sigaction() - Unix, Linux System Call
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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);
}
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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sa_restorer element is obsolete and should not be used.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.
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 execution 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 behaviour of the signal handling
process. It is formed by the bitwise OR of zero or more of the following:
Tag | Description |
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 transform 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 obsolete, non-standard synonym for this flag.
|
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_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.)
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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 */
}
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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 signal.
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 | 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() or tgkill() (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 |
RETURN VALUE
sigaction() returns 0 on success and -1 on error.
ERRORS
Tag | Description |
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.
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NOTES
According to POSIX, the behaviour of a process is undefined after it
ignores a
SIGFPE, SIGILL, or
SIGSEGV signal that was not generated by
kill() or
raise(). 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 behaviours 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 perform a
wait(2)
or similar.
POSIX.1-1990 only specified
SA_NOCLDSTOP. POSIX.1-2001 added
SA_NOCLDWAIT, 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 implementation
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 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.
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.
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.
CONFORMING TO
POSIX.1-2001, SVr4.
UNDOCUMENTED
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.
SEE ALSO
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