SIGNAL(3)						   BSD Library Functions Manual 						 SIGNAL(3)

NAME

     signal -- simplified software signal facilities

LIBRARY

     Standard C Library (libc, -lc)

SYNOPSIS

     #include <signal.h>

     void
     (*signal(int sig, void (*func)(int)))(int);

     or in FreeBSD's equivalent but easier to read typedef'd version:

     typedef void (*sig_t) (int);

     sig_t
     signal(int sig, sig_t func);

DESCRIPTION

     This signal() facility is a simplified interface to the more general sigaction(2) facility.

     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' char-
     acter.  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 sig-
     nals are defined in the file <signal.h>:

     Num   Name 	Default Action	     Description
     1	   SIGHUP	terminate process    terminal line hangup
     2	   SIGINT	terminate process    interrupt program
     3	   SIGQUIT	create core image    quit program
     4	   SIGILL	create core image    illegal instruction
     5	   SIGTRAP	create core image    trace trap
     6	   SIGABRT	create core image    abort program (formerly SIGIOT)
     7	   SIGEMT	create core image    emulate instruction executed
     8	   SIGFPE	create core image    floating-point exception
     9	   SIGKILL	terminate process    kill program
     10    SIGBUS	create core image    bus error
     11    SIGSEGV	create core image    segmentation violation
     12    SIGSYS	create core image    non-existent system call invoked
     13    SIGPIPE	terminate process    write on a pipe with no reader
     14    SIGALRM	terminate process    real-time timer expired
     15    SIGTERM	terminate process    software termination signal
     16    SIGURG	discard signal	     urgent condition present on socket
     17    SIGSTOP	stop process	     stop (cannot be caught or ignored)
     18    SIGTSTP	stop process	     stop signal generated from keyboard
     19    SIGCONT	discard signal	     continue after stop
     20    SIGCHLD	discard signal	     child status has changed
     21    SIGTTIN	stop process	     background read attempted from control terminal
     22    SIGTTOU	stop process	     background write attempted to control terminal
     23    SIGIO	discard signal	     I/O is possible on a descriptor (see fcntl(2))
     24    SIGXCPU	terminate process    cpu time limit exceeded (see setrlimit(2))
     25    SIGXFSZ	terminate process    file size limit exceeded (see setrlimit(2))
     26    SIGVTALRM	terminate process    virtual time alarm (see setitimer(2))
     27    SIGPROF	terminate process    profiling timer alarm (see setitimer(2))
     28    SIGWINCH	discard signal	     Window size change
     29    SIGINFO	discard signal	     status request from keyboard
     30    SIGUSR1	terminate process    User defined signal 1
     31    SIGUSR2	terminate process    User defined signal 2
     32    SIGTHR	terminate process    thread interrupt
     33    SIGLIBRT	terminate process    real-time library interrupt

     The sig argument specifies which signal was received.  The 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, func should be SIG_DFL.  A SIG_DFL resets the default action.  To ignore the
     signal 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 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.  Unlike
     previous signal facilities, the handler func() remains installed after a signal has been delivered.

     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.  Any handler installed with signal(3) will have the SA_RESTART flag set, meaning that any restartable system call will not return
     on receipt of a signal.  The affected system calls include read(2), write(2), sendto(2), recvfrom(2), sendmsg(2) and recvmsg(2) on a communi-
     cations 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 subse-
     quently 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 ECHILD.

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

RETURN VALUES

     The previous action is returned on a successful call.  Otherwise, SIG_ERR is returned and the global variable errno is set to indicate the
     error.

ERRORS

     The signal() function will fail and no action will take place if one of the following occur:

     [EINVAL]		The sig argument is not a valid signal number.

     [EINVAL]		An attempt is made to ignore or supply a handler for SIGKILL or SIGSTOP.

SEE ALSO

     kill(1), kill(2), ptrace(2), sigaction(2), sigaltstack(2), sigprocmask(2), sigsuspend(2), wait(2), fpsetmask(3), setjmp(3), siginterrupt(3),
     tty(4)

HISTORY

     The signal facility appeared in 4.0BSD.  The option to avoid the creation of child zombies through ignoring SIGCHLD appeared in FreeBSD 5.0.

BSD
								   June 7, 2004 							       BSD