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NAME [Toc] [Back]
sigvector - software signal facilities
SYNOPSIS [Toc] [Back]
const struct sigvec *vec,
struct sigvec *ovec
DESCRIPTION [Toc] [Back]
The system defines a set of signals that can be delivered to a
process. The set of signals is defined in signal(5), along with the
meaning and side effects of each signal. This manual entry, along
with those for sigblock(2), sigsetmask(2), sigpause(3C), and
sigspace(2), defines an alternate mechanism for handling these signals
that ensures the delivery of signals and the integrity of signal
handling procedures. The facilities described here should not be used
in the same program as signal(2).
With the sigvector() interface, 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 can specify a handler function to be invoked when a
signal is delivered, or specify that a signal should be blocked or
ignored. A process can also specify that a default action should be
taken by the system when a signal occurs. It is possible to ensure a
minimum amount of stack space for processing signals using sigspace()
All signals have the same priority. Signal routines execute with the
signal that causes their invocation to be blocked, although other
signals can yet occur. A global 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 0). It
can be changed with a sigblock(), sigsetmask(), or sigpause() call, or
when a signal is delivered to the process.
A signal mask is represented as a long, with one bit representing each
signal being blocked. The following macro defined in <signal.h> is
used to convert a signal number to its corresponding bit in the mask:
#define sigmask(signo) (1L << (signo-1))
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 blocked by the process, it is delivered to the process.
When a signal is delivered, the current state of the process is saved,
a new signal mask is calculated (as described below), and the signal
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handler is invoked. The call to the handler is arranged so that if
the signal handling routine returns normally, the process resumes
execution in the same context as before the signal's delivery. If the
process wishes to resume in a different context, 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
sigblock() or sigsetmask() call is made). This mask is formed by
taking the current signal mask, computing the bit-wise inclusive OR
with the value of vec.sv_mask (see below) from the most recent call to
sigvector() for the signal to be delivered, and, unless the
SV_RESETHAND flag is set (see below), setting the bit corresponding to
the signal being delivered. When the user's signal handler returns
normally, the original mask is restored.
sigvector() assigns a handler for the signal specified by sig. vec
and ovec are pointers to sigvec structures that include the following
If vec is non-zero, it specifies a handler routine (sv_handler), a
mask (sv_mask) that the system should use when delivering the
specified signal, and a set of flags (sv_flags) that modify the
delivery of the signal. If ovec is non-zero, the previous handling
information for the signal is returned to the user. If vec is zero,
signal handling is unchanged. Thus, the call can be used to enquire
about the current handling of a given signal. If vec and ovec point
to the same structure, the value of vec is read prior to being
The sv_flags field can be used to modify the receipt of signals. The
following flag bits are defined:
SV_ONSTACK Use the sigspace() allocated space.
SV_BSDSIG Use the Berkeley signal semantics.
SV_RESETHAND Use the semantics of signal(2).
If SV_ONSTACK is set, the system uses or permits the use of the space
reserved for signal processing in the sigspace() system call.
If SV_BSDSIG is set, the signal is given the Berkeley semantics. The
following signal is affected by this flag:
SIGCLD In addition to being sent when a child process
dies, the signal is also sent when any child's
status changes from running to stopped. This
would normally be used by a program such as csh
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(see csh(1)) when maintaining process groups under
Berkeley job control.
If SV_RESETHAND is set, the signal handler is installed with the same
semantics as a handler installed with signal(2). This affects signal
mask set-up during the signal handler (see above) and whether the
handler is reset after a signal is caught (see below).
If SV_RESETHAND is not set, once a signal handler is installed, it
remains installed until another sigvector() call is made or an exec()
system call is performed (see exec(2)). If SV_RESETHAND is set and
the signal is not one of those marked "not reset when caught" under
signal(5), the default action is reinstated when the signal is caught,
prior to entering the signal-catching function. The "not reset when
caught" distinction is not significant when sigvector() is called and
SV_RESETHAND is not set.
The default action for a signal can be reinstated by setting
sv_handler to SIG_DFL; this default usually results in termination of
the process. If sv_handler is SIG_IGN the signal is usually
subsequently ignored, and pending instances of the signal are
discarded. The exact meaning of SIG_DFL and SIG_IGN for each signal
is discussed in signal(5).
Certain system calls can be interrupted by a signal; all other system
calls complete before the signal is serviced. The scp pointer
described in signal(5) is never null if sigvector() is supported. scp
points to a machine-dependent sigcontext structure. All
implementations of this structure include the fields:
The value SYS_NOTSYSCALL for the sc_syscall field indicates that the
signal is not interrupting a system call; any other value indicates
which system call it is interrupting.
If a signal that is being caught occurs during a system call that can
be interrupted, the signal handler is immediately invoked. If the
signal handler exits normally, the value of the sc_syscall_action
field is inspected; if the value is SIG_RETURN, the system call is
aborted and the interrupted program continues past the call. The
result of the interrupted call is -1 and errno is set to EINTR. If
the value of the sc_syscall_action field is SIG_RESTART, the call is
restarted. A call is restarted if, in the case of a read() or write()
system call (see read(2) or write(2)), it had transferred no data. If
some data had been transferred, the operation is considered to have
completed with a partial transfer, and the sc_syscall value is
SYS_NOTSYSCALL. Other values are undefined and reserved for future
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Exiting the handler abnormally (such as with longjmp() - see
setjmp(3C)) aborts the call, leaving the user responsible for the
context of further execution. The value of scp->sc_syscall_action is
ignored when the value of scp->sc_syscall is SYS_NOTSYSCALL.
scp->sc_syscall_action is always initialized to SIG_RETURN before
invocation of a signal handler. When an system call that can be
interrupted is interrupted by multiple signals, if any signal handler
returns a value of SIG_RETURN in scp->sc_syscall_action, all
subsequent signal handlers are passed a value of SYS_NOTSYSCALL in
Note that calls to read(), write(), or ioctl() on fast devices (such
as disks) cannot be interrupted, but I/O to a slow device (such as a
printer) can be interrupted. Other system calls, such as those used
for networking, also can be interrupted on some implementations. In
these cases additional values can be specified for scp->sc_syscall.
Programs that look at the values of scp->sc_syscall always should
compare them to these symbolic constants; the numerical values
represented by these constants might vary among implementations.
System calls that can be interrupted and their corresponding values
for scp->sc_syscall are listed below:
Call | sc_syscall value
read (slow devices) | SYS_READ
readv (slow devices) | SYS_READV
write (slow devices) | SYS_WRITE
writev (slow devices) | SYS_WRITEV
open (slow devices) | SYS_OPEN
ioctl (slow requests) | SYS_IOCTL
close (slow requests) | SYS_CLOSE
wait | SYS_WAIT
select | SYS_SELECT
pause | SYS_PAUSE
sigpause | SYS_SIGPAUSE
semop | SYS_SEMOP
msgsnd | SYS_MSGSND
msgrcv | SYS_MSGRCV
These system calls are not defined if the preprocessor macro _XPG2 is
defined when <signal.h> is included. This is because the X/Open
Portability Guide, Issue 2 specifies a different meaning for the
symbol SYS_OPEN (see limits(5)).
After a fork() or vfork() system call, the child inherits all signals,
the signal mask, and the reserved signal stack space.
exec(2) resets all caught signals to the default action; ignored
signals remain ignored, the signal mask remains unchanged, and the
reserved signal stack space is released.
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The mask specified in vec is not allowed to block signals that cannot
be ignored, as defined in signal(5). This is enforced silently by the
If sigvector() is called to catch SIGCLD in a process that currently
has terminated (zombie) children, a SIGCLD signal is delivered to the
calling process immediately, or as soon as SIGCLD is unblocked if it
is currently blocked. Thus, in a process that spawns multiple
children and catches SIGCLD, it is sometimes advisable to reinstall
the handler for SIGCLD after each invocation in case there are
multiple zombies present. This is true even though the handling of
the signal is not reset by the system, as with signal(2), because
deaths of multiple processes while SIGCLD is blocked in the handler
result in delivery of only a single signal. Note that the function
must reinstall itself after it has called wait() or wait3().
Otherwise the presence of the child that caused the original signal
always causes another signal to be delivered.
RETURN VALUE [Toc] [Back]
Upon successful completion, sigvector() returns 0; otherwise, it
returns -1 and sets errno to indicate the reason.
ERRORS [Toc] [Back]
sigvector() fails and no new signal handler is installed if any of the
following conditions are encountered:
[EFAULT] Either vec or ovec points to memory that is
not a valid part of the process address
space. Reliable detection of this error is
[EINVAL] sig is not a valid signal number.
[EINVAL] An attempt was made to ignore or supply a
handler for a signal that cannot be caught or
ignored; see signal(5).
WARNINGS [Toc] [Back]
Restarting a select(2) call can sometimes cause unexpected results.
If the select() call has a timeout specified, the timeout is restarted
with the call, ignoring any portion that had elapsed prior to
interruption by the signal. Normally this simply extends the timeout
and is not a problem. However, if a handler repeatedly catches
signals, and the timeout specified to select() is longer than the time
between those signals, restarting the select() call effectively
renders the timeout infinite.
sigvector() should not be used in conjunction with the facilities
described under sigset(3C).
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Obsolescent Interfaces [Toc] [Back]
sigvector() is to be obsoleted at a future date.
APPLICATION USAGE [Toc] [Back]
The signal disposition (such as catch/ignore/default) established by
sigvector() is shared by all threads in the process. Each thread
maintains its own blocked signal mask. For more information regarding
signals and threads, refer to signal(5).
AUTHOR [Toc] [Back]
sigvector() was developed by HP and the University of California,
SEE ALSO [Toc] [Back]
kill(1), kill(2), ptrace(2), sigblock(2), signal(2), sigpause(3C),
sigsetmask(2), sigspace(2), setjmp(3C), signal(5), termio(7).
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