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 ter-
 mination 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 receiv-
 ing 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 sig-
 nal 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 previ-
 ous signal facilities, the handler func() remains installed after a sig-
 nal 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 commu-
 nications 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 seman-
 tics 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.
BSD June 7, 2004 BSD