sigaction, rt_sigaction - examine and change a signal action
#include <signal.h>
int sigaction(int signum, const struct sigaction *act,
struct sigaction *oldact);
Feature Test Macro Requirements for glibc (see
feature_test_macros(7)):
sigaction(): _POSIX_C_SOURCE
siginfo_t: _POSIX_C_SOURCE >= 199309L
The
sigaction() system call is used to change the action taken by a
process on receipt of a specific signal. (See
signal(7) for an overview
of signals.)
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 field is not intended for application use. (POSIX does
not specify a
sa_restorer field.) Some further details of the purpose
of this field can be found in
sigreturn(2).
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 three arguments, as described below.
sa_mask specifies a mask of signals which should be blocked (i.e., added
to the signal mask of the thread in which the signal handler is invoked)
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 behavior of the
signal. 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)). This flag is meaningful only
when establishing a handler for SIGCHLD.
- SA_NOCLDWAIT (since Linux 2.6)
- If signum is SIGCHLD, do not transform children into zombies
when they terminate. See also waitpid(2). This flag is meaningful
only when establishing a handler for SIGCHLD, or when setting that
signal's disposition to SIG_DFL.
- If the SA_NOCLDWAIT flag is set when establishing a handler for
SIGCHLD, POSIX.1 leaves it unspecified whether a SIGCHLD
signal is generated when a child process terminates. On Linux, a
SIGCHLD signal is generated in this case; on some other
implementations, it is not.
- SA_NODEFER
- Do not prevent the signal from being received from within its own signal
handler. This flag is meaningful only when establishing a signal handler.
SA_NOMASK is an obsolete, nonstandard 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. This flag is meaningful only when establishing a
signal handler.
- SA_RESETHAND
- Restore the signal action to the default upon entry to the signal handler.
This flag is meaningful only when establishing a signal handler.
SA_ONESHOT is an obsolete, nonstandard synonym for this flag.
- SA_RESTART
- Provide behavior compatible with BSD signal semantics by making certain
system calls restartable across signals. This flag is meaningful only when
establishing a signal handler. See signal(7) for a discussion of
system call restarting.
- SA_RESTORER
- Not intended for application use. This flag is used by C libraries
to indicate that the sa_restorer field contains the address of a
"signal trampoline". See sigreturn(2) for more
details.
- SA_SIGINFO (since Linux 2.2)
- The signal handler takes three arguments, not one. In this case,
sa_sigaction should be set instead of sa_handler. This flag
is meaningful only when establishing a signal handler.
When the
SA_SIGINFO flag is specified in
act.sa_flags, the signal
handler address is passed via the
act.sa_sigaction field. This handler
takes three arguments, as follows:
void
handler(int sig, siginfo_t *info, void *ucontext)
{
...
}
These three arguments are as follows
- sig
- The number of the signal that caused invocation of the handler.
- info
- A pointer to a siginfo_t, which is a structure containing further
information about the signal, as described below.
- ucontext
- This is a pointer to a ucontext_t structure, cast to
void *. The structure pointed to by this field contains
signal context information that was saved on the user-space stack by the
kernel; for details, see sigreturn(2). Further information about
the ucontext_t structure can be found in getcontext(3).
Commonly, the handler function doesn't make any use of the third
argument.
The
siginfo_t data type is a structure with the following fields:
siginfo_t {
int si_signo; /* Signal number */
int si_errno; /* An errno value */
int si_code; /* Signal code */
int si_trapno; /* Trap number that caused
hardware-generated signal
(unused on most architectures) */
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 */
int si_overrun; /* Timer overrun count;
POSIX.1b timers */
int si_timerid; /* Timer ID; POSIX.1b timers */
void *si_addr; /* Memory location which caused fault */
long si_band; /* Band event (was int in
glibc 2.3.2 and earlier) */
int si_fd; /* File descriptor */
short si_addr_lsb; /* Least significant bit of address
(since Linux 2.6.32) */
void *si_lower; /* Lower bound when address violation
occurred (since Linux 3.19) */
void *si_upper; /* Upper bound when address violation
occurred (since Linux 3.19) */
int si_pkey; /* Protection key on PTE that caused
fault (since Linux 4.6) */
void *si_call_addr; /* Address of system call instruction
(since Linux 3.5) */
int si_syscall; /* Number of attempted system call
(since Linux 3.5) */
unsigned int si_arch; /* Architecture of attempted system call
(since Linux 3.5) */
}
si_signo,
si_errno and
si_code are defined for all signals.
(
si_errno is generally unused on Linux.) The rest of the struct may be
a union, so that one should read only the fields that are meaningful for the
given signal:
- *
- Signals sent with kill(2) and sigqueue(3) fill in
si_pid and si_uid. In addition, signals sent with
sigqueue(3) fill in si_int and si_ptr with the values
specified by the sender of the signal; see sigqueue(3) for more
details.
- *
- Signals sent by POSIX.1b timers (since Linux 2.6) fill in
si_overrun and si_timerid. The si_timerid field is an
internal ID used by the kernel to identify the timer; it is not the same
as the timer ID returned by timer_create(2). The si_overrun
field is the timer overrun count; this is the same information as is
obtained by a call to timer_getoverrun(2). These fields are
nonstandard Linux extensions.
- *
- Signals sent for message queue notification (see the description of
SIGEV_SIGNAL in mq_notify(3)) fill in
si_int/si_ptr, with the sigev_value supplied to
mq_notify(3); si_pid, with the process ID of the message
sender; and si_uid, with the real user ID of the message
sender.
- *
- SIGCHLD fills in si_pid, si_uid, si_status,
si_utime, and si_stime, providing information about the
child. The si_pid field is the process ID of the child;
si_uid is the child's real user ID. The si_status field
contains the exit status of the child (if si_code is
CLD_EXITED), or the signal number that caused the process to change
state. The si_utime and si_stime contain the user and system
CPU time used by the child process; these fields do not include the times
used by waited-for children (unlike getrusage(2) and
times(2)). In kernels up to 2.6, and since 2.6.27, these fields
report CPU time in units of sysconf(_SC_CLK_TCK). In 2.6 kernels
before 2.6.27, a bug meant that these fields reported time in units of the
(configurable) system jiffy (see time(7)).
- *
- SIGILL, SIGFPE, SIGSEGV, SIGBUS, and
SIGTRAP fill in si_addr with the address of the fault. On
some architectures, these signals also fill in the si_trapno
field.
- Some suberrors of SIGBUS, in particular BUS_MCEERR_AO and
BUS_MCEERR_AR, also fill in si_addr_lsb. This field
indicates the least significant bit of the reported address and therefore
the extent of the corruption. For example, if a full page was corrupted,
si_addr_lsb contains log2(sysconf(_SC_PAGESIZE)). When
SIGTRAP is delivered in response to a ptrace(2) event
(PTRACE_EVENT_foo), si_addr is not populated, but si_pid and
si_uid are populated with the respective process ID and user ID
responsible for delivering the trap. In the case of seccomp(2), the
tracee will be shown as delivering the event. BUS_MCEERR_* and
si_addr_lsb are Linux-specific extensions.
- The SEGV_BNDERR suberror of SIGSEGV populates
si_lower and si_upper.
- The SEGV_PKUERR suberror of SIGSEGV populates
si_pkey.
- *
- SIGIO/SIGPOLL (the two names are synonyms on Linux) fills in
si_band and si_fd. The si_band event is a bit mask
containing the same values as are filled in the revents field by
poll(2). The si_fd field indicates the file descriptor for
which the I/O event occurred; for further details, see the description of
F_SETSIG in fcntl(2).
- *
- SIGSYS, generated (since Linux 3.5) when a seccomp filter returns
SECCOMP_RET_TRAP, fills in si_call_addr, si_syscall,
si_arch, si_errno, and other fields as described in
seccomp(2).
The
si_code field inside the
siginfo_t argument that is passed to
a
SA_SIGINFO signal handler is a value (not a bit mask) indicating why
this signal was sent. For a
ptrace(2) event,
si_code will
contain
SIGTRAP and have the ptrace event in the high byte:
(SIGTRAP | PTRACE_EVENT_foo << 8).
For a non-
ptrace(2) event, the values that can appear in
si_code
are described in the remainder of this section. Since glibc 2.20, the
definitions of most of these symbols are obtained from
<signal.h>
by defining feature test macros (before including
any header file) as
follows:
- *
- _XOPEN_SOURCE with the value 500 or greater;
- *
- _XOPEN_SOURCE and _XOPEN_SOURCE_EXTENDED; or
- *
- _POSIX_C_SOURCE with the value 200809L or greater.
For the
TRAP_* constants, the symbol definitions are provided only in the
first two cases. Before glibc 2.20, no feature test macros were required to
obtain these symbols.
For a regular signal, the following list shows the values which can be placed in
si_code for any signal, along with the reason that the signal was
generated.
- SI_USER
- kill(2).
- SI_KERNEL
- Sent by the kernel.
- SI_QUEUE
- sigqueue(3).
- SI_TIMER
- POSIX timer expired.
- SI_MESGQ (since Linux 2.6.6)
- POSIX message queue state changed; see mq_notify(3).
- SI_ASYNCIO
- AIO completed.
- SI_SIGIO
- Queued SIGIO (only in kernels up to Linux 2.2; from Linux 2.4
onward SIGIO/SIGPOLL fills in si_code as described
below).
- SI_TKILL (since Linux 2.4.19)
- tkill(2) or tgkill(2).
The following values can be placed in
si_code for a
SIGILL signal:
- 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.
The following values can be placed in
si_code for a
SIGFPE signal:
- 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.
The following values can be placed in
si_code for a
SIGSEGV
signal:
- SEGV_MAPERR
- Address not mapped to object.
- SEGV_ACCERR
- Invalid permissions for mapped object.
- SEGV_BNDERR (since Linux 3.19)
- Failed address bound checks.
- SEGV_PKUERR (since Linux 4.6)
- Access was denied by memory protection keys. See pkeys(7). The
protection key which applied to this access is available via
si_pkey.
The following values can be placed in
si_code for a
SIGBUS signal:
- BUS_ADRALN
- Invalid address alignment.
- BUS_ADRERR
- Nonexistent physical address.
- BUS_OBJERR
- Object-specific hardware error.
- BUS_MCEERR_AR (since Linux 2.6.32)
- Hardware memory error consumed on a machine check; action required.
- BUS_MCEERR_AO (since Linux 2.6.32)
- Hardware memory error detected in process but not consumed; action
optional.
The following values can be placed in
si_code for a
SIGTRAP
signal:
- TRAP_BRKPT
- Process breakpoint.
- TRAP_TRACE
- Process trace trap.
- TRAP_BRANCH (since Linux 2.4, IA64 only))
- Process taken branch trap.
- TRAP_HWBKPT (since Linux 2.4, IA64 only))
- Hardware breakpoint/watchpoint.
The following values can be placed in
si_code for a
SIGCHLD
signal:
- 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 (since Linux 2.6.9)
- Stopped child has continued.
The following values can be placed in
si_code for a
SIGIO/
SIGPOLL signal:
- 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 following value can be placed in
si_code for a
SIGSYS signal:
- SYS_SECCOMP (since Linux 3.5)
- Triggered by a seccomp(2) filter rule.
sigaction() returns 0 on success; on error, -1 is returned, and
errno is set to indicate the error.
- 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, POSIX.1-2008, SVr4.
A child created via
fork(2) inherits a copy of its parent's signal
dispositions. During an
execve(2), the dispositions of handled signals
are reset to the default; the dispositions of ignored signals are left
unchanged.
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 and later allow 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 perform a
wait(2) or similar.
POSIX.1-1990 specified only
SA_NOCLDSTOP. POSIX.1-2001 added
SA_NOCLDSTOP,
SA_NOCLDWAIT,
SA_NODEFER,
SA_ONSTACK,
SA_RESETHAND,
SA_RESTART, and
SA_SIGINFO. Use of these latter values in
sa_flags may be less
portable in applications intended for older UNIX implementations.
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.
See
signal-safety(7) for a list of the async-signal-safe functions that
can be safely called inside from inside a signal handler.
The glibc wrapper function for
sigaction() gives an error (
EINVAL)
on attempts to change the disposition of the two real-time signals used
internally by the NPTL threading implementation. See
nptl(7) for
details.
On architectures where the signal trampoline resides in the C library, the glibc
wrapper function for
sigaction() places the address of the trampoline
code in the
act.sa_restorer field and sets the
SA_RESTORER flag
in the
act.sa_flags field. See
sigreturn(2).
The original Linux system call was named
sigaction(). However, with the
addition of real-time signals in Linux 2.2, the fixed-size, 32-bit
sigset_t type supported by that system call was no longer fit for
purpose. Consequently, a new system call,
rt_sigaction(), was added to
support an enlarged
sigset_t type. The new system call takes a fourth
argument,
size_t sigsetsize, which specifies the size in bytes of the
signal sets in
act.sa_mask and
oldact.sa_mask. This argument is
currently required to have the value
sizeof(sigset_t) (or the error
EINVAL results). The glibc
sigaction() wrapper function hides
these details from us, transparently calling
rt_sigaction() when the
kernel provides it.
Before the introduction of
SA_SIGINFO, it was also possible to get some
additional information about the signal. This was done by providing an
sa_handler signal handler with a second argument of type
struct
sigcontext, which is the same structure as the one that is passed in the
uc_mcontext field of the
ucontext structure that is passed (via
a pointer) in the third argument of the
sa_sigaction handler. See the
relevant Linux kernel sources for details. This use is obsolete now.
In kernels up to and including 2.6.13, specifying
SA_NODEFER in
sa_flags prevents not only the delivered signal from being masked
during execution of the handler, but also the signals specified in
sa_mask. This bug was fixed in kernel 2.6.14.
See
mprotect(2).
kill(1),
kill(2),
pause(2),
pidfd_send_signal(2)
restart_syscall(2),
seccomp(2)
sigaltstack(2),
signal(2),
signalfd(2),
sigpending(2),
sigprocmask(2),
sigreturn(2),
sigsuspend(2),
wait(2),
killpg(3),
raise(3),
siginterrupt(3),
sigqueue(3),
sigsetops(3),
sigvec(3),
core(5),
signal(7)