8.2 Generating Signals

Every signal has a symbolic name starting with SIG . The signal names are defined in signal.h , which must be included by any C program that uses signals. The names of the signals represent small integers greater than 0. Table 8.1 describes the required POSIX signals and lists their default actions. Two signals, SIGUSR1 and SIGUSR2 , are available for users and do not have a preassigned use. Some signals such as SIGFPE or SIGSEGV are generated when certain errors occur; other signals are generated by specific calls such as alarm .

Table 8.1. The POSIX required signals.

signal	description	default action
SIGABRT	process abort	implementation dependent
SIGALRM	alarm clock	abnormal termination
SIGBUS	access undefined part of memory object	implementation dependent
SIGCHLD	child terminated , stopped or continued	ignore
SIGCONT	execution continued if stopped	continue
SIGFPE	error in arithmetic operation as in division by zero	implementation dependent
SIGHUP	hang-up (death) on controlling terminal (process)	abnormal termination
SIGILL	invalid hardware instruction	implementation dependent
SIGINT	interactive attention signal (usually Ctrl-C)	abnormal termination
SIGKILL	terminated (cannot be caught or ignored)	abnormal termination
SIGPIPE	write on a pipe with no readers	abnormal termination
SIGQUIT	interactive termination: core dump (usually Ctrl- )	implementation dependent
SIGSEGV	invalid memory reference	implementation dependent
SIGSTOP	execution stopped (cannot be caught or ignored)	stop
SIGTERM	termination	abnormal termination
SIGTSTP	terminal stop	stop
SIGTTIN	background process attempting to read	stop
SIGTTOU	background process attempting to write	stop
SIGURG	high bandwidth data available at a socket	ignore
SIGUSR1	user -defined signal 1	abnormal termination
SIGUSR2	user-defined signal 2	abnormal termination

Generate signals from the shell with the kill command. The name kill derives from the fact that, historically, many signals have the default action of terminating the process. The signal_name parameter is a symbolic name for the signal formed by omitting the leading SIG from the corresponding symbolic signal name.

  SYNOPSIS  kill -s signal_name pid...     kill -l [exit_status]     kill [-signal_name] pid...     kill [-signal_number] pid...  POSIX:Shell and Utilities  

The last two lines of the synopsis list the traditional forms of the kill command. Despite the fact that these two forms do not follow the POSIX guidelines for command-line arguments, they continue to be included in the POSIX standard because of their widespread use. The last form of kill supports only the signal_number values of for signal , 1 for signal SIGHUP , 2 for signal SIGINT , 3 for signal SIGQUIT , 6 for signal SIGABRT , 9 for signal SIGKILL , 14 for signal SIGALRM and 15 for signal SIGTERM .

Example 8.1

The following command is the traditional way to send signal number 9 ( SIGKILL ) to process 3423.

 kill -9 3423 

Example 8.2

The following command sends the SIGUSR1 signal to process 3423 .

 kill -s USR1 3423 

Example 8.3

The kill -l command gives a list of the available symbolic signal names. A system running Sun Solaris produced the following sample output.

 % kill -l HUP INT QUIT ILL TRAP ABRT EMT FPE KILL BUS SEGV SYS PIPE ALRM TERM USR1 USR2 CLD PWR WINCH URG POLL STOP TSTP CONT TTIN TTOU VTALRM PROF XCPU XFSZ WAITING LWP FREEZE THAW CANCEL LOST XRES RTMIN RTMIN+1 RTMIN+2 RTMIN+3 RTMAX-3 RTMAX-2 RTMAX-1 RTMAX 

Call the kill function in a program to send a signal to a process. The kill function takes a process ID and a signal number as parameters. If the pid parameter is greater than zero, kill sends the signal to the process with that ID. If pid is 0, kill sends the signal to members of the caller's process group . If the pid parameter is -1, kill sends the signal to all processes for which it has permission to send. If the pid parameter is another negative value, kill sends the signal to the process group with group ID equal to pid . Section 11.5 discusses process groups.

  SYNOPSIS  #include <signal.h>    int kill(pid_t pid, int sig);  POSIX:CX  

If successful, kill returns 0. If unsuccessful , kill returns “1 and sets errno . The following table lists the mandatory errors for kill .

errno	cause
EINVAL	sig is an invalid or unsupported signal
EPERM	caller does not have the appropriate privileges
ESRCH	no process or process group corresponds to pid

A user may send a signal only to processes that he or she owns. For most signals, kill determines permissions by comparing the user IDs of caller and target. SIGCONT is an exception. For SIGCONT , user IDs are not checked if kill is sent to a process that is in the same session. Section 11.5 discusses sessions. For security purposes, a system may exclude an unspecified set of processes from receiving the signal.

Example 8.4

The following code segment sends SIGUSR1 to process 3423 .

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

Normally, programs do not hardcode specific process IDs such as 3423 in the kill function call. The usual way to find out relevant process IDs is with getpid , getppid , getgpid or by saving the return value from fork .

Example 8.5

This scenario sounds grim, but a child process can kill its parent by executing the following code segment.

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

A process can send a signal to itself with the raise function. The raise function takes just one parameter, a signal number.

  SYNOPSIS  #include <signal.h>   int raise(int sig);  POSIX:CX  

If successful, raise returns 0. If unsuccessful, raise returns a nonzero error value and sets errno . The raise function sets errno to EINVAL if sig is invalid.

Example 8.6

The following statement causes a process to send the SIGUSR1 signal to itself.

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

A key press causes a hardware interrupt that is handled by the device driver for the keyboard. This device driver and its associated modules may perform buffering and editing of the keyboard input. Two special characters , the INTR and QUIT characters, cause the device driver to send a signal to the foreground process group. A user can send the SIGINT signal to the foreground process group by entering the INTR character. This user-settable character is often Ctrl-C. The user-settable QUIT character sends the SIGQUIT signal.

Example 8.7

The stty -a command reports on the characteristics of the device associated with standard input, including the settings of the signal-generating characters. A system running Sun Solaris produced the following output.

 % stty -a speed 9600 baud; rows = 57; columns = 103; ypixels = 0; xpixels = 0; eucw 1:0:0:0, scrw 1:0:0:0 intr = ^c; quit = ^; erase = ^?; kill = ^u; eof = ^d; eol = <undef>; eol2 = <undef>; swtch = <undef>; start = ^q; stop = ^s; susp = ^z; dsusp = ^y; rprnt = ^r; flush = ^o; werase = ^w; lnext = ^v; -parenb -parodd cs8 -cstopb hupcl cread -clocal -loblk -crtscts -parext -ignbrk brkint ignpar -parmrk -inpck -istrip -inlcr -igncr icrnl -iuclc ixon -ixany -ixoff imaxbel isig icanon -xcase echo echoe echok -echonl -noflsh -tostop echoctl -echoprt echoke -defecho -flusho -pendin iexten opost -olcuc onlcr -ocrnl -onocr -onlret -ofill -ofdel 

The terminal in Example 8.7 interprets Ctrl-C as the INTR character. The QUIT character (Ctrl- above) generates SIGQUIT . The SUSP character (Ctrl-Z above) generates SIGSTOP , and the DSUSP character (Ctrl-Y above) generates SIGCONT .

The alarm function causes a SIGALRM signal to be sent to the calling process after a specified number of real seconds has elapsed. Requests to alarm are not stacked , so a call to alarm before the previous timer expires causes the alarm to be reset to the new value. Call alarm with a zero value for seconds to cancel a previous alarm request.

  SYNOPSIS  #include <unistd.h>   unsigned alarm(unsigned seconds);  POSIX  

The alarm function returns the number of seconds remaining on the alarm before the call reset the value, or 0 if no previous alarm was set. The alarm function never reports an error.

Example 8.8 simplealarm.c

Since the default action for SIGALRM is to terminate the process, the following program runs for approximately ten seconds of wall-clock time.

 #include <unistd.h> int main(void) {    alarm(10);    for ( ; ; ) ; }