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SIGNAL(3)              FreeBSD 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'
     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 <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
     32    SIGTHR           terminate process    thread 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.  (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 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.

FreeBSD 11.0-PRERELEASE          June 7, 2004          FreeBSD 11.0-PRERELEASE

NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUES | ERRORS | SEE ALSO | HISTORY

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