4.3BSD-Reno/src/lib/libc/sys/sigaction.2
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.\" @(#)sigaction.2 6.1 (Berkeley) 7/1/90
.\"
.TH SIGACTION 2 "July 1, 1990"
.UC 7
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.SH NAME
sigaction \- software signal facilities
.SH SYNOPSIS
.nf
.B #include <signal.h>
.PP
.B struct sigaction {
.B void (*sa_handler)();
.B sigset_t sa_mask;
.B int sa_flags;
.B };
.PP
.B sigaction(sig, act, oact)
.B int sig;
.B struct sigaction *act, *oact;
.fi
.SH DESCRIPTION
The system defines a set of signals that may be delivered to a process.
Signal delivery resembles the occurence of a hardware interrupt:
the signal is blocked from further occurrence, the current process
context is saved, and a new one is built. A process may specify a
.I handler
to which a signal is delivered, or specify that a signal is to be
.IR ignored .
A process may also specify that a default action is to be taken
by the system when a signal occurs.
A signal may also be
.IR blocked ,
in which case its delivery is postponed until it is
.IR unblocked .
The action to be taken on delivery is determined at the time
of delivery.
Normally, signal handlers execute on the current stack
of the process. This may be changed, on a per-handler basis,
so that signals are taken on a special
.IR "signal stack" .
.PP
Signal routines execute with the signal that caused their
invocation
.IR blocked ,
but other signals may yet occur.
A global
.I "signal mask"
defines the set of signals currently blocked from delivery
to a process. The signal mask for a process is initialized
from that of its parent (normally empty). It
may be changed with a
.IR sigprocmask (2)
call, or when a signal is delivered to the process.
.PP
When a signal
condition arises for a process, the signal is added to a set of
signals pending for the process.
If the signal is not currently
.I blocked
by the process then it is delivered to the process.
Signals may be delivered any time a process enters the operating system
(e.g., during a system call, page fault or trap, or clock interrupt).
If multiple signals are ready to be delivered at the same time,
any signals that could be caused by traps are delivered first.
Additional signals may be processed at the same time, with each
appearing to interrupt the handlers for the previous signals
before their first instructions.
The set of pending signals is returned by the
.IR sigpending (2)
function.
When a caught signal
is delivered, the current state of the process is saved,
a new signal mask is calculated (as described below),
and the signal handler is invoked. The call to the handler
is arranged so that if the signal handling routine returns
normally the process will resume execution in the context
from before the signal's delivery.
If the process wishes to resume in a different context, then it
must arrange to restore the previous context itself.
.PP
When a signal is delivered to a process a new signal mask is
installed for the duration of the process' signal handler
(or until a
.I sigprocmask
call is made).
This mask is formed by taking the union of the current signal mask set,
the signal to be delivered, and
the signal mask associated with the handler to be invoked.
.PP
.I Sigaction
assigns an action for a specific signal.
If
.I act
is non-zero, it
specifies an action (SIG_DFL, SIG_IGN, or a handler routine) and mask
to be used when delivering the specified signal.
If
.I oact
is non-zero, the previous handling information for the signal
is returned to the user.
.PP
Once a signal handler is installed, it remains installed
until another
.I sigaction
call is made, or an
.IR execve (2)
is performed.
The default action for a signal may be reinstated by setting
.I sa_handler
to SIG_DFL.
The default actions are termination, possibly with a core image;
no action; stopping the process; or continuing the process.
See the signal list below for each signal's default action.
If
.I sa_handler
is SIG_IGN the signal is subsequently ignored,
and pending instances of the signal are discarded.
.PP
Options may be specified by setting
.IR sa_flags .
If the SA_NOCLDSTOP bit is set when installing a catching function
for the SIGCHLD signal,
the SIGCHLD signal will be generated only when a child process exits,
not when a child process stops.
Further, if the SA_ONSTACK bit is set in
.I sa_flags,
the system will deliver the signal to the process on a
.IR "signal stack" ,
specified with
.IR sigstack (2).
.PP
If a caught signal occurs during certain system calls,
the call may be forced to terminate prematurely
with an EINTR error return,
or the call may be restarted.
Restart of pending calls is requested
by setting the SA_RESTART bit in
.I sa_flags.
The affected system calls include
.IR read (2),
.IR write (2),
.IR sendto (2),
.IR recvfrom (2),
.IR sendmsg (2)
and
.IR recvmsg (2)
on a communications channel or a slow device (such as a terminal,
but not a regular file)
and during a
.IR wait (2)
or
.IR ioctl (2).
However, calls that have already committed are not restarted,
but instead return a partial success (for example, a short read count).
.PP
After a
.IR fork (2)
or
.IR vfork (2)
the child inherits
all signals, the signal mask, the signal stack,
and the restart/interrupt flags.
.PP
.IR Execve (2)
resets all
caught signals to default action and
resets all signals to be caught on the user stack.
Ignored signals remain ignored;
the signal mask remains the same;
signals that restart pending system calls continue to do so.
.PP
The following is a list of all signals
with names as in the include file
.RI < signal.h >:
.LP
.nf
.RS
.ta \w'SIGVTALRM\0\0\0'u +\w'15*\*p\0\0'u
SIGHUP 1 hangup
SIGINT 2 interrupt
SIGQUIT 3* quit
SIGILL 4* illegal instruction
SIGTRAP 5*\*p trace trap
SIGABRT 6* \fIabort\fP() call (formerly SIGIOT)
SIGEMT 7*\*p EMT instruction
SIGFPE 8* floating point exception
SIGKILL 9 kill (cannot be caught, blocked, or ignored)
SIGBUS 10*\*p bus error
SIGSEGV 11* segmentation violation
SIGSYS 12*\*p 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\*b\*p urgent condition present on socket
SIGSTOP 17\*d stop (cannot be caught, blocked, or ignored)
SIGTSTP 18\*d stop signal generated from keyboard
SIGCONT 19\*b continue after stop
SIGCHLD 20\*b child status has changed
SIGTTIN 21\*d background read attempted from control terminal
SIGTTOU 22\*d background write attempted to control terminal
SIGIO 23\*b\*p i/o is possible on a descriptor (see \fIfcntl\fP(2))
SIGXCPU 24\*p cpu time limit exceeded (see \fIsetrlimit\fP(2))
SIGXFSZ 25\*p file size limit exceeded (see \fIsetrlimit\fP(2))
SIGVTALRM 26\*p virtual time alarm (see \fIsetitimer\fP(2))
SIGPROF 27\*p profiling timer alarm (see \fIsetitimer\fP(2))
SIGWINCH 28\*b\*p window size change
SIGINFO 29\*b\*p status request from keyboard
SIGUSR1 30 user-defined signal 1
SIGUSR2 31 user-defined signal 2
.RE
.fi
.PP
The default signal action is termination
if the signal is not caught or ignored,
except for signals marked with \*b or \*d.
The starred signals in the list above cause termination with a core image.
Signals marked with \*b are discarded if the action
is SIG_DFL; signals marked
with \*d cause the process to stop.
The signals marked with \*p are not defined by POSIX.
.SH NOTES
The mask specified in
.I act
is not allowed to block SIGKILL or SIGSTOP.
This is done silently by the system.
.SH "RETURN VALUE
A 0 value indicated that the call succeeded. A \-1 return value
indicates an error occurred and
.I errno
is set to indicated the reason.
.SH ERRORS
.I Sigaction
will fail and no new signal handler will be installed if one
of the following occurs:
.TP 15
[EFAULT]
Either
.I act
or
.I oact
points to memory that is not a valid part of the process
address space.
.TP 15
[EINVAL]
.I Sig
is not a valid signal number.
.TP 15
[EINVAL]
An attempt is made to ignore or supply a handler for SIGKILL
or SIGSTOP.
.SH STANDARDS
The
.I sigaction
function is defined by POSIX.1.
The SA_ONSTACK and SA_RESTART flags are Berkeley extensions,
as are the signals marked with \*p.
Most of those signals are available on most BSD-derived systems.
.SH "SEE ALSO"
kill(1), ptrace(2), kill(2),
sigaction(2), sigprocmask(2), sigsetops(2), sigsuspend(2),
sigblock(2), sigsetmask(2), sigpause(2),
sigstack(2), sigvec(2), setjmp(3), siginterrupt(3), tty(4)
.SH "NOTES (VAX-11)"
The handler routine can be declared:
.PP
void handler(sig, code, scp)
int sig, code;
struct sigcontext *scp;
.PP
Here
.I sig
is the signal number, into which the hardware faults and traps are
mapped as defined below.
.I Code
is a parameter that is either a constant
as given below or, for compatibility mode faults, the code provided by
the hardware (Compatibility mode faults are distinguished from the
other SIGILL traps by having PSL_CM set in the psl).
.I Scp
is a pointer to the
.I sigcontext
structure (defined in
.RI < signal.h >),
used to restore the context from before the signal.
.PP
The following defines the mapping of hardware traps to signals
and codes. All of these symbols are defined in
.RI < signal.h >:
.LP
.ta \w' Floating/decimal divide by zero 'u +\w'15* 'u +8n
.nf
Hardware condition Signal Code
Arithmetic traps:
Integer overflow SIGFPE FPE_INTOVF_TRAP
Integer division by zero SIGFPE FPE_INTDIV_TRAP
Floating overflow trap SIGFPE FPE_FLTOVF_TRAP
Floating/decimal division by zero SIGFPE FPE_FLTDIV_TRAP
Floating underflow trap SIGFPE FPE_FLTUND_TRAP
Decimal overflow trap SIGFPE FPE_DECOVF_TRAP
Subscript-range SIGFPE FPE_SUBRNG_TRAP
Floating overflow fault SIGFPE FPE_FLTOVF_FAULT
Floating divide by zero fault SIGFPE FPE_FLTDIV_FAULT
Floating underflow fault SIGFPE FPE_FLTUND_FAULT
Length access control SIGSEGV
Protection violation SIGBUS
Reserved instruction SIGILL ILL_RESAD_FAULT
Customer-reserved instr. SIGEMT
Reserved operand SIGILL ILL_PRIVIN_FAULT
Reserved addressing SIGILL ILL_RESOP_FAULT
Trace pending SIGTRAP
Bpt instruction SIGTRAP
Compatibility-mode SIGILL hardware supplied code
Chme SIGSEGV
Chms SIGSEGV
Chmu SIGSEGV
.fi
.SH BUGS
This manual page is still confusing.