
User level
Process A
Process C
Process B
kernel
Pending for A
Pending for B
0
Pending for C
Blocked for A
Blocked for B
Blocked for C
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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
Icurr Inext
(2) Control passes to signal handler
(4) Signal handler
returns to
next instruction
(3) Signal handler runs
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Signal 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
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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 delivered ▪ 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.
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Signal Concepts: Sending a Signal
User level
Process A
Process C
Process B
kernel
Pending for A
Pending for B
1
Pending for C
Blocked for A
Blocked for B
Blocked for C
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Sending Signals: Process Groups
Every process belongs to exactly one process group
pid=10
pgid=10
Shell
Fore- pgid=20 ground
job
pid=20
Back- ground job #1
Background process group 32
pid=32
pgid=32
Back- ground job #2
Background process group 40
pid=40
pgid=40
Child
pid=21 pgid=20
Child
pid=22 pgid=20
Foreground process group 20
getpgrp()
Return process group of current process
setpgid()
Change process group of a process (see text for details)
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Sending 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
linux> ps
PID TTY
24788 pts/2
24820 pts/2
linux> /bin/kill -9 -24817
linux> ps
TIME CMD
00:00:00 tcsh
24818 pts/2 00:00:02 forks
24819 pts/2 00:00:02 forks
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PID TTY 24788 pts/2 24823 pts/2 linux>
TIME CMD
00:00:00 tcsh
00:00:00 ps
00:00:00 ps
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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
pid=10 pgid=10
Shell
Fore- pgid=20 ground
job
pid=20
Back- ground job #1
Background process group 32
pid=32
pgid=32
Back- ground job #2
Background process group 40
pid=40
pgid=40
Child
pid=21
pgid=20
Child
pid=22
pgid=20
Foreground
process group 20
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Example 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
Suspended
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 0:00 ps w
TIME COMMAND
0:00 -tcsh
0:00 ps w
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Receiving Signals
Suppose kernel is returning from an exception handler
and is ready to pass control to process p Process q Process p
Time
context switch
context switch
user code
kernel code
user code
kernel code
user code
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Receiving Signals
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
Else
▪ 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
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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
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Quiz Time!
Check out:
https://canvas.cmu.edu/courses/17808
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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
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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”); sleep(2);
printf(“Well…”);
fflush(stdout);
sleep(1);
printf(“OK. :-)\n”);
exit(0);
}
int main(int argc, char** argv)
{
/* Install the SIGINT handler */
if (signal(SIGINT, sigint_handler) == SIG_ERR) unix_error(“signal error”);
/* Wait for the receipt of a signal */
pause();
return 0; }
sigint.c
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Signals Handlers as Concurrent Flows
A signal handler is a separate logical flow (not process) that runs concurrently with the main program
But, this flow exists only until returns to main program
Process A Process A
while (1) handler(){ ;…
}
Process B
Time
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Another View of Signal Handlers as Concurrent Flows
Signal delivered to process A
Signal received by process A
context switch
context switch
Process A
Process B
Icurr
user code (main)
kernel code
user code (main)
kernel code
user code (handler)
kernel code
Inext
user code (main)
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Nested Signal Handlers
Handlers can be interrupted by other handlers
Main program Handler S Handler T (2) Control passes
(1) Program catches signal s
(7) Main program resumes
Icurr Inext
to handler S
(3) Program catches signal t
(6) Handler S returns to main program
(4) Control passes to handler T
(5) Handler T returns to handler S
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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
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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 52
…
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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.
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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
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Async-Signal-Safety
Function is async-signal-safe if either reentrant (e.g., all variables stored on stack frame, CS:APP3e 12.7.2) or non- interruptible by signals.
Posix guarantees 117 functions to be async-signal-safe ▪ Source: “man 7 signal-safety”
▪ 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
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Safe Formatted Output: Option #1
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) ▪ void sio_error(char s[])
/* Put long */
/* Put msg & exit */
void sigint_handler(int sig) /* Safe SIGINT handler */ {
}
sigintsafe.c
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);
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Safe Formatted Output: Option #2
Use the new & improved reentrant sio_printf ! ▪ Handles restricted class of printf format strings
▪ Recognizes: %c %s %d %u %x %% ▪ Size designators ‘l’ and ‘z’
void sigint_handler(int sig) /* Safe SIGINT handler */ {
Sio_printf(“So you think you can stop the bomb”
” (process %d) with ctrl-%c, do you?\n”,
(int) getpid(), ‘c’);
sleep(2); Sio_puts(“Well…”); sleep(1); Sio_puts(“OK. :-)\n”); _exit(0);
}
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sigintsafe.c
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volatile 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");
sleep(1);
Correct Signal Handling
errno = olderrno; }
void fork14() {
pid_t pid[N];
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.
This code is incorrect!
int i;
ccount = N;
Signal(SIGCHLD, child_handler);
for (i = 0; i < N; i++) {
if ((pid[i] = Fork()) == 0) {
Sleep(1);
exit(0); /* Child exits */
}
}
while (ccount > 0) /* Parent spins */
;
}
forks.c
58
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N == 5
whaleshark> ./forks 14 Handler reaped child 23240 Handler reaped child 23241 . . .(hangs)
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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) {
ccount–;
Sio_puts(“Handler reaped child “);
Sio_putl((long)pid);
Sio_puts(” \n”);
}
if (errno != ECHILD)
Sio_error(“wait error”);
errno = olderrno;
}
whaleshark> ./forks 15 Handler reaped child 23246 Handler reaped child 23247 Handler reaped child 23248 Handler reaped child 23249 Handler reaped child 23250 whaleshark>
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Synchronizing to Avoid Parent-Child Race
int main(int argc, char **argv)
{
int pid;
sigset_t mask_all, mask_one, prev_one; int n = N; /* N = 5 */ Sigfillset(&mask_all); Sigemptyset(&mask_one); Sigaddset(&mask_one, SIGCHLD); Signal(SIGCHLD, handler);
initjobs(); /* Initialize the job list */
while (n–) {
Sigprocmask(SIG_BLOCK, &mask_one, &prev_one); /* Block SIGCHLD */ if ((pid = Fork()) == 0) { /* Child process */
Sigprocmask(SIG_SETMASK, &prev_one, NULL); /* Unblock SIGCHLD */
Execve(“/bin/date”, argv, NULL);
}
Sigprocmask(SIG_BLOCK, &mask_all, NULL); /* Parent process */ addjob(pid); /* Add the child to the job list */ Sigprocmask(SIG_SETMASK, &prev_one, NULL); /* Unblock SIGCHLD */
} exit(0);
} procmask2.c Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition
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Explicitly Waiting for Signals
Handlers for program explicitly waiting for SIGCHLD to arrive.
volatile sig_atomic_t pid;
void sigchld_handler(int s)
{
int olderrno = errno;
pid = Waitpid(-1, NULL, 0); /* Main is waiting for nonzero pid */
errno = olderrno;
}
void sigint_handler(int s) {
}
Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition 63
waitforsignal.c
Explicitly Waiting for Signals
int main(int argc, char **argv) {
sigset_t mask, prev;
int n = N; /* N = 10 */
Signal(SIGCHLD, sigchld_handler);
Signal(SIGINT, sigint_handler);
Sigemptyset(&mask);
Sigaddset(&mask, SIGCHLD);
while (n–) {
Sigprocmask(SIG_BLOCK, &mask, &prev); /* Block SIGCHLD */
if (Fork() == 0) /* Child */
exit(0);
/* Parent */
pid = 0;
Sigprocmask(SIG_SETMASK, &prev, NULL); /* Unblock SIGCHLD */
/* Wait for SIGCHLD to be received (wasteful!) */
while (!pid) ;
/* Do some work after receiving SIGCHLD */
printf(“.”);
}
printf(“\n”);
exit(0);
}
Carnegie Mellon
Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition
64
Similar to a shell waiting for a foreground job to terminate.
waitforsignal.c
Carnegie Mellon
Explicitly Waiting for Signals
Program is correct, but very wasteful ▪ Program in busy-wait loop
Possible race condition
▪ Between checking pid and starting pause, might receive signal
Safe, but slow
▪ Will take up to one second to respond
while (!pid)
;
while (!pid) /* Race! */
pause();
while (!pid) /* Too slow! */
sleep(1);
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Waiting for Signals with sigsuspend int sigsuspend(const sigset_t *mask)
Equivalent to atomic (uninterruptable) version of:
sigprocmask(SIG_SETMASK, &mask, &prev); pause();
sigprocmask(SIG_SETMASK, &prev, NULL);
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Waiting for Signals with sigsuspend
int main(int argc, char **argv) { sigset_t mask, prev;
int n = N; /* N = 10 */ Signal(SIGCHLD, sigchld_handler); Signal(SIGINT, sigint_handler); Sigemptyset(&mask); Sigaddset(&mask, SIGCHLD);
}
sigsuspend.c
while (n–) {
Sigprocmask(SIG_BLOCK, &mask, &prev); /* Block SIGCHLD */
if (Fork() == 0) /* Child */
exit(0);
/* Wait for SIGCHLD to be received */
pid = 0;
while (!pid)
Sigsuspend(&prev);
/* Optionally unblock SIGCHLD */
Sigprocmask(SIG_SETMASK, &prev, NULL);
/* Do some work after receiving SIGCHLD */
printf(“.”);
} printf(“\n”); exit(0);
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Summary
Signals provide process-level exception handling ▪ Can generate from user programs
▪ Can define effect by declaring signal handler
▪ Be very careful when writing signal handlers
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Additional slides
Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition 69
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Sending Signals with kill Function
void fork12() {
pid_t pid[N];
int i;
int child_status;
for (i = 0; i < N; i++)
if ((pid[i] = fork()) == 0) {
/* Child: Infinite Loop */
while(1) ;
}
for (i = 0; i < N; i++) {
printf("Killing process %d\n", pid[i]); kill(pid[i], SIGINT);
}
for (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));
else
printf("Child %d terminated abnormally\n", wpid); forks.c
} }
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Nonlocal Jumps: setjmp/longjmp Powerful (but dangerous) user-level mechanism for
transferring control to an arbitrary location
▪ Controlled to way to break the procedure call / return discipline ▪ Useful for error recovery and signal handling
int setjmp(jmp_buf j) ▪ Must be called before longjmp
▪ Identifies a return site for a subsequent longjmp ▪ Called once, returns one or more times
Implementation:
▪ Remember where you are by storing the current register context,
stack pointer, and PC value in jmp_buf
▪ Return 0
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setjmp/longjmp (cont)
void longjmp(jmp_buf j, int i)
▪ Meaning:
▪ return from the setjmp remembered by jump buffer j again ... ▪ ... this time returning i instead of 0
▪ Called after setjmp
▪ Called once, but never returns
longjmpImplementation:
▪ Restore register context (stack pointer, base pointer, PC value) from
jump buffer j
▪ Set %eax (the return value) to i
▪ Jump to the location indicated by the PC stored in jump buf j
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setjmp/longjmp Example
Goal: return directly to original caller from a deeply-
nested function
/* Deeply nested function foo */
void foo(void)
{
if (error1)
longjmp(buf, 1);
bar(); }
void bar(void)
{
if (error2) longjmp(buf, 2);
}
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jmp_buf buf;
int error1 = 0;
int error2 = 1;
void foo(void), bar(void);
int main() {
switch(setjmp(buf)) {
case 0:
foo();
break; case 1:
printf("Detected an error1 condition in foo\n");
break;
case 2:
printf("Detected an error2 condition in foo\n");
break;
default:
printf("Unknown error condition in foo\n"); }
exit(0); }
Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition 74
setjmp/longjmp Example (cont)
Carnegie Mellon
Limitations of Nonlocal Jumps Works within stack discipline
▪ Can only long jump to environment of function that has been called
but not yet completed
jmp_buf env;
P1() {
if (setjmp(env)) {
/* Long Jump to here */
} else { P2();
} }
P2()
{ ...P2();...P3();}
P3() {
longjmp(env, 1);
}
env
Before longjmp
After longjmp
P1
P1
P2
P2
P2
P3
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Limitations of Long Jumps (cont.) Works within stack discipline
▪ Can only long jump to environment of function that has been called but not yet completed
P1
P2
jmp_buf env;
P1() {
P2(); P3(); }
P2() {
if (setjmp(env)) {
env
/* Long Jump to here */ X }
}
P3() {X
longjmp(env, 1);
}
At setjmp
P1
P2
env
P1
P3
P2 returns
env
Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition
76
At longjmp
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Putting It All Together: A Program That Restarts Itself When ctrl-c’d
#include "csapp.h" sigjmp_buf buf;
void handler(int sig)
{
siglongjmp(buf, 1);
}
int main() {
if (!sigsetjmp(buf, 1)) {
Signal(SIGINT, handler);
Sio_puts("starting\n");
}
else
Sio_puts("restarting\n");
while(1) {
Sleep(1);
Sio_puts("processing...\n"); }
exit(0); /* Control never reaches here */
} restart.c Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition
77
greatwhite> ./restart starting processing… processing… processing… restarting processing… processing… restarting processing… processing… processing…
Ctrl-c
Ctrl-c