Originální popis anglicky:
sigaction, sigprocmask, sigpending, sigsuspend - POSIX signal handling functions
Návod, kniha: Linux Programmer's Manual
#include <signal.h>
int sigaction(int signum, const struct sigaction
*act, struct sigaction *oldact);
int sigprocmask(int how, const sigset_t *set,
sigset_t *oldset);
int sigpending(sigset_t *set);
int sigsuspend(const sigset_t *mask);
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 something like
struct sigaction {
void (*sa_handler)(int);
void (*sa_sigaction)(int, siginfo_t *, void *);
sigset_t sa_mask;
int sa_flags;
void (*sa_restorer)(void);
}
On some architectures a union is involved - do not assign to both
sa_handler and
sa_sigaction.
The
sa_restorer element is obsolete and should not be used. POSIX does
not specify a
sa_restorer element.
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. This function receives the
signal number as its only argument.
sa_sigaction also specifies the action to be associated with
signum. This function receives the signal number as its first argument,
a pointer to a
siginfo_t as its second argument and a pointer to a
ucontext_t (cast to void *) as its third argument.
sa_mask gives a mask of signals which should be blocked during execution
of the signal handler. In addition, the signal 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.
- SA_ONSTACK
- Call the signal handler on an alternate signal stack
provided by sigaltstack(2). If an alternate stack is not available,
the default stack will be used.
- SA_RESTART
- Provide behaviour compatible with BSD signal semantics by
making certain system calls restartable across signals.
- SA_NOMASK or SA_NODEFER
- Do not prevent the signal from being received from within
its own signal handler.
- SA_SIGINFO
- The signal handler takes 3 arguments, not one. In this
case, sa_sigaction should be set instead of sa_handler. (The
sa_sigaction field was added in Linux 2.1.86.)
The
siginfo_t parameter to
sa_sigaction is a struct with the
following elements
siginfo_t {
int si_signo; /* Signal number */
int si_errno; /* An errno value */
int si_code; /* Signal code */
pid_t si_pid; /* Sending process ID */
uid_t si_uid; /* Real user ID of sending process */
int si_status; /* Exit value or signal */
clock_t si_utime; /* User time consumed */
clock_t si_stime; /* System time consumed */
sigval_t si_value; /* Signal value */
int si_int; /* POSIX.1b signal */
void * si_ptr; /* POSIX.1b signal */
void * si_addr; /* Memory location which caused fault */
int si_band; /* Band event */
int si_fd; /* File descriptor */
}
si_signo,
si_errno and
si_code are defined for all signals.
The rest of the struct may be a union, so that one should only read the fields
that are meaningful for the given signal.
kill(2), POSIX.1b signals and
SIGCHLD fill in
si_pid and
si_uid. SIGCHLD also fills in
si_status,
si_utime and
si_stime.
si_int and
si_ptr are specified by the sender of the POSIX.1b signal. SIGILL,
SIGFPE, SIGSEGV and SIGBUS fill in
si_addr with the address of the
fault. SIGPOLL fills in
si_band and
si_fd.
si_code indicates why this signal was sent. It is a value, not a bitmask.
The values which are possible for any signal are listed in this table:
| si_code |
|
| Value |
Signal origin |
| SI_USER |
kill, sigsend or raise |
| SI_KERNEL |
The kernel |
| SI_QUEUE |
sigqueue |
| SI_TIMER |
timer expired |
| SI_MESGQ |
mesq state changed |
| SI_ASYNCIO |
AIO completed |
| SI_SIGIO |
queued SIGIO |
| SIGILL |
|
| ILL_ILLOPC |
illegal opcode |
| ILL_ILLOPN |
illegal operand |
| ILL_ILLADR |
illegal addressing mode |
| ILL_ILLTRP |
illegal trap |
| ILL_PRVOPC |
privileged opcode |
| ILL_PRVREG |
privileged register |
| ILL_COPROC |
coprocessor error |
| ILL_BADSTK |
internal stack error |
| SIGFPE |
|
| FPE_INTDIV |
integer divide by zero |
| FPE_INTOVF |
integer overflow |
| FPE_FLTDIV |
floating point divide by zero |
| FPE_FLTOVF |
floating point overflow |
| FPE_FLTUND |
floating point underflow |
| FPE_FLTRES |
floating point inexact result |
| FPE_FLTINV |
floating point invalid operation |
| FPE_FLTSUB |
subscript out of range |
| SIGSEGV |
|
| SEGV_MAPERR |
address not mapped to object |
| SEGV_ACCERR |
invalid permissions for mapped object |
| SIGBUS |
|
| BUS_ADRALN |
invalid address alignment |
| BUS_ADRERR |
non-existent physical address |
| BUS_OBJERR |
object specific hardware error |
| SIGTRAP |
|
| TRAP_BRKPT |
process breakpoint |
| TRAP_TRACE |
process trace trap |
| SIGCHLD |
|
| CLD_EXITED |
child has exited |
| CLD_KILLED |
child was killed |
| CLD_DUMPED |
child terminated abnormally |
| CLD_TRAPPED |
traced child has trapped |
| CLD_STOPPED |
child has stopped |
| CLD_CONTINUED |
stopped child has continued |
| SIGPOLL |
|
| POLL_IN |
data input available |
| POLL_OUT |
output buffers available |
| POLL_MSG |
input message available |
| POLL_ERR |
i/o error |
| POLL_PRI |
high priority input available |
| POLL_HUP |
device disconnected |
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 signals (ones which
have been raised while blocked). The signal 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.
The functions
sigaction,
sigprocmask, and
sigpending return
0 on success and -1 on error. The function
sigsuspend always returns
-1, normally with the error
EINTR.
- EFAULT
- act, oldact, set, oldset or
mask point to memory which is not a valid part of the process
address space.
- EINTR
- System call was interrupted.
- 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.
It is not possible to block
SIGKILL or
SIGSTOP with the
sigprocmask 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.
The
SA_SIGINFO flag is specified by POSIX.1b. Support for it was added in
Linux 2.2.
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.
POSIX, SVr4. SVr4 does not document the EINTR condition.
Before the introduction of
SA_SIGINFO it was also possible to get some
additional information, namely by using a sa_handler with second argument of
type
struct sigcontext. See the relevant kernel sources for details.
This use is obsolete now.
kill(1),
kill(2),
killpg(2),
pause(2),
sigaltstack(2),
signal(2),
sigvec(2),
raise(3),
siginterrupt(3),
sigsetops(3),
signal(7)
