NAME
sigaction, sigprocmask, sigpending, sigsuspend - POSIX sig-
nal handling functions.
SYNOPSIS
#include <signal.h>
int sigaction(int signum, const struct sigaction *act,
struct sigaction *oldact));
int sigprocmask(int how, const sigset_t *set, sigset_t *old-
set));
int sigpending(sigset_t *set));
int sigsuspend(const sigset_t *mask));
DESCRIPTION
The sigaction system call is used to change the action taken
by a process on receipt of a specific signal.
signum specifies the signal and can be any valid signal
except SIGKILL and SIGSTOP.
If act is non-null, the new action for signal signum is
installed from act. If oldact is non-null, the previous
action is saved in oldact.
The sigaction structure is defined as
struct sigaction {
void (*sa_handler)(int);
sigset_t sa_mask;
int sa_flags;
void (*sa_restorer)(void);
}
sa_handler specifies the action to be associated with signum
and may be SIG_DFL for the default action, SIG_IGN to ignore
this signal, or a pointer to a signal handling function.
sa_mask gives a mask of signals which should be blocked dur-
ing execution of the signal handler. In addition, the sig-
nal which triggered the handler will be blocked, unless the
SA_NODEFER or SA_NOMASK flags are used.
sa_flags specifies a set of flags which modify the behaviour
of the signal handling process. It is formed by the bitwise
OR of zero or more of the following:
SA_NOCLDSTOP
If signum is SIGCHLD, do not receive notification
when child processes stop (i.e., when child
processes receive one of SIGSTOP, SIGTSTP, SIGTTIN
or SIGTTOU).
SA_ONESHOT or SA_RESETHAND
Restore the signal action to the default state
once the signal handler has been called. (This is
the default behavior of the signal(2) system
call.)
SA_RESTART
Provide behaviour compatible with BSD signal
semantics by making certain system calls restart-
able across signals.
SA_NOMASK or SA_NODEFER
Do not prevent the signal from being received from
within its own signal handler.
The sa_restorer element is obsolete and should not be used.
The sigprocmask call is used to change the list of currently
blocked signals. The behaviour of the call is dependent on
the value of how, as follows.
SIG_BLOCK
The set of blocked signals is the union of the
current set and the set argument.
SIG_UNBLOCK
The signals in set are removed from the current
set of blocked signals. It is legal to attempt to
unblock a signal which is not blocked.
SIG_SETMASK
The set of blocked signals is set to the argument
set.
If oldset is non-null, the previous value of the signal mask
is stored in oldset.
The sigpending call allows the examination of pending sig-
nals (ones which have been raised while blocked). The sig-
nal mask of pending signals is stored in set.
The sigsuspend call temporarily replaces the signal mask for
the process with that given by mask and then suspends the
process until a signal is received.
RETURN VALUES
sigaction, sigprocmask, sigpending and sigsuspend return 0
on success and -1 on error.
ERRORS
EINVAL
An invalid signal was specified. This will also be
generated if an attempt is made to change the action
for SIGKILL or SIGSTOP, which cannot be caught.
EFAULT
act, oldact, set or oldset point to memory which is not
a valid part of the process address space.
EINTR
System call was interrupted.
NOTES
It is not possible to block SIGKILL or SIGSTOP with the sig-
procmask call. Attempts to do so will be silently ignored.
According to POSIX, the behaviour of a process is undefined
after it ignores a SIGFPE, SIGILL, or SIGSEGV signal that
was not generated by the kill() or the raise() functions.
Integer division by zero has undefined result. On some
architectures it will generate a SIGFPE signal. (Also
dividing the most negative integer by -1 may generate
SIGFPE.) Ignoring this signal might lead to an endless
loop.
POSIX (B.3.3.1.3) disallows setting the action for SIGCHLD
to SIG_IGN. The BSD and SYSV behaviours differ, causing BSD
software that sets the action for SIGCHLD to SIG_IGN to fail
on Linux.
The POSIX spec only defines SA_NOCLDSTOP. Use of other
sa_flags is non-portable.
The SA_RESETHAND flag is compatible with the SVr4 flag of
the same name.
The SA_NODEFER flag is compatible with the SVr4 flag of the
same name under kernels 1.3.9 and newer. On older kernels
the Linux implementation allowed the receipt of any signal,
not just the one we are installing (effectively overriding
any sa_mask settings).
The SA_RESETHAND and SA_NODEFER names for SVr4 compatibility
are present only in library versions 3.0.9 and greater.
sigaction can be called with a null second argument to query
the current signal handler. It can also be used to check
whether a given signal is valid for the current machine by
calling it with null second and third arguments.
See sigsetops(3) for details on manipulating signal sets.
CONFORMING TO
POSIX, SVr4. SVr4 does not document the EINTR condition.
SEE ALSO
kill(1), kill(2), killpg(2), siginterrupt(3), signal(2),
signal(7), sigvec(2)