CS 3733 Operating Systems Notes: USP Chapter 8 - Signals

You can send a signal to a process from the command line using kill
kill -l will list the signals the system understands
kill [-signal] pid will send a signal to a process.
The optional argument may be a name or a number.
The default is SIGTERM.
To unconditionally kill a process, use:
kill -9 pid which is
kill -KILL pid.

From a program you can use the kill system call:

#include <sys/types.h>
#include <signal.h>
int kill(pid_t pid, int sig);

   if (kill(3423, SIGUSR1) == -1)
      perror("Failed to send the SIGUSR1 signal");

   if (kill(getppid(), SIGTERM) == -1)
       perror ("Failed to kill parent");

   if (raise(SIGUSR1) != 0)
      perror("Failed to raise SIGUSR1");

int main(void) {
   alarm(10);
   for ( ; ; ) ;
}

Signal Mask and Signal Sets
How do you deal with a set of signals?
Originally, an int would hold a collection of bits, one per signal.
Now, signal sets of type sigset_t are used.

Here are the routines for handling sets of signals.
You should compare these to the way select handles sets of file descriptors.

  #include <signal.h>

  int sigemptyset(sigset_t *set);
  int sigfillset(sigset_t *set);
  int sigaddset(sigset_t *set, int signo);
  int sigdelset(sigset_t *set, int signo);
  int sigismember(const sigset_t *set, int signo);

The process signal mask is a set of signals that are blocked.
A blocked signal remains pending after it is generated until the signal is unblocked.
The process signal mask is modified with the sigprocmask system call.

   #include <signal.h>
   int sigprocmask(int how, const sigset_t *restrict set,
                   sigset_t *restrict oset);

The how can be:

• SIG_BLOCK
• SIG_UNBLOCK
• SIG_SETMASK

Either set or oset may be NULL.

Example 8.9: initialize a signal set:

   if ((sigemptyset(&twosigs) == -1) ||
       (sigaddset(&twosigs, SIGINT) == -1)  ||
       (sigaddset(&twosigs, SIGQUIT) == -1))
       perror("Failed to set up signal mask");

   sigset_t newsigset;
 
   if ((sigemptyset(&newsigset) == -1) ||
       (sigaddset(&newsigset, SIGINT) == -1))
      perror("Failed to initialize the signal set");
   else if (sigprocmask(SIG_BLOCK, &newsigset, NULL) == -1)
      perror("Failed to block SIGINT");

Program 8.2 on page 264 blocks a signal while creating two pipes.

Program 8.3 on page 265 blocks all signals before a fork and exec.

Program 8.4 on page 266 blocks some signals while getting a password.

The process signal mask is inherited by the child process.

Catching and Ignoring Signals: sigaction

   #include <signal.h>

   int sigaction(int signo, const struct sigaction *act,
                             struct sigaction *oact);
   struct sigaction {
      void (*sa_handler)(int); /* SIG_DFL, SIG_IGN or pointer to function */
      sigset_t sa_mask;        /* additional signals to be blocked
                                     during execution of handler */
      int sa_flags;            /* special flags and options */
      void(*sa_sigaction) (int, siginfo_t *, void *); /* realtime handler */
   };

If the SA_SIGINFO flag of the sa_flags field is clear, sa_handler specifies the action to be taken.

void (*sa_handler)() means a pointer to a function that has not return value.

Example 8.15: ignore SIGINT if the default action is set:

struct sigaction act;
 
if (sigaction(SIGINT, NULL, &act) == -1)  /* Find current SIGINT handler */
   perror("Failed to get old handler for SIGINT");
else if (act.sa_handler == SIG_DFL) {    /* if SIGINT handler is default */
   act.sa_handler = SIG_IGN;         /* set new SIGINT handler to ignore */
   if (sigaction(SIGINT, &act, NULL) == -1)
      perror("Failed to ignore SIGINT");
}

Example 8.16: set up a signal handler for SIGINT:

   void catchctrlc(int signo) {
      char handmsg[] = "I found Ctrl-C\n";
      int msglen = sizeof(handmsg);
 
      write(STDERR_FILENO, handmsg, msglen);
   }
   ...
   struct sigaction act;
   act.sa_handler = catchctrlc;
   act.sa_flags = 0;
   if ((sigemptyset(&act.sa_mask) == -1) ||
       (sigaction(SIGINT, &act, NULL) == -1))
      perror("Failed to set SIGINT to handle Ctrl-C");

Example 8.18: set the action of SIGINT to the default:

   struct sigaction newact;
 
   newact.sa_handler = SIG_DFL;    /* new handler set to default */
   newact.sa_flags = 0;            /* no special options */
   if ((sigemptyset(&newact.sa_mask) == -1) ||
       (sigaction(SIGINT, &newact, NULL) == -1))
      perror("Failed to set SIGINT to the default action");

Example 8.19: see if a signal is ignored:

int testignored(int signo) {
   struct sigaction act;
   if ((sigaction(signo, NULL, &act) == -1) || (act.sa_handler != SIG_IGN))
      return 0;
   return 1;
}

Program 8.5: A program that terminates gracefully on ctrl-C

Program 8.6: A program to estimate the average value of sin(x)

Waiting for signals
Old way: pause

#include <unistd.h>
int pause(void);

   static volatile sig_atomic_t sigreceived = 0;

   while(sigreceived == 0)
       pause();

   static volatile sig_atomic_t sigreceived = 0;

   int signum;
   sigset_t sigset;
 
   sigemptyset(&sigset);
   sigaddset(&sigset, signum);
   sigprocmask(SIG_BLOCK, &sigset, NULL);
   while(sigreceived == 0)
      pause();

#include <signal.h>
int sigsuspend(const sigset_t *sigmask);

Example 8.25: a correct way to wait for a signal:

 1. static volatile sig_atomic_t sigreceived = 0;

 3. sigset_t maskblocked, maskold, maskunblocked;
 4. int signum = SIGUSR1;

 6. sigprocmask(SIG_SETMASK, NULL, &maskblocked);
 7. sigprocmask(SIG_SETMASK, NULL, &maskunblocked);
 8. sigaddset(&maskblocked, signum);
 9. sigdelset(&maskunblocked, signum);
10. sigprocmask(SIG_BLOCK, &maskblocked, &maskold);
11. while(sigreceived == 0)
12.    sigsuspend(&maskunblocked);
13. sigprocmask(SIG_SETMASK, &maskold, NULL);

Program 8.7, simplesuspend, shows how to safely block on a specific signal.

Program 8.8 shows how to use Program 8.7.

Programs 8.9 and 8.10 implement a simple biff.

sigwait
This provides an alternative way to wait for signals.
Idea:

• Block all signals
• Put the signals you want to wait for in a sigset_t
• call sigwait
• sigwait blocks the process until at least one of these signals is pending.
• It removes one of the pending signals and gives you the corresponding signal number in the second parameter..
• Do what you want: no signal handler needed.
• It returns 0 on success and -1 on error with errno set.

  #include <signal.h>

  int sigwait(const sigset_t *restrict sigmask, int *restrict signo);

Errors and async-signal safety

Three issues in handling signals:

1. Certain blocking system calls will return -1 with errno set to EINTR if a signal is caught while the function is blocked.
   See the man page to see of a system call can set errno to EINTR.
   In this case you should usually restart the function since a real error has not occurred.

   The restart library handles this for many of the most important system calls.
   Look at the functions in the restart library.

2. Handling errors in signal handlers.
   If you use a function that can modify errno in a signal handler, you must make sure that it does not interfere with error handling in the main part of the program.
   There is only one errno and it is used by both the main program and by the signal handler.
   Solution: in the signal handler, save and restore errno.

      void myhandler(int signo) {
         int esaved;
         esaved = errno;
         write(STDOUT_FILENO, "Got a signal\n", 13);
         errno = esaved;
      }
   

3. Async-signal safety.
   
   • Only certain system calls and library functions may be used safely in a signal handler.
   • The strtok function is an example of a function that might have a problem.
   • In fact, the POSIX standard only graruantees that a small number of functions are async-signal safe, that is safe to use in a signal handler and the main program.
   • Other functions may be async-signal safe in some implementations.
   • Almost none of the functions in the standard C library are on the list.

Realtime Signals: From Section 9.4
Recall the sigaction structure:

   struct sigaction {
      void (*sa_handler)(int); /* SIG_DFL, SIG_IGN or pointer to function */
      sigset_t sa_mask;        /* additional signals to be blocked
                                     during execution of handler */
      int sa_flags;            /* special flags and options */
      void(*sa_sigaction) (int, siginfo_t *, void *); /* realtime handler */
   };

This defines a new type a signal handler that takes three parameters instead of one.
It also defines a new data type: siginfo_t which is a structure containing at least the following:

    int si_signo;            /* signal number */
    int si_code;             /* cause of the signal */
    union sigval si_value;   /* signal value */

    int sival_int;
    void *sival_ptr;

Signals set up with this type of signal handler are queued.
The system call to send one of these signals is called sigqueue rather than kill:

  #include <signal.h>

  int sigqueue(pid_t pid, int signo, const union sigval value);

On some systems, only certain signals can be queued, those between SIGRTMIN and SIGRTMAX.

Program 9.9, page 323 shows a program to send a queued signal to a process. Note that there is no command line function similar to kill.

Program 9.10, page 324, gives an example program that can receive SIGUSR1 signals.