Apue Signal

28 September 2014

###Signal Generate

The terminal-generated signals occur when users press certain terminal keys.
Hardware exceptions generate signals, detected by the hardware, and the kernel is notified.such as divide by 0, invalid memory reference, and the like.
The kill(2) function allows a process to send any signal to another process or process group.
The kill(1) command allows us to send signals to other processes.
Software conditions can generate signals when a process should be notified of various events.

Signals are classic examples of asynchronous events. They occur at what appear to ss can’t simply test a variable (such as errno) to see whether a signal has occurred; instead, the process has to tell the kernel “if and when this signal occurs, do the following.”random times to the process.

###Action Associated With A Signal

Ignore the signal. SIGKILL and SIGSTOP can never be ignored. hardware exception is undefined.
Catch the signal. SIGKILL and SIGSTOP can’t be caught.
Let the default action apply. default action for most signals is to terminate the process.

###Core File The core file will not be generated

the process was set-user-ID and the current user is not the owner of the program file
the process was set-group-ID and the current user is not the group owner of the file
the user does not have permission to write in the current working directory
the file already exists and the user does not have permission to write to it
the file is too big

SIGABRT abort SIGALRM alarm setitimer SIGCHLD child process terminates or stops, send to parent process SIGCONT job-control signal SIGBUS hardware error SIGEMT hardware error SIGFPE divide by 0, floating-point overflow SIGIOT implementation-defined hardware fault. SIGILL an illegal hardware instruction SIGINT Control-C SIGIO an asynchronous shiaf?/* event SIGIOT Hardware fault SIGPIPE write to a pipeline but the reader has terminated SIGPOLL It can be generated when a specific event occurs on a pollable device. SIGPROF setitimer SIGQUIT Control-backslash SIGSTOP job-control signal stops a process,cannot be caught or ignored SIGSYS an invalid system call. SIGTERM kill SIGTRAP hardware error SIGTSTP Control-Z SIGTTIN generate by terminal driver when a process in background process group tries to read from its control terminal. SIGTTOU generate by terminal driver when a process in background process group tries to write from its control terminal. SIGURG otifies the process that an urgent condition has occurred. It is optionally generated when out-of-band data is received on a network connection. SIGUSR1 user-defined signal SIGUSR2 user-defined signal SIGVTALRM a virtual interval timer set by the setitimer(2) function expires SIGWINCH ioctl set-window-size SIGXFSZ process exceeds its soft file size limit

###unreliable signal

signal get lost :a signal could occur and the process would never know about it
signal of control is weak : a process could catch the signal or ignore it.
a signal was reset to its default each time the signal occurred(now it catch ever time)
unable to turn a signal off when it didn’t want the signal to occur.

###Reentrant Functions

The Single UNIX Specification specifies the functions that are guaranteed to be safe to call from within a signal handler. These functions are reentrant and are called async-signal safe by the Single UNIX Specification.

Besides being reentrant, they block any signals during operation if delivery of a signal might cause inconsistencies.

Cannot Reentrant Function

use static data structure
call malloc or free
part of the standard I/O library

Note

even though we call printf from signal handlers in some of our examples, it is
not guaranteed to produce the expected results,signal handler can interrupt a
call to printf from our main program

as a general rule, when calling the reentrant functions from a signal handler, we should save and restore errno.

###SIGCLD Function

Two signals that continually generate confusion are SIGCLD and SIGCHLD. The name SIGCLD (without the H) is from System V, and this signal has different semantics from the BSD signal, named SIGCHLD. The POSIX.1 signal is also named SIGCHLD.

###Reliable-Signal Terminology and Semantics

A process has the option of blocking the delivery of a signal. If a signal that is blocked is generated for a process, and if the action for that signal is either the default action or to catch the signal, then the signal remains pending for the process until the process either (a) unblocks the signal or (b) changes the action to ignore the signal. The system determines what to do with a blocked signal when the signal is delivered, not when it’s generated. This allows the process to change the action for the signal before it’s delivered. The sigpending function (Section 10.13) can be called by a process to determine which signals are blocked and pending.

Each process has a signal mask that defines the set of signals currently blocked from delivery to that process. We can think of this mask as having one bit for each possible signal. If the bit is on for a given signal, that signal is currently blocked. A process can examine and change its current signal mask by calling sigprocmask.

###kill raise function

    #include <signal.h>
    int kill(pid_t pid, int signo);
    int raise(int signo);
            Both return: 0 if OK, −1 on error

If the signo argument is 0, then the normal error checking is performed by kill, but no signal is sent. This technique is often used to determine if a specific process still exists. that the test for process existence is not atomic. By the time that kill returns the answer to the caller, the process in question might have exited, so the answer is of limited value.

####alarm

    #include <unistd.h>
    unsigned int alarm(unsigned int seconds);
            Returns: 0 or number of seconds until previously set alarm

case 1

If, when we call alarm, a previously registered alarm clock for the process has not yet expired, the number of seconds left for that alarm clock is returned as the value of this function. That previously registered alarm clock is replaced by the new value.

case 2

If a previously registered alarm clock for the process has not yet expired and if the seconds value is 0, the previous alarm clock is canceled. The number of seconds left for that previous alarm clock is still returned as the value of the function.

case 3

If we intend to catch SIGALRM, we need to be careful to install its signal handler before calling alarm. If we call alarm first and are sent SIGALRM before we can install the signal handler, our process will terminate.

#include <stdio.h>
#include <string.h>
#include <unistd.h>
#include <signal.h>
#include <sys/types.h>
#include <pwd.h>

static void my_alram(int signo)
{
    struct  passwd      *rootptr;
    printf("signal hander\n");
    alarm(2);
}

int main(int argc, char **argv)
{
    struct passwd       *ptr;
    /*uncomment for test case3
    alarm(1);
    sleep(2);
    */

    signal(SIGALRM,my_alram);
    alarm(6);//test case 1
    /*
    uncomment for testing case2
    alarm(0);
    */
    for(;;)
    {
        printf("wait signal\n");
        sleep(1);
    }
}

####pause

suspends the calling process until a signal is caught.

    #include <unistd.h>
    int pause(void);
            Returns: −1 with errno set to EINTR

###Signal Set

    #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);
        All four return: 0 if OK, −1 on errorint sigismember()

    int sigismember(const sigset_t *set, int signo);
        Returns: 1 if true, 0 if false, −1 on error

All applications have to call either sigemptyset or sigfillset once for each signal set, before using the signal set, because we cannot assume that the C initialization for external and static variables (0) corresponds to the implementation of signal sets on a given system.

    int sigprocmask(int how, const sigset_t *restrict set, sigset_t *restrict oset,)
        Returns: 0 if OK, −1 on error

The sigprocmask function is defined only for single-threaded processes.

    int sigpending(sigset_t *set);
        Returns: 0 if OK, −1 on error

signals are not queued on this system.

    int sigaction(int signo, const struct sigaction *restrict act,
            struct sigaction *restrict oact);
        Returns: 0 if OK, −1 on error