CS 3733 Operating Systems Notes: Signals

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Symbol	Meaning
SIGABRT	abnormal termination as initiated by abort
SIGALRM	timeout signal as initiated by alarm
SIGFPE	error in arithmetic operation as in division by zero
SIGHUP	hang up (death) on controlling terminal (process)
SIGILL	invalid hardware instruction
SIGINT	interactive attention signal (CTRL-C)
SIGKILL	terminate (cannot be caught or ignored)
SIGPIPE	write on a pipe with no readers
SIGQUIT	interactive termination: dump core (CTRL-|)
SIGSEGV	invalid memory reference
SIGTERM	termination
SIGUSR1	user-defined signal 1
SIGUSR2	user-defined signal 2

Table 5.1, Page 170: POSIX.1 required signals.

Note that SIGKILL is special. It cannot be caught or ignored.

Table 5.2, Page 171: The job control signals control foreground and background processes.

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);

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 *set, sigset_t *oset);

The how can be:

• SIG_BLOCK
• SIG_UNBLOCK
• SIG_SETMASK

Either set or oset may be NULL.

Example 5.8, page 175, initializes a set to contain 2 signals.
Example 5.9, page 176, blocks SIGINT
Example 5.10, pages 176-177, blocks and unblocks SIGINT in a loop
Example 5.11, pages 177-178, blocks signals while a pipe is created
Example 5.12, pages 178-179, blocks signals while child is created

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)(); /* 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_handler)() means a pointer to a function that has not return value.

Example 5.13, page 180: Set up a handler for SIGINT
Example 5.14, pages 180-181: Ignore SIGINT if using the default action
Example 5.15, page 181: Catch SIGINT
Example 5.16, pages 181-182: Set SIGINT to default action
Example 5.17, page 182: return 1 if signal is ignored

Waiting for signals
Old way: pause

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

What you need to do: atomically unblock the signal and suspend the process.

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

Example 5.20, page 184 shows a correct way to wait for a signal.

A biff example

Program 5.1, pages 186-188) uses signals to turn on or off the notification of mail.

System Calls and Signals
Two problems:
system call interruption and
non-reentrant system calls.

System Call Interruption:

• Some system calls that can block indefinitely long will return when a signal is delivered.
• These system calls return -1 and set errno to EINTR
• You can tell which system calls can do this my looking at the return values listed on their man pages.
• Two of the system calls that need special attention are read and write.
• Here is a program segment to restart a read if it is interrupted by a signal:

     while (retval=read(fd, buf,size), retval == -1 && errno == EINTR) ;
     if (retval == -1)
        handle real errors here
  

• Note the use fo the 3rd comma in the while statement.
• The write system call can be handled in a similar way.
• Recall that you also need to handle a write that does not write as many bytes as requested.

Non-reentrant system calls:

• A function that can be safely called from a signal handler is called async-signal-safe
• Recall that functions like strtok are not async-signal-safe
• The table below gives a complete list of those function calls that must be async-signal-safe if the POSIX standard is followed.
• Note that the C I/O functions such as fprintf are not included.
• These may be safe under some implementations but not under others.
• Under Solaris, almost everything is async-signal-safe if it can be.
• For compatibility, you should not assume that the ones not in this list are safe.

Realtime Signals
In 1993 a new signal interface was added to the POSIX standard.

• This is commonly known as the Realtime Signal Extension and is defined in POSIX.1b.
• Some of the enhancements include queueing of signals and the passing of information to the signal handler.

  #include <signal.h>

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

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

    int sival_int;
    void *sival_ptr;

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 5.3, page 195 shows a program to send a queued signal to a process. Note that there is no command line function similar to kill.

Program 5.4, page 196, gives an example program that can receive SIGUSR1 signals.