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NAME

signal

 - simplified software signal facilities

 

LIBRARY

Lb libc

 

SYNOPSIS

void * Ns Fo signal Fa int sig Fa void *funcint Fc Ns int ;

or in Fx Ap s equivalent but easier to read typedef'd version: typedef void *sig_t int ;

sig_t signal (int sig sig_t func);
 

DESCRIPTION

Signals allow the manipulation of a process from outside its domain as well as allowing the process to manipulate itself or copies of itself (children). There are two general types of signals: those that cause termination of a process and those that do not. Signals which cause termination of a program might result from an irrecoverable error or might be the result of a user at a terminal typing the `interrupt' character. Signals are used when a process is stopped because it wishes to access its control terminal while in the background (see tty(4)). Signals are optionally generated when a process resumes after being stopped, when the status of child processes changes, or when input is ready at the control terminal. Most signals result in the termination of the process receiving them if no action is taken; some signals instead cause the process receiving them to be stopped, or are simply discarded if the process has not requested otherwise. Except for the SIGKILL and SIGSTOP signals, the signal ();
function allows for a signal to be caught, to be ignored, or to generate an interrupt. These signals are defined in the file In signal.h :

Name  Default Action  Description

1 Ta SIGHUP Ta terminate process Ta terminal line hangup

2 Ta SIGINT Ta terminate process Ta interrupt program

3 Ta SIGQUIT Ta create core image Ta quit program

4 Ta SIGILL Ta create core image Ta illegal instruction

5 Ta SIGTRAP Ta create core image Ta trace trap

6 Ta SIGABRT Ta create core image Ta abort program

(formerly
SIGIOT

7 Ta SIGEMT Ta create core image Ta emulate instruction executed

8 Ta SIGFPE Ta create core image Ta floating-point exception

9 Ta SIGKILL Ta terminate process Ta kill program

10 Ta SIGBUS Ta create core image Ta bus error

11 Ta SIGSEGV Ta create core image Ta segmentation violation

12 Ta SIGSYS Ta create core image Ta non-existent system call invoked

13 Ta SIGPIPE Ta terminate process Ta write on a pipe with no reader

14 Ta SIGALRM Ta terminate process Ta real-time timer expired

15 Ta SIGTERM Ta terminate process Ta software termination signal

16 Ta SIGURG Ta discard signal Ta urgent condition present on socket

17 Ta SIGSTOP Ta stop process Ta stop (cannot be caught or ignored)

18 Ta SIGTSTP Ta stop process Ta stop signal generated from keyboard

19 Ta SIGCONT Ta discard signal Ta continue after stop

20 Ta SIGCHLD Ta discard signal Ta child status has changed

21 Ta SIGTTIN Ta stop process Ta background read attempted from

control terminal

22 Ta SIGTTOU Ta stop process Ta background write attempted to

control terminal

23 Ta SIGIO Ta discard signal Ta I/O

is possible on a descriptor (see fcntl(2))

24 Ta SIGXCPU Ta terminate process Ta cpu time limit exceeded (see

setrlimit(2))

25 Ta SIGXFSZ Ta terminate process Ta file size limit exceeded (see

setrlimit(2))

26 Ta SIGVTALRM Ta terminate process Ta virtual time alarm (see

setitimer(2))

27 Ta SIGPROF Ta terminate process Ta profiling timer alarm (see

setitimer(2))

28 Ta SIGWINCH Ta discard signal Ta Window size change

29 Ta SIGINFO Ta discard signal Ta status request from keyboard

30 Ta SIGUSR1 Ta terminate process Ta User defined signal 1

31 Ta SIGUSR2 Ta terminate process Ta User defined signal 2

32 Ta SIGTHR Ta terminate process Ta thread interrupt

The Fa sig argument specifies which signal was received. The Fa func procedure allows a user to choose the action upon receipt of a signal. To set the default action of the signal to occur as listed above, Fa func should be SIG_DFL A SIG_DFL resets the default action. To ignore the signal Fa func should be SIG_IGN This will cause subsequent instances of the signal to be ignored and pending instances to be discarded. If SIG_IGN is not used, further occurrences of the signal are automatically blocked and Fa func is called.

The handled signal is unblocked when the function returns and the process continues from where it left off when the signal occurred. Bf -symbolic Unlike previous signal facilities, the handler func() remains installed after a signal has been delivered. Ef

For some system calls, if a signal is caught while the call is executing and the call is prematurely terminated, the call is automatically restarted. (The handler is installed using the SA_RESTART flag with sigaction(2).) The affected system calls include read(2), write(2), sendto(2), recvfrom(2), sendmsg(2) and recvmsg(2) on a communications channel or a low speed device and during a ioctl(2) or wait(2). However, calls that have already committed are not restarted, but instead return a partial success (for example, a short read count). These semantics could be changed with siginterrupt(3).

When a process which has installed signal handlers forks, the child process inherits the signals. All caught signals may be reset to their default action by a call to the execve(2) function; ignored signals remain ignored.

If a process explicitly specifies SIG_IGN as the action for the signal SIGCHLD the system will not create zombie processes when children of the calling process exit. As a consequence, the system will discard the exit status from the child processes. If the calling process subsequently issues a call to wait(2) or equivalent, it will block until all of the calling process's children terminate, and then return a value of -1 with errno set to Er ECHILD .

See sigaction(2) for a list of functions that are considered safe for use in signal handlers.  

RETURN VALUES

ERRORS

Bq Er EINVAL

The Fa sig argument is not a valid signal number.

Bq Er EINVAL

An attempt is made to ignore or supply a handler for SIGKILL or SIGSTOP

SEE ALSO

HISTORY

Index

NAME

LIBRARY

SYNOPSIS

DESCRIPTION

RETURN VALUES

ERRORS

SEE ALSO

HISTORY

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