UNIX System Programming Overview Signals 1. Definition 2. Signal Types 3. Generating a Signal 4. Responding to a Signal Objectives 5. Common Uses of Signals – Introduce signals 6. Implementing a read() – Concentrate on sigaction() function Timeout continued 1730 UNIX System Programming: Signals Maria Hybinette 1 1730 UNIX System Programming: Signals Maria Hybinette 2 1. Definition 7. POSIX Signal Functions A signal is an asynchronous event which is 8. Interrupted System Calls delivered to a process. 9. System Calls inside Handlers 10. More Information Asynchronous means that the event can occur at any time – may be unrelated to the execution of the process – e.g. user types ctrl-C, or the modem hangs 1730 UNIX System Programming: Signals Maria Hybinette 3 1730 UNIX System Programming: Signals Maria Hybinette 4 2. Signal Types (31 in POSIX) Signal Sources terminal memory Name Description Default Action driver management shell command SIGINT Interrupt character typed terminate process SIGQUIT Quit character typed (^\) create core image SIGINT SIGHUP SIGKILL kill -9 terminate process SIGQUIT SIGSEGV Invalid memory reference create core image SIGKILL kernel SIGPIPE SIGPIPE Write on pipe but no reader terminate process SIGALRM alarm() clock ‘rings’ terminate process SIGUSR1 user-defined signal type terminate process SIGWINCH SIGALRM SIGUSR2 user-defined signal type terminate process window : manager a process SIGUSR1 See man 7 signal other user processes 1730 UNIX System Programming: Signals Maria Hybinette 5 1730 UNIX System Programming: Signals Maria Hybinette 6 UNIX: Signals Maria Hybinette 1 3. Generating a Signal kill() Use the UNIX command: $ kill -KILL 4481 Send a signal to a process (or group of processes). – send a SIGKILL signal to pid 4481 – check #include <signal.h> ps –l int kill( pid_t pid, int signo ); – to make sure process died kill is not a good name; send_signal Return 0 if ok, -1 on error. might be better. 1730 UNIX System Programming: Signals Maria Hybinette 7 1730 UNIX System Programming: Signals Maria Hybinette 8 4. Responding to a Signal Some pid Values A process can: – ignore/discard the signal (not possible with pid Meaning SIGKILL or SIGSTOP) > 0 send signal to process pid – execute a signal handler function, and then possibly resume execution or terminate == 0 send signal to all processes whose process group ID – carry out the default action for that signal equals the sender’s pgid. e.g. parent kills all children The choice is called the process’ signal disposition 1730 UNIX System Programming: Signals Maria Hybinette 9 1730 UNIX System Programming: Signals Maria Hybinette 10 signal(): library call Actual Prototype The actual prototype, listed in the “man” page is a bit Specify a signal handler function to deal with a signal type. perplexing but is an expansion of the Sigfunc type: #include <signal.h> void (*signal(int signo, void(*handler)(int)))(int); typedef void Sigfunc(int); /* my defn */ In Linux: Sigfunc *signal( int signo, Sigfunc *handler ); typedef void (*sighandler_t)(int); – signal returns a pointer to a function that returns an int (i.e. it sig_handler_t signal(int signo, sighandler_t handler); returns a pointer to Sigfunc Signal returns a pointer to a function that returns an int Returns previous signal disposition if ok, SIG_ERR on error. 1730 UNIX System Programming: Signals Maria Hybinette 11 1730 UNIX System Programming: Signals Maria Hybinette 12 UNIX: Signals Maria Hybinette 2 The signal function itself returnsSignal a pointer toHandling a function. Example The return type is the same as the function that is passedThe signal in, toThe be handler function int main() { i.e., aUse function the signal that handling takes caughtlibrary: an or ignored signal.hReceives a single integer signal( SIGINT, foo ); int and returns a void is given as argumentArgument and returns void : Then can use the signal call: sig /* do usual things until SIGINT */ #include <signal.h> return 0; } void (*signal( int sig, void (*handler)(int))) (int) ; void foo( int signo ) signal returns a pointer to the PREVIOUS signal { handler : /* deal with SIGINT signal */ The function to be called Signal #includeis a function <signal.h> when the specified signal that takestypedef two void Sigfunc(int); is /* received my defnThe */is returnedgiven as functiona return; /* return to program */ Sigfunc *signal( int signo, Sigfunc *handler ); } arguments: pointer totakes the function a integer sig and handler handler parameter. 1730 UNIX System Programming: Signals Maria Hybinette 13 1730 UNIX System Programming: Signals Maria Hybinette 14 sig_examp.c #include <stdio.h> #include <unistd.h> : #include <signal.h> while(1) { void sig_usr( int signo ); /* handles two signals */ printf( “%2d\n“, I ); pause(); int main() { /* pause until signal handler int i = 0; * has processed signal */ if( signal( SIGUSR1,sig_usr ) == SIG_ERR ) i++; printf( “Cannot catch SIGUSR1\n” ); } if( signal( SIGUSR2,sig_usr ) == SIG_ERR ) return 0; printf(“Cannot catch SIGUSR2\n”); } : continued continued 1730 UNIX System Programming: Signals Maria Hybinette 15 1730 UNIX System Programming: Signals Maria Hybinette 16 Usage void sig_usr( int signo ) /* argument is signal number */ { $ sig_examp & if( signo == SIGUSR1 ) [1] 4720 printf(“Received SIGUSR1\n”); 0 else if( signo == SIGUSR2 ) $ kill -USR1 4720 printf(“Received SIGUSR2\n”); Received SIGUSR1 else 1 printf(“Error: received signal $ kill -USR2 4720 %d\n”, signo); Received SIGUSR2 2 return; $ kill 4720 /* send SIGTERM */ } [1] + Terminated sig_examp & $ 1730 UNIX System Programming: Signals Maria Hybinette 17 1730 UNIX System Programming: Signals Maria Hybinette 18 UNIX: Signals Maria Hybinette 3 Special Sigfunc * Values Multiple Signals If many signals of the same type are waiting Value Meaning to be handled (e.g. two SIGINTs), then most UNIXs will only deliver one of them. SIG_IGN Ignore / discard the signal. SIG_IGN – the others are thrown away SIG_DFL Use default action to handle signal. SIG_ERR Returned by signal() as an error. If many signals of different types are waiting to be handled (e.g. a SIGINT, SIGSEGV, SIGUSR1), they are not delivered in any fixed order. 1730 UNIX System Programming: Signals Maria Hybinette 19 1730 UNIX System Programming: Signals Maria Hybinette 20 pause() The Reset Problem Suspend the calling process until a signal is caught. In Linux (and many other UNIXs), the #include <unistd.h> signal disposition in a process is reset to its int pause(void); signal disposition in a process is reset to its default action immediately after the signal Returns -1 with errno assigned EINTR. has been delivered. (Linux assigns it ERESTARTNOHAND). pause() only returns after a signal handler has Must call signal() again to reinstall the returned. signal handler function. 1730 UNIX System Programming: Signals Maria Hybinette 21 1730 UNIX System Programming: Signals Maria Hybinette 22 To keep catching Reset Problem the signal with this Reset Problem Example function, must call : the signal system int main() void ouch( int sig ) call again. { { signal(SIGINT, foo); printf( "OUCH! - I got signal %d\n", sig ); : (void) signal(SIGINT, ouch); /* do usual things until SIGINT */ } } Problem: from the time int main() that the interrupt function { starts to just before the void foo(int signo) signal handler is re-established (void) signal( SIGINT, ouch ); { the signal will not be while(1) handled. signal(SIGINT, foo); /* reinstall */ { : printf("Hello World!\n"); return; sleep(1); If another SIGINT signal is } } received during this time, default behavior will be done, } 1730 UNIX System Programming: Signals Maria Hybinette 23 1730 UNIX System Programming: Signals Maria Hybinette i.e., program will terminate.24 UNIX: Signals Maria Hybinette 4 Re-installation may be too 5. Common Uses of slow! Signals There is a (very) small time period in foo() 5.1. Ignore a Signal when a new SIGINT signal will cause the default action to be carried out -- process 5.2. Clean up and Terminate termination. 5.3. Dynamic Reconfiguration 5.4. Report Status With signal() there is no answer to this problem. 5.5. Turn Debugging on/off – POSIX signal functions solve it (and some 5.6. Restore Previous Handler other later UNIXs) 1730 UNIX System Programming: Signals Maria Hybinette 25 1730 UNIX System Programming: Signals Maria Hybinette 26 5.2. Clean up and 5.1. Ignore a Signal Terminate : int main() : { /* global variables */ signal(SIGINT, SIG_IGN); signal(SIGINT, SIG_IGN); int my_children_pids; signal(SIGQUIT, SIG_IGN); : : /* do work without interruptions */ void clean_up(int signo); } int main() Cannot ignore/handle SIGKILL or SIGSTOP { Should check for SIG_ERR signal(SIGINT, clean_up); : } continued 1730 UNIX System Programming: Signals Maria Hybinette 27 1730 UNIX System} Programming: Signals Maria Hybinette 28 Problems void clean_up(int signo) { If a program is run in the background then unlink(“/tmp/work-file”); the interrupt and quit signals (SIGINT, kill(my_children_pids, SIGTERM); SIGQUIT) are automatically ignored. wait((int *)0); fprintf(stderr, “Program terminated\n”); Your code should not override these exit(1); changes: } – check if the signal dispositions are SIG_IGN 1730 UNIX System Programming: Signals Maria Hybinette 29 1730 UNIX System Programming: Signals Maria Hybinette 30 UNIX: Signals Maria Hybinette 5 Checking the Disposition 5.3. Dynamic new disposition old disposition Reconfiguration : : if( signal(SIGINT, SIG_IGN ) != SIG_IGN ) void read_config(int signo);