sigaction(2) System Calls Manual sigaction(2)
NAME
sigaction, rt_sigaction - examine and change a signal action
LIBRARY
Standard C library (libc, -lc)
SYNOPSIS
#include <signal.h>
int sigaction(int signum,
const struct sigaction *_Nullable restrict act,
struct sigaction *_Nullable restrict oldact);
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
sigaction():
_POSIX_C_SOURCE
siginfo_t:
_POSIX_C_SOURCE >= 199309L
DESCRIPTION
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_han-
dler 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 can be
one of the following:
• SIG_DFL for the default action.
• SIG_IGN to ignore this signal.
• 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 sig-
nal. 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 han-
dler 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 implemen-
tations, it is not.
SA_NODEFER
Do not add the signal to the thread's signal mask while the han-
dler is executing, unless the signal is specified in act.sa_mask.
Consequently, a further instance of the signal may be delivered
to the thread while it is executing the 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 de-
fault stack will be used. This flag is meaningful only when es-
tablishing 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 sig-
nal(7) for a discussion of system call restarting.
SA_RESTORER
Not intended for application use. This flag is used by C li-
braries to indicate that the sa_restorer field contains the ad-
dress of a "signal trampoline". See sigreturn(2) for more de-
tails.
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.
SA_UNSUPPORTED (since Linux 5.11)
Used to dynamically probe for flag bit support.
If an attempt to register a handler succeeds with this flag set
in act->sa_flags alongside other flags that are potentially un-
supported by the kernel, and an immediately subsequent sigac-
tion() call specifying the same signal number and with a non-NULL
oldact argument yields SA_UNSUPPORTED clear in oldact->sa_flags,
then oldact->sa_flags may be used as a bitmask describing which
of the potentially unsupported flags are, in fact, supported.
See the section "Dynamically probing for flag bit support" below
for more details.
SA_EXPOSE_TAGBITS (since Linux 5.11)
Normally, when delivering a signal, an architecture-specific set
of tag bits are cleared from the si_addr field of siginfo_t. If
this flag is set, an architecture-specific subset of the tag bits
will be preserved in si_addr.
Programs that need to be compatible with Linux versions older
than 5.11 must use SA_UNSUPPORTED to probe for support.
The siginfo_t argument to a SA_SIGINFO handler
When the SA_SIGINFO flag is specified in act.sa_flags, the signal han-
dler 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 infor-
mation that was saved on the user-space stack by the kernel; for
details, see sigreturn(2). Further information about the ucon-
text_t structure can be found in getcontext(3) and signal(7).
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 */
union sigval 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 re-
turned 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)). Up
to Linux 2.6, and since Linux 2.6.27, these fields report CPU time in
units of sysconf(_SC_CLK_TCK). In Linux 2.6 kernels before Linux
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 ex-
tent 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 deliv-
ering the event. BUS_MCEERR_* and si_addr_lsb are Linux-specific ex-
tensions.
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 val-
ues 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_er-
rno, and other fields as described in seccomp(2).
The si_code field
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 SIG-
TRAP 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 de-
scribed in the remainder of this section. Since glibc 2.20, the defini-
tions 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 re-
quired 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 up to Linux 2.2; from Linux 2.4 onward SI-
GIO/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 re-
quired.
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 sig-
nal:
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.
Dynamically probing for flag bit support
The sigaction() call on Linux accepts unknown bits set in act->sa_flags
without error. The behavior of the kernel starting with Linux 5.11 is
that a second sigaction() will clear unknown bits from oldact->sa_flags.
However, historically, a second sigaction() call would typically leave
those bits set in oldact->sa_flags.
This means that support for new flags cannot be detected simply by test-
ing for a flag in sa_flags, and a program must test that SA_UNSUPPORTED
has been cleared before relying on the contents of sa_flags.
Since the behavior of the signal handler cannot be guaranteed unless the
check passes, it is wise to either block the affected signal while reg-
istering the handler and performing the check in this case, or where
this is not possible, for example if the signal is synchronous, to issue
the second sigaction() in the signal handler itself.
In kernels that do not support a specific flag, the kernel's behavior is
as if the flag was not set, even if the flag was set in act->sa_flags.
The flags SA_NOCLDSTOP, SA_NOCLDWAIT, SA_SIGINFO, SA_ONSTACK,
SA_RESTART, SA_NODEFER, SA_RESETHAND, and, if defined by the architec-
ture, SA_RESTORER may not be reliably probed for using this mechanism,
because they were introduced before Linux 5.11. However, in general,
programs may assume that these flags are supported, since they have all
been supported since Linux 2.6, which was released in the year 2003.
See EXAMPLES below for a demonstration of the use of SA_UNSUPPORTED.
RETURN VALUE
sigaction() returns 0 on success; on error, -1 is returned, and errno is
set to indicate the 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.
VERSIONS
C library/kernel differences
The glibc wrapper function for sigaction() gives an error (EINVAL) on
attempts to change the disposition of the two real-time signals used in-
ternally 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 pur-
pose. 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 cur-
rently 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.
STANDARDS
POSIX.1-2008.
HISTORY
POSIX.1-2001, SVr4.
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)). Neverthe-
less, 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_NOCLD-
WAIT, SA_NODEFER, SA_ONSTACK, SA_RESETHAND, SA_RESTART, and SA_SIGINFO
as XSI extensions. POSIX.1-2008 moved SA_NODEFER, SA_RESETHAND,
SA_RESTART, and SA_SIGINFO to the base specifications. 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 later. On older kernels the Linux implementa-
tion allowed the receipt of any signal, not just the one we are in-
stalling (effectively overriding any sa_mask settings).
NOTES
A child created via fork(2) inherits a copy of its parent's signal dis-
positions. During an execve(2), the dispositions of handled signals are
reset to the default; the dispositions of ignored signals are left un-
changed.
According to POSIX, the behavior of a process is undefined after it ig-
nores 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.
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-safety(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 about the signal. This was done by providing an
sa_handler signal handler with a second argument of type struct sigcon-
text, 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.
BUGS
When delivering a signal with a SA_SIGINFO handler, the kernel does not
always provide meaningful values for all of the fields of the siginfo_t
that are relevant for that signal.
Up to and including Linux 2.6.13, specifying SA_NODEFER in sa_flags pre-
vents 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 Linux 2.6.14.
EXAMPLES
See mprotect(2).
Probing for flag support
The following example program exits with status EXIT_SUCCESS if SA_EX-
POSE_TAGBITS is determined to be supported, and EXIT_FAILURE otherwise.
#include <signal.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
void
handler(int signo, siginfo_t *info, void *context)
{
struct sigaction oldact;
if (sigaction(SIGSEGV, NULL, &oldact) == -1
|| (oldact.sa_flags & SA_UNSUPPORTED)
|| !(oldact.sa_flags & SA_EXPOSE_TAGBITS))
{
_exit(EXIT_FAILURE);
}
_exit(EXIT_SUCCESS);
}
int
main(void)
{
struct sigaction act = { 0 };
act.sa_flags = SA_SIGINFO | SA_UNSUPPORTED | SA_EXPOSE_TAGBITS;
act.sa_sigaction = &handler;
if (sigaction(SIGSEGV, &act, NULL) == -1) {
perror("sigaction");
exit(EXIT_FAILURE);
}
raise(SIGSEGV);
}
SEE ALSO
kill(1), kill(2), pause(2), pidfd_send_signal(2), restart_syscall(2),
seccomp(2), sigaltstack(2), signal(2), signalfd(2), sigpending(2), sig-
procmask(2), sigreturn(2), sigsuspend(2), wait(2), killpg(3), raise(3),
siginterrupt(3), sigqueue(3), sigsetops(3), sigvec(3), core(5), sig-
nal(7)
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