sigaction
SIGACTION(2) Linux Programmer's Manual 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 pro-
cess 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The sa_restorer element is obsolete and should not be used.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.
sa_mask gives a mask of signals which should be blocked during execu-
tion 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 pro-
vided 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_sta-
tus, 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 bit-
mask. 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 sig-
nals is stored in set.
The sigsuspend call temporarily replaces the signal mask for the pro-
cess with that given by mask and then suspends the process until a sig-
nal is received.
RETURN VALUE
The functions sigaction, sigprocmask, and sigpending return 0 on suc-
cess and -1 on error. The function sigsuspend always returns -1, nor-
mally with the error EINTR.
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, oldset or mask 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 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 sig-
nal. (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 implementa-
tion 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 cur-
rent signal handler. It can also be used to check whether a given sig-
nal 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.
UNDOCUMENTED
Before the introduction of SA_SIGINFO it was also possible to get some
additional information, namely by using a sa_handler with second argu-
ment of type struct sigcontext. See the relevant kernel sources for
details. This use is obsolete now.
SEE ALSO
kill(1), kill(2), killpg(2), pause(2), sigaltstack(2), raise(3), sigin-
terrupt(3), signal(2), signal(7), sigsetops(3), sigvec(2)
Linux 2.4 2001-12-29 SIGACTION(2)
