signal - simplified software signal facilities

#include <signal.h>

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

Signal is a simplified interface to the more general sigvec(2)

A signal is generated by some abnormal event, initiated by a user
at a terminal (quit, interrupt, stop), by a program error (bus
error, etc.), by request of another program (kill), or 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 cause termination of the receiving
process 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 call allows signals either to be
ignored or to cause an interrupt to a specified location. The
following is a list of all signals with names as in the include
file <signal.h>:

SIGHUP 1 hangup
SIGINT 2 interrupt
SIGQUIT 3* quit
SIGILL 4* illegal instruction
SIGTRAP 5* trace trap
SIGIOT 6* IOT instruction
SIGEMT 7* EMT instruction
SIGFPE 8* floating point exception
SIGKILL 9 kill (cannot be caught or ignored)
SIGBUS 10* bus error
SIGSEGV 11* segmentation violation
SIGSYS 12* bad argument to system call
SIGPIPE 13 write on a pipe with no one to read it
SIGALRM 14 alarm clock
SIGTERM 15 software termination signal
SIGURG 16@ urgent condition present on socket
SIGSTOP 17+ stop (cannot be caught or ignored)
SIGTSTP 18+ stop signal generated from keyboard
SIGCONT 19@ continue after stop
SIGCHLD 20@ child status has changed
SIGTTIN 21+ background read attempted from control terminal
SIGTTOU 22+ background write attempted to control terminal
SIGIO 23@ i/o is possible on a descriptor (see fcntl(2))
SIGXCPU 24 cpu time limit exceeded (see setrlimit(2)*)
SIGXFSZ 25 file size limit exceeded (see setrlimit(2)*)
SIGVTALRM 26 virtual time alarm (see setitimer(2))
SIGPROF 27 profiling timer alarm (see setitimer(2))
SIGWINCH 28@ Window size change
SIGUSR1 30 User defined signal 1
SIGUSR2 31 User defined signal 2

The starred signals in the list above cause a core image if not
caught or ignored.

If func is SIG_DFL, the default action for signal sig is
reinstated; this default is termination (with a core image for
starred signals) except for signals marked with @ or +. Signals
marked with @ are discarded if the action is SIG_DFL; signals
marked with + cause the process to stop. If func is SIG_IGN the
signal is subsequently ignored and pending instances of the signal
are discarded. Otherwise, when the signal occurs further
occurrences of the signal are automatically blocked and func is

A return from the function unblocks the handled signal and
continues the process at the point it was interrupted. Unlike
previous signal facilities, the handler func remains installed
after a signal has been delivered.

If a caught signal occurs during certain system calls, causing the
call to terminate prematurely, the call is automatically restarted.
In particular this can occur during a read or write(2) on a slow
device (such as a terminal; but not a file) and during a wait(2).

The value of signal is the previous (or initial) value of func for
the particular signal.

After a fork(2) or vfork(2) the child inherits all signals.
Execve(2) resets all caught signals to the default action; ignored
signals remain ignored.

The previous action is returned on a successful call. Otherwise,
-1 is returned and errno is set to indicate the error.

Signal will fail and no action will take place if one of the
following occur:

[EINVAL] Sig is not a valid signal number.

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

[EINVAL] An attempt is made to ignore SIGCONT (by default SIGCONT
is ignored).

kill(1), ptrace(2), kill(2), sigvec(2), sigblock(2), sigsetmask(2),
sigpause(2), sigstack(2), setjmp(3), tty(4)

The handler routine can be declared:

void handler(sig, code, scp)

Here sig is the signal number, into which the hardware faults and
traps are mapped as defined below. Code is a parameter which is
either a constant as given below or, for compatibility mode faults,
the code provided by the hardware. Scp is a pointer to the struct
sigcontext used by the system to restore the process context from
before the signal. Compatibility mode faults are distinguished
from the other SIGILL traps by having PSL_CM set in the psl. See
sigvec(2) for additional details.

* Not currently supported under MachTen Personal Unix