2.11BSD/man/cat2/sigaction.0
SIGACTION(2) UNIX Programmer's Manual SIGACTION(2)
NAME
sigaction - software signal facilities
SYNOPSIS
#include <signal.h>
struct sigaction {
int (*sa_handler)();
sigset_t sa_mask;
int sa_flags;
};
sigaction(sig, act, oact)
_i_n_t _s_i_g;
_s_t_r_u_c_t _s_i_g_a_c_t_i_o_n *_a_c_t;
_s_t_r_u_c_t _s_i_g_a_c_t_i_o_n *_o_a_c_t;
DESCRIPTION
The system defines a set of signals that may be delivered to
a process. Signal delivery resembles the occurrence 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 sigprocmask(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
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signals is returned by the 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.
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 sigprocmask 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.
Sigaction 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 sigaction call is made, or an execve(2) is
performed. A signal-specific default action may be reset by
setting _s_a__h_a_n_d_l_e_r to SIG_DFL. The defaults are process
termination, possibly with core dump; 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_DFL, the default action for the signal is to discard the
signal, and if a signal is pending, the pending signal is
discarded even if the signal is masked. If _s_a__h_a_n_d_l_e_r is
set to SIG_IGN current and pending instances of the signal
are ignored and 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 sigstack(2).
If a signal is caught during the system calls listed below,
the call may be forced to terminate with the error EINTR,
the call may return with a data transfer shorter than
requested, 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 open(2),
read(2), write(2), sendto(2), recvfrom(2), sendmsg(2) and
recvmsg(2) on a communications channel or a slow device
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(such as a terminal, but not a regular file) and during a
wait(2) or ioctl(2). However, calls that have already com-
mitted are not restarted, but instead return a partial suc-
cess (for example, a short read count).
After a fork(2) or vfork(2) all signals, the signal mask,
the signal stack, and the restart/interrupt flags are inher-
ited by the child.
Execve(2) reinstates the default action for all signals
which were caught 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>:
NAME Action Description
SIGHUP terminate terminal line hangup
SIGINT terminate interrupt program
SIGQUIT core quit program
SIGILL core illegal instruction
SIGTRAP core trace trap
SIGIOT core abort(2) call (same as SIGABRT)
SIGEMT core emulate instruction executed
SIGFPE core floating-point exception
SIGKILL terminate kill program
SIGBUS core bus error
SIGSEGV core segmentation violation
SIGSYS core system call given invalid argu-
ment
SIGPIPE terminate write on a pipe with no reader
SIGALRM terminate real-time timer expired
SIGTERM terminate software termination signal
SIGURG discard urgent condition present on
socket
SIGSTOP stop stop (cannot be caught or
ignored)
SIGTSTP stop stop generated from keyboard
SIGCONT discard continue after stop
SIGCHLD discard child status has changed
SIGTTIN stop background read attempted on
control terminal
SIGTTOU stop background write attemped to
control terminal
SIGIO discard I/O is possible on a descriptor
(see fcntl(2))
SIGXCPU terminate cpu time limit exceeded (see
setrlimit(2))
SIGXFSZ terminate file size limit exceeded (see
setrlimit(2))
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SIGACTION(2) UNIX Programmer's Manual SIGACTION(2)
SIGVTALRM terminate virtual time alarm (see setiti-
mer(2))
SIGPROF terminate profiling timer alarm (see seti-
timer(2))
SIGWINCH discard Window size change
SIGINFO discard status request from keyboard
SIGUSR1 terminate User defined signal 1
SIGUSR2 terminate User defined signal 2
NOTE
The mask specified in _a_c_t is not allowed to block SIGKILL or
SIGSTOP. This is done silently by the system.
RETURN VALUES
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.
EXAMPLE
The handler routine can be declared:
int 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. _C_o_d_e is a parameter that is
either a constant or the code provided by the hardware. _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 sig-
nal.
ERRORS
Sigaction 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.
STANDARDS
The sigaction function is defined by IEEE Std1003.1-1988
(``POSIX''). The SA_ONSTACK and SA_RESTART flags are Berke-
ley extensions, as are the signals, SIGTRAP, SIGEMT, SIGBUS,
SIGSYS, SIGURG, SIGIO, SIGXCPU, SIGXFSZ, SIGVTALRM, SIGPROF,
SIGWINCH, and SIGINFO. Those signals are available on most
BSD-derived systems.
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BUGS
The networking related syscalls are not properly restarted
in 2.11BSD. The SIGINFO signal is not implemented in
2.11BSD.
SEE ALSO
kill(1), fcntl(2), ptrace(2), kill(2), setitimer(2),
setrlimit(2), sigaction(2), sigprocmask(2), sigsuspend(2),
sigblock(2), sigsetmask(2), sigpause(2), sigstack(2),
sigvec(2), setjmp(3), siginterrupt(3), sigsetops(3), tty(4)
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