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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 the 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' character.  Signals are used when a process is stopped because it wishes to access its con-trol control
     trol 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 <signal.h>:

     No    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

     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 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 sig-nals signals
     nals 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 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