sigaction(2)
NAME
sigaction, sigprocmask, sigpending, sigsuspend - POSIX
signal 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
*oldset);
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);
void (*sa_sigaction)(int, siginfo_t *, void *);
sigset_t sa_mask;
int sa_flags;
void (*sa_restorer)(void);
}
The sa_restorer element is obsolete and should not be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.
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 notifi
cation 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
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_sigac
tion 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 sig
nals. 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-existant 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 cur
rently 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 cur
rent 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
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.
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
sigprocmask call. Attempts to do so will be silently
ignored.
According to POSIX, the behaviour of a process is unde
fined after it ignores a SIGFPE, SIGILL, or SIGSEGV signal
that was not generated by the kill() or the raise() func
tions. 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 gener
ate SIGFPE.) Ignoring this signal might lead to an end
less 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 (effec
tively overriding any sa_mask settings).
The SA_RESETHAND and SA_NODEFER names for SVr4 compatibil
ity 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 argu
ments.
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), pause(2), raise(3), siginter
rupt(3), signal(2), signal(7), sigsetops(3), sigvec(2)
Man(1) output converted with
man2html