4.3BSD-Reno/share/man/cat2/sigaction.0
SIGACTION(2) 1990 SIGACTION(2)
NNAAMMEE
sigaction - software signal facilities
SSYYNNOOPPSSIISS
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DDEESSCCRRIIPPTTIIOONN
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 _h_a_n_d_l_e_r to which a
signal is delivered, or specify that a signal is to be
_i_g_n_o_r_e_d. 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 _b_l_o_c_k_e_d, in which case its delivery is postponed
until it is _u_n_b_l_o_c_k_e_d. 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 _s_i_g_n_a_l _s_t_a_c_k.
Signal routines execute with the signal that caused their
invocation _b_l_o_c_k_e_d, but other signals may yet occur. A glo-
bal _s_i_g_n_a_l _m_a_s_k 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 _s_i_g_p_r_o_c_m_a_s_k(2) call, or when a signal
is delivered to the process.
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 _b_l_o_c_k_e_d 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 sig-
nals 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 _s_i_g_p_e_n_d_i_n_g(2) function. When a
Printed 7/27/90 July 1
SIGACTION(2) 1990 SIGACTION(2)
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.
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 _s_i_g_p_r_o_c_m_a_s_k call is made). This mask is formed
by taking the union of the current signal mask set, the sig-
nal to be delivered, and the signal mask associated with the
handler to be invoked.
_S_i_g_a_c_t_i_o_n assigns an action for a specific signal. If _a_c_t
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 _o_a_c_t is non-zero, the previous han-
dling information for the signal is returned to the user.
Once a signal handler is installed, it remains installed
until another _s_i_g_a_c_t_i_o_n call is made, or an _e_x_e_c_v_e(2) is
performed. The default action for a signal may be rein-
stated by setting _s_a__h_a_n_d_l_e_r 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 _s_a__h_a_n_d_l_e_r is SIG_IGN the signal is subsequently ignored,
and pending instances of the signal are discarded.
Options may be specified by setting _s_a__f_l_a_g_s. 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 _s_a__f_l_a_g_s,
the system will deliver the signal to the process on a _s_i_g_-
_n_a_l _s_t_a_c_k, specified with _s_i_g_s_t_a_c_k(2).
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
_s_a__f_l_a_g_s. The affected system calls include _r_e_a_d(2),
_w_r_i_t_e(2), _s_e_n_d_t_o(2), _r_e_c_v_f_r_o_m(2), _s_e_n_d_m_s_g(2) and _r_e_c_v_m_s_g(2)
on a communications channel or a slow device (such as a ter-
minal, but not a regular file) and during a _w_a_i_t(2) or
_i_o_c_t_l(2). However, calls that have already committed are
not restarted, but instead return a partial success (for
example, a short read count).
Printed 7/27/90 July 2
SIGACTION(2) 1990 SIGACTION(2)
After a _f_o_r_k(2) or _v_f_o_r_k(2) the child inherits all signals,
the signal mask, the signal stack, and the restart/interrupt
flags.
_E_x_e_c_v_e(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.
The following is a list of all signals with names as in the
include file <_s_i_g_n_a_l._h>:
SIGHUP 1 hangup
SIGINT 2 interrupt
SIGQUIT 3* quit
SIGILL 4* illegal instruction
SIGTRAP 5*'|=' trace trap
SIGABRT 6* _a_b_o_r_t() call (formerly SIGIOT)
SIGEMT 7*'|=' EMT instruction
SIGFPE 8* floating point exception
SIGKILL 9 kill (cannot be caught, blocked, 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, blocked, 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 _f_c_n_t_l(2))
SIGXCPU 24'|=' cpu time limit exceeded (see _s_e_t_r_l_i_m_i_t(2))
SIGXFSZ 25'|=' file size limit exceeded (see _s_e_t_r_l_i_m_i_t(2))
SIGVTALRM 26'|=' virtual time alarm (see _s_e_t_i_t_i_m_e_r(2))
SIGPROF 27'|=' profiling timer alarm (see _s_e_t_i_t_i_m_e_r(2))
SIGWINCH 28@'|=' window size change
SIGINFO 29@'|=' status request from keyboard
SIGUSR1 30 user-defined signal 1
SIGUSR2 31 user-defined signal 2
The default signal action is termination if the signal is
not caught or ignored, except for signals marked with @ or
'|+'. The starred signals in the list above cause termina-
tion with a core image. Signals marked with @ are discarded
if the action is SIG_DFL; signals marked with '|+' cause the
process to stop. The signals marked with '|=' are not
defined by POSIX.
Printed 7/27/90 July 3
SIGACTION(2) 1990 SIGACTION(2)
NNOOTTEESS
The mask specified in _a_c_t is not allowed to block SIGKILL or
SIGSTOP. This is done silently by the system.
RREETTUURRNN VVAALLUUEE
A 0 value indicated that the call succeeded. A -1 return
value indicates an error occurred and _e_r_r_n_o is set to indi-
cated the reason.
EERRRROORRSS
_S_i_g_a_c_t_i_o_n will fail and no new signal handler will be
installed if one of the following occurs:
[EFAULT] Either _a_c_t or _o_a_c_t points to memory that is
not a valid part of the process address
space.
[EINVAL] _S_i_g is not a valid signal number.
[EINVAL] An attempt is made to ignore or supply a
handler for SIGKILL or SIGSTOP.
SSTTAANNDDAARRDDSS
The _s_i_g_a_c_t_i_o_n function is defined by POSIX.1. The
SA_ONSTACK and SA_RESTART flags are Berkeley extensions, as
are the signals marked with '|='. Most of those signals are
available on most BSD-derived systems.
SSEEEE AALLSSOO
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), siginter-
rupt(3), tty(4)
NNOOTTEESS ((VVAAXX--1111))
The handler routine can be declared:
void handler(sig, code, scp)
int sig, code;
struct sigcontext *scp;
Here _s_i_g is the signal number, into which the hardware
faults and traps are mapped as defined below. _C_o_d_e 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). _S_c_p is a
pointer to the _s_i_g_c_o_n_t_e_x_t structure (defined in <_s_i_g_n_a_l._h>),
used to restore the context from before the signal.
The following defines the mapping of hardware traps to sig-
nals and codes. All of these symbols are defined in
Printed 7/27/90 July 4
SIGACTION(2) 1990 SIGACTION(2)
<_s_i_g_n_a_l._h>:
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
BBUUGGSS
This manual page is still confusing.
Printed 7/27/90 July 5