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NAME
sigaction - examine and change a signal action
SYNOPSIS
#include <signal.h>
int sigaction(int signum, const struct sigaction *act,
struct sigaction *oldact);
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 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The sa_restorer element is obsolete and should not be used.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 sig‐
nal number as its only argument.
If SA_SIGINFO is specified in sa_flags, then sa_sigaction (instead of
sa_handler) specifies the signal-handling function for 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 execu‐
tion of the signal handler. In addition, the signal which triggered
the handler will be blocked, unless the SA_NODEFER flag is used.
sa_flags specifies a set of flags which modify the behavior of the sig‐
nal 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 they receive one of
SIGSTOP, SIGTSTP, SIGTTIN or SIGTTOU) or resume (i.e.,
they receive SIGCONT) (see wait(2)).
SA_NOCLDWAIT
(Linux 2.6 and later) If signum is SIGCHLD, do not trans‐
form children into zombies when they terminate. See also
waitpid(2).
SA_RESETHAND
Restore the signal action to the default state once the
signal handler has been called. SA_ONESHOT is an obso‐
lete, non-standard synonym for this flag.
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 behavior compatible with BSD signal semantics by
making certain system calls restartable across signals.
SA_NODEFER
Do not prevent the signal from being received from within
its own signal handler. SA_NOMASK is an obsolete, non-
standard synonym for this flag.
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. (si_signo
is unused on Linux.) The rest of the struct may be a union, so that
one should only read the fields that are meaningful for the given sig‐
nal:
* 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.
See sigqueue(2) for more details.
* 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(2) or raise(3) |
+-----------+-------------------------------------------------------+
|SI_KERNEL | The kernel |
+-----------+-------------------------------------------------------+
|SI_QUEUE | sigqueue(2) |
+-----------+-------------------------------------------------------+
|SI_TIMER | POSIX timer expired |
+-----------+-------------------------------------------------------+
|SI_MESGQ | POSIX message queue state changed (since Linux 2.6.6) |
+-----------+-------------------------------------------------------+
|SI_ASYNCIO | AIO completed |
+-----------+-------------------------------------------------------+
|SI_SIGIO | queued SIGIO |
+-----------+-------------------------------------------------------+
|SI_TKILL | tkill(2) or tgkill(2) (since Linux 2.4.19) |
+-----------+-------------------------------------------------------+
+-------------------------------------+
| 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 (since Linux 2.6.9) |
+--------------+-------------------------------------------------+
+-----------------------------------------+
| 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 |
+---------+-------------------------------+
sigaction() returns 0 on success and -1 on error.
ERRORS
EFAULT act or oldact points to memory which is not a valid part of the
process address space.
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 or ignored.
POSIX.1-2001, SVr4.
NOTES
According to POSIX, the behavior of a process is undefined after it
ignores a SIGFPE, SIGILL, or SIGSEGV signal that was not generated by
kill(2) or raise(3). 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.1-1990 disallowed setting the action for SIGCHLD to SIG_IGN.
POSIX.1-2001 allows this possibility, so that ignoring SIGCHLD can be
used to prevent the creation of zombies (see wait(2)). Nevertheless,
the historical BSD and System V behaviors for ignoring SIGCHLD differ,
so that the only completely portable method of ensuring that terminated
children do not become zombies is to catch the SIGCHLD signal and per‐
form a wait(2) or similar.
POSIX.1-1990 only specified SA_NOCLDSTOP. POSIX.1-2001 added SA_NOCLD
WAIT, SA_RESETHAND, SA_NODEFER, and SA_SIGINFO. Use of these latter
values in sa_flags may be less portable in applications intended for
older Unix implementations.
Support for SA_SIGINFO was added in Linux 2.2.
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).
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.
It is not possible to block SIGKILL or SIGSTOP (by specifying them in
sa_mask). Attempts to do so are silently ignored.
See sigsetops(3) for details on manipulating signal sets.
See signal(7) for a list of the async-signal-safe functions that can be
safely called inside from inside a signal handler.
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.
BUGS
In kernels up to and including 2.6.13, specifying SA_NODEFER in
sa_flags preventing not only the delivered signal from being masked
during execution of the handler, but also the signals specified in
sa_mask. This bug is was fixed in kernel 2.6.14.
EXAMPLE
See mprotect(2).
kill(1), kill(2), pause(2), sigaltstack(2), signal(2), sigpending(2),
sigprocmask(2), sigqueue(2), sigsuspend(2), wait(2), killpg(3),
raise(3), siginterrupt(3), sigsetops(3), sigvec(3), core(5), signal(7)